[Instreng.com]All Interview Instrumentation Questions Answered by Carl Ellis

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© 2014 Carl Ellis, Measure First

This document includes Carl Ellis’s responses to the questions posed on Allinterview.com’s

instrumentation forum.

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Q #

9880

warning lamp does not light, but it does light when the wire from the lamp going to

the sending unit is grounded (the part that plugs into the sending unit) what does this

indicate)?

Answ

er

If the lamp lights when the wire from lamp to the sending

unit is grounded, then the other side of the lamp has

power. The circuit is designed for the lamp to light when

the sending unit provides a complete electrical circuit.

The lamp lighting proves that

- the bulb is good

- there is power at the lamp.

If the warning lamp does not light when it should, then the

sending unit is faulty.

Carl Ellis

Measure First

0 2014

-04-

18

Q #

6054

6

Could anybody tell how much amps will a single input to plc will consume?

Answ

er

The spec sheets on my desk have these specs:

for an AC voltage digital input:

2 mA nominal @ 230 VAC, 50 Hz

for a DC voltage digital input:

2.3 mA @ 12 VDC

6.9 mA @ 24 VDC nominal

for a dry contact digital input (internal 15Vdc source):

2.6 mA nominal

Interviewees should be able to convert from milliamps to amps.

Normally, power (watts) is 'consumed'.

Current (amps) is a 'draw' on the power supply.

Carl Ellis

Measure First

0 2014

-04-

06

2


Q #

1008

74

how in the 2 - wire transmitter, 24v supply and the 4ma signal gets in the same path?

Answ

er

The 2 wire transmitter, DC power supply and the load (some

receiver, an analog input on a

PLC/DCS/Controller/recorder/indicator) are wired in series.

The installer 'gets them in the same path', an electrical

series circuit.

The transmitter uses about 3.5-3.6 mA of current as power to

run its own electronics that excite and/or read the sensor,

interpret the sensor's signal and create a scaled output.

A 2 wire transmitter acts like a variable resistor, or

transistor, in that it regulates its internal resistance so

that the output is scaled 4.0-20.0 mA, proportional to the

process variable it is sensing, with the loop current the

ranging 4-20mA.

The power supply voltage must be high enough to drive 20mA

(21.xmA if upscale failsafe is used/selected) through the

load (the input's resistance, wire resistance, any stray

resistance (terminal block). 24Vdc power supplies are the

most widely used power supplies, but most 2 wire

transmitters can operate with power supplies up to about

32-36 volts DC.

Carl Ellis

Measure First

0 2014

-01-

06

Q #

1045

60

how to check the leak in thermowells

Answ

er

From Wika's FAQ:

What tests and inspections are stipulated for thermowells?

In accordance with DIN 43772 Point 4.6, all tests and

certifications should be agreed between the manufacturer and

operator.

What tests are usual or possible for thermowells?

Common non-destructive tests are the pressure test and, for

welded thermowells, the liquid penetrant test. In addition,

to test the centrality of the bore, ultrasound or X-ray

testing is possible. To test the sealing, helium leak

testing is an option. The surface finish or surface

hardness may also be tested. A material test would be

Positive Material Identification (PMI test).

What does ZFP, NDE or NDT mean?

ZFP is the German abbreviation for "Zerstörungsfreie

0 2013

-12-

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Prüfungen" (non-destructive examinations). The abbreviations

NDE or NDT stand for "Non-Destructive Examination" or

"Non-Destructive Testing", respectively. This is used to

refer to non-destructive inspections or tests on components

in general.

What is a hydrostatic pressure test?

The hydrostatic pressure test is a pressure and strength

test of the components of a thermowell in accordance with

the AD2000 data sheet HP30. For the test, the thermowell is

clamped into a test fixture and loaded at room temperature

with a defined test pressure and duration (e.g. three

minutes). In general, one differentiates between external

and internal pressure testing. Typical test pressures are

1.5 times the nominal pressure of the flange with external

pressure, or 500 bar with internal pressure. The test is

performed with water with a chloride content < 15 ppm. After

passing the hydrostatic pressure test, the thermowell is

marked with a "P".

What is a helium leak test?

For leak testing in accordance with DIN EN 1779 (1999) / EN

13185, helium 4.6 is used as a test gas. The test is able to

detect minimal leakage rates and is considered the most

sensitive test method for leak testing. In general, one

should distinguish between an integral and local test

method. In the integral test, leak rates (e.g. 1x10-7 mbar *

l / s) can be determined, while the local testing enables

the location of the leak to be determined using a spray

probe. After passing a helium leak test, the thermowell is

labeled with a corresponding sticker.

What is a PMI test?

The PMI (positive material identification) test proves which

alloy constituents exist in the material. There are various

common test procedures. With optical emission spectrometry

(OES) in accordance with DIN 51008-1 and -2, an arc is

generated between the thermowell surface and the test

equipment, and the spectrum of this arc enables the alloy’s

elements to be identified – both qualitatively and

quantitatively. A characteristic feature of this procedure

is the fire mark that is left on the workpiece. A test

procedure which doesn’t damage the surface is X-ray

analysis; during the X-ray the atoms of the thermowell

material are energised until they radiate themselves. The

wavelength and intensity of the emitted radiation is again a

measure of the alloy’s constituent elements and their

concentrations. Following a successful PMI test / positive

material identification test, the thermowell is marked with

"PMI".

What is a dye penetrant test?

With the penetrant test in accordance with DIN EN 571-1,

fine surface cracks and porosities in weld seams can be made

visible. After cleaning the surface to be inspected, a

contrast agent (red or fluorescent) is sprayed on. Through

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the capillary effect, this agent penetrates any surface

defects there might be. After re-cleaning the surface, a

developer (white) is then sprayed on, which extracts the

contrast agent (from any hairline cracks, etc.) and through

color contrast, enables an easy evaluation of the defects.

After passing a liquid penetration test, the thermowell is

marked with "PT".

What is a X-ray testing?

Through an X-ray test to EN 1435 or ASME Section V, Article 2, Edition

2010, for example, full penetration welds on thermowells can be

investigated with respect to irregularities (cracks, voids, insufficient

bonding). Here,depending on the dimensions of the thermowell, up to five

X-ray images may be necessary to determine irregularities with sizes <

0.5 mm in the full-penetration weld. An X-ray examination can also be

used to record the bore centrality in solid body material thermowells.

For this purpose, two images of the thermowell tip at 90° to each other

are required.

What is an ultrasonic test?

Through an ultrasonic test to DIN EN ISO 17640, for example,full

penetration welds on thermowells can be investigated with respect to

irregularities (cracks, voids, insufficient bonding). To do this, the

reflections of a radiated ultrasonic signal from the interfaces of

irregularities are measured. To determine the position of the

irregularities, the ultrasound machine is set in advance with the aid of

a reference body. The ultrasonic method can also be used to measure the

wall thickness of a solid body material thermowell, in order to determine

the bore centrality.

Carl Ellis

Measure First

Q #

1121

84

how to use K thermocouple instead E- type without compesation cable

Answ

er

Any thermocouple measuement requires 2 temperature measurements

- hot junction measurement

- cold junction measurement

A cold junction is created at the point where non-compensation wire is

connected to the thermocouple wire.

A measurement without compensation wire AND without a corresponding cold

junction measurement is not valid.

The only way to take a K type thermocouple measurement (or any type

thermocouple measurement) without compensating cable is to use a

temperature transmitter configured for the type of thermocouple because

the transmitter will perform the cold junction measurement at its wiring

terminals.

Carl Ellis

Measure First

0 2014

-03-

04

5


Q #

1125

76

how can we measure the pressure in underground cooling water pipeline by using

pressure transmitter?what about the tapping position?

Answ

er

If the transmitter is not directly mounted on the

underground pipe, which is not unusual unless there is

vault access, then an impulse tube/pipe has to come up out

of the ground. The pressure measurement will be in error,

low, by the difference in elevation between the pipe's

elevation and the pressure transmitter's elevation.

Suppose the pipe is 2.1m below the grade level and the

pressure transmitter is 1.6m above grade. The elevation

difference is 3.7m.

The measurement is 3.7m or 0.362 bar low.

It is not unusual to have water supplied at pressure

between at 4-8 bar. At 4 bar pressure, an error of 0.36m

water column is a 9% error, a vary large value for an

insturment capable of 0.25% or better accuracy.

The commissioning of the unit should include the

appropriate zero correction to compensate for such a large

error.

Carl Ellis, Measure First

0 2013

-10-

20

Q #

1158

79

Dear friends, I have a question: We work on a project which has some boilers to

provide steam for desalination unit. The boiler area is safe and the only hazard source

is the fuel lines of boilers. I want to know according to NFPA standard which type of

instrument should be selected on the fuel line: Eex Type or Non-Eex type? Thanks in

advance

Answ

er

I totally disagree. Because NFPA 86 (2007) specifically

states that there is no need to impose requirements for

wiring methods appropriate for a hazardous (classified

area).

------- NFPA 86 (2007) ------------

A.4.1.3.3 Unless otherwise required by the local

environment, ovens and furnaces and the surrounding area

are not classified as a hazardous (classified) location.

The primary source of ignition associated with an oven

installation is the oven heating system or equipment or

materials heated. The presence of these ignition sources

precludes the need for imposing requirements for wiring

methods appropriate for a hazardous (classified) location.

Refer to Section 3.3 of NFPA497, Recommended Practice for

the Classification of Flammable Liquids, Gases, or Vapors

and of Hazardous (Classified) Locations for Electrical

Installations in Chemical Process Areas, and Section 3.3

------------ end ---------------

The 'gas train' or 'fuel train' is a pipe carrying fuel.

It is assumed that the fuel stays in the pipe until it gets

to the burner or is bypassed to an equally secure pilot

fuel line.

0 2013

-12-

03

6


Control components in the pipeline typically need agency

approvals for conformance to failsafe conditions, but not

for explosion protection.

Carl Ellis

Measure First

Q #

1202

66

How flow control valve is working with pnuematic positioner and I/P convreter from

command is given from remote

Answ

er

A controller (the remote device) generates an electronic

4-20mA output signal which tells the valve what its position

should be.

The 4-20mA signal is an input to an I/P

(current-to-pneumatic) transducer, which creates a pneumatic

output signal proportional to the 4-20mA electronic signal

from the controller, probably a 3-15psi pneumatic signal.

The pneumatic control signal is an input to the pneumatic

positioner, which mechanically senses the valve's position.

The positioner drives the actuator with as much supply air

pressure as is necessary to drive the valve to position

demanded by the control signal.

Carl Ellis

Measure First

0 2014

-07-

05

Q #

1220

28

What are the most frequent problem faces with Vortex flow meter?

Answ

er

Installation error - there is insufficient upstream and

downstream straight run piping for flow conditioning.

Failure to understand that all vortex do low flow cutoff at

approximately 10% of flow because there are not enough

vortices formed to produce an accurate flow rate

measurement. Low velocity flow rate in a steam header can

indicate zero flow even when the boiler is operating with

reasonable loads.

Corrosive fluid destroys the thin sensor membranes.

Carl Ellis

Measure First

0 2014

-02-

18

Q #

1238

38

What is output of Thermocouple at 25 deg c

Answ

er

Question: What is output of Thermocouple at 25 deg C?

I'd flunk all of you except B0nz0 because only B0nz0

0 2013

-08-

17

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understands thermocouple thermometry.

The answer is, you don't know because the cold junction

temperature is not specified. The question attempts to

find out if the concepts of a thermal gradient and cold

junction compensation are understood. Which is why, I

assume, the question does not specify the type thermocouple.

B0nz0 is correct; any type thermocouple sitting on a desk

at 25 deg C; hot end = 25 Deg C, cold end = 25 Deg C, will

read 0.0mV because there is no thermal gradient.

The cold junction temperature is the same as the hot end

temperature. Doesn't matter if it's a Type K or J or S.

With the same temperature at both ends, the output is 0mV.

But without specifying what the cold end temperature is, an

answer of 0mV assumes that the cold end is at 25 Deg C.

In the real world, it usually isn't.

The hot end can be at 25 Deg C with the cold end on a

terminal strip in control panel that's at 45 Deg C because

of the heat generated by the electronics in the control

panel. Then you'll read a negative voltage because the

cold end is hotter than the hot end!

The hot end can be at 25 Deg C and the cold end is

connected to handheld temperature calibrator in northern

Canada in the winter where the cold end terminal connection

might be at -30 Deg C.

In each case, it's the cold junction compensation that

corrects the temperature reading by measuring the

temperature of the cold end connection and taking that

temperature into account.

If I were to ask the question, I'd ask, "What is the output

of a thermocouple, both ends of which are at 25 deg C, and

why?". The answer to the first part is 0mV.

Tell 'em Carl sent you.

Q #

1250

88

I have observed in field instrumentation work... I remove the fuse of an analog

transmitter then it shows 7.75 kg/sq cm (which is practically wrong value)in DCS and

when put the fuse it shows 0 in dcs. Then cheach the field transmitter termination and

found moister. clean that and then it shows 1.1 kg/ sq cm which is right value. Please

describe the above phenomenon.

Answ

er

Reason: ground current added to loop current through a

ground created by moisture.

The analog input on the DCS references ground at some point.

The moisture in the junction box or remote terminal panel

makes a connection to a local ground at that point. The

difference in potentials between the grounds causes a ground

loop - current added to the modulated loop current. The

DCS's analog input 'sees' this current, even though the

0 2013

-12-

26

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transmitter is turned off, because the current is driven by

the potential of the ground loop, not the potential from the

power supply.

When the moisture (and hopefully, the dirt) is removed, the

local ground connection is broken and the loop is isolated,

except for the DCS's AI reference to ground. So the DCS AI

'sees' only the loop current modulated by the pressure

transmitter and reports the correct loop pressure.

Some handheld calibration instruments can detect current

leakage in loop powered loops. An example is the Practical

Instrument Electronics (PIE) 334 Plus calibrator.

http://www.piecal.com/products/evolution/334Plus_mA.php

Carl Ellis

Measure First

Q #

1271

79

pls any1 sugges me book for plc,dcs & scada..

Answ

er

Tony Kuphaldt's "Lessons in Industrial Instrumentation"

download URL:

http://www.ibiblio.org/kuphaldt/socratic/sinst/book/liii.pdf

If the link breaks, search for the title and the author.

Better than the textbooks that cost a week's wages.

Carl Ellis

Measure First

0 2014

-05-

25

Q #

1279

97

i want to calibrate a transmitter in the range of -100 to 300 mmwc. How it is possible

to measure -100 to 300?.

Answ

er

Atmospheric pressure equals zero pressure (gauge).

Vacuum pressures are less than atmospheric so are negative with respect

to atmospheric pressure. -100mm would be a vacuum pressure.

Pressures above atmospheric are positive with respect to atmospheric

pressure.

Carl Ellis

Measure First

0 2014

-04-

28

Q #

1287

62

what is difference between E/P positioner and I/P positioner. Why E/P fixed in below

actuator and why I/P fixed in other place

Answ

er

An E/P transducer is a voltage to position transducer, so its input

signal from the control system is a voltage 0 2014

-07-

9


signal, like 0-5Vdc. Its output is a proportional pneumatic signal.

An I/P transducer is a current to position transducer, so its input

signal from the control system is a current signal, like 4-20mA. Its

output is a proportional pneumatic signal.

E/P and I/P are not positioners and are not called positioners because

each is a transducer that converts an input signal to a proportional

pneumatic signal.

A positioner is a much more than a transducer; a positioner is a

controller.

A positioner senses valve position and regulates its pneumatic output to

control the valve position to the setpoint defined by the input signal.

Where an E/P or an I/P is mounted is a matter of opinion,plant standard

or convenience, it is not dependent on its function.

A positioner must receive a valve position feedback signal, so a

positioner must be mounted so mechanical linkage can sense valve position

or when mounted remotely from the valve (like in high ambient temperature

applications) a remote position sensor is used to feed a signal to the

positioner.

Carl Ellis

Measure First

02

Q #

1293

83

1)unmovable objects have frequency or not?if that object have frequency,how do we

measure and transmit? 2)how do u measure the continuity without multimeter? 3)we

put 24volt dc supply in railway track how many distance it is traveled without voltage

drop?

Answ

er

How do you measure continuity without a multimeter?

Answer: with a continuity tester, which consists of a power

source (a battery)and an indicator like a lamp or LED

0 2014

-04-

18

Q #

1308

02

How to measure density of oil in your kitchen. i.e indirectly to verify the quality of

oil using min. gadgets available in the kitchen/house.

Answ

er

To determine the density/specific gravity of the oil:

Weigh an empty, clean measuring cup. The larger the volume of the

measuring cup, the better the precision.

Fill the measuring cup with the oil to a marked level, say, 500mL.

Weigh the measuring cup and oil.

Subtract the weight of the measuring cup.

The ratio of the weight of the oil to the weight of an equivalent volume

of water (1 gram/mL) is the oil's specific gravity: weight oil/weight of

water

Carl Ellis

0 2014

-05-

20

10


Measure First

Q #

1379

28

i want to control the temp of sand(welding flux) between the range of 150deg C to

170deg C. sugges me sum temp control method without PID controller.

Answ

er

An On-off controller with a setpoint of 160 Deg C and a

hysteresis of ±10 Deg C will do this type of control.

Carl Ellis

Measure First

0 2014

-05-

15

Q #

1472

54

whats the problem will arrise if we rotate the hosing screw of the differential

pressure transmitter?

Answ

er

Rotating the electronics housing screw loosens or tightens

the screw that holds the housing in place in relation to the

pressure body.

Rotating the electronics housing (for access to the screw

covers or to orient the indicator display) with respect to

the housing body has to be done with care to avoid twisting,

crimping, bending, breaking, stretching or stressing the

wiring/cables that connect the sensor(s) elements in the

pressure body to the electronics in the transmitter head.

Most transmitters allow a +/- 180 degree rotation from a

position with the length of the head in axial alignment

with the bore of the pressure ports.

Carl Ellis

Measure First

0 2014

-01-

05

Q #

1474

80

Q- Which of the following is Close loop system - a-)respiratory System b-)traffic

light system c-)washing machine d-)Room heater my option is A, what is our take on

option B & C

Answ

er

A room heater has a thermostat which senses room

temperature and turns the heater elements on or off, as

needed. It is close loop control.

A washing machine can have a closed loop feedback loop, for

example, water level, where a knob sets water level desired

[setpoint], a sensor detects level and turns off water

valve when satisfied. But the overall wash, soak, rinse,

empty, fill, spin functions are sequenced on a timed basis,

which is classic, open loop, non-feedback control.

Likewise a traffic light system can sense the presence of a

vehicle at an intersection, which enables an upcoming light

change, but the sequences of light changes are timer based,

open loop, non-feedback control.

Carl Ellis

Measure First

0 2013

-09-

06

11


Q #

1479

15

Can DP transmitter with orifice plate works on Beta ratio 0f 0.3 and differential

pressure of 100 MMWC in liquid service application?

Answ

er

An orifice plate (OP) creates a pressure drop in the flow

stream.

The question is whether an orifice plate 'works'. Works how?

An orifice plate will always 'work' by creating a pressure

drop; the proper questions are whether the permanent

pressure loss is excessive, whether the orifice plate has

been 'sized' for a specific medium at correct design

conditions, and whether the pressure drop (DP) is adequate

for instrumentation that senses the DP.

Orifice plates typically drop 100 inches water column at

100% flow rate, about 25 times the DP of 100mm water column.

A DP of 100mm water column at 100% flow rate is more

typical of an averaging pitot tube than an orifice plate. A

full scale flow rate of 100mm w.c. requires a low range

pressure transmitter, or one with a high turndown ratio.

A 0.3 Beta is acceptable.

Carl Ellis

Measure First

0 2014

-01-

01

Q #

1494

90

what is difference between two wire &three wire pt 100 ?

Answ

er

A 2 wire RTD has no means of compensating for lead wire

resistance so the reading from a 2 wire RTD will always have

some positive error due to lead wire resistance.

A 3 wire RTD can be connected to an appropriate circuit that

compensates for the resistance of the lead wires by

accounting for the resistance of the 3rd wire, which is

presumably the same length and gauge as the other two wires

across the resistance element.

Carl Ellis

Measure First

0 2014

-06-

13

Q #

1498

81

wht is snap action switch?where it is used?is there any diff between snap action

switch and normal pressure switch?

Answ

er

A snap action switch requires very little mechanical

movement (a very short linear distance) in order to actuate

the electrical switch contacts. Internally, the movement

and mechanical advantage of a lever is used to activate,

make or break, switch contacts.

The sensor elements in pressure switches: diaphragms,

pistons, bellows, all have move relatively small distances

0 2014

-01-

28

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with changes in pressure. A snap action switch is used as

the electrical component in a pressure switch because it can

be actuated/tripped by the small movement of pressure switch

sensor.

Mechanical adjustments allow for an adjustable setpoint by

moving the snap action switch with respect to the sensor

element.

The name of the firm that invented snap action switches is

Microswitch, so snap switches are frequently called

microswitches, which is technically a brand name.

In contrast, ordinary switches, like a toggle switch or a

rotary switch, rely on the relatively large movement of an

operator handle/lever to actuate the electrical contacts.

Carl Ellis

Measure First

Q #

1540

28

I have a steam flow DP transmitter that has output swinging. Steam valve is in

manual.I have good steam pressure. The impulse lines have steam tracing on them.

The steam tracing has a hole in it that is blowing raw steam on the high side impulse

line only. Could this be causing the swinging?

Answ

er

Answer:

Yes, live steam on an impulse line could cause pressure swings.

Blowing live steam on an impulse line could conceivably heat

the condensate in the impulse tubing enough to boil it.

Even if it doesn't boil, a deadheaded impulse line with have

thermal siphon flow occurring as the hot spot condensate

rises and its place is taken by cooler condensate.

If boiling is occurring, then it could be intermittent

boiling with associated pressure pulses: as the steam

bubble(s) move away from the hot spot and cooler condensate

takes it place.

None of which should be happening in an impulse line.

The high side is 'seeing' these artifacts as forces on its

diaphragm which is creating pressure swings.

Carl Ellis

Measure First

0 2014

-01-

07

Q #

1540

42

could someone explain for me what is 32-bit Floating Point ? Thanks

Answ

er

Floating point numbers are also called 'real' numbers.

Floating point refers to the inclusion of a decimal point

and values to the right of the decimal point. The number

is expressed in engineering or exponential notation, a

signed mantissa with an exponent.

0 2014

-03-

17

13


The IEEE Standard for Floating-Point Arithmetic (IEEE 754)

is a technical standard for floating-point computation

established in 1985 by the Institute of Electrical and

Electronics Engineers (IEEE).

A single precision IEEE 754 floating point is a 32 bit word

A double precision IEEE 754 floating point is 64 bit word.

A single precision FP consists of a one bit algebraic sign,

an 8 bit exponent and a 23 bit mantissa.

Floating point calculations can be done in hardware or software.

Floating point is widely used for Process Automation

Controllers (PAC) and in DCS controllers.

Double precision is frequently used in flow computers.

Single precision is typically the standard for other data an

operations. Single precision floating point is widely used

by serial data communication protocols like Modbus.

Carl Ellis

Measure First

Q #

1547

84

How can corrosion alter the accuracy and precision of a C- Bourdon tube pressure

gauge?

Answ

er

The issue with corrosion of a bourdon tube is not accuracy,

it's safety.

Corrosion failure of a bourdon tube occurs when the elastic

element (Bourdon tube) has been weakened .

The potential corrosion hazard is leakage of the process

fluid, which may be hot, toxic or corrosive, from a

measuring instrument, process pipe or tank. The other

corrosion hazard is catastrophic failure of the bourdon

tube, and the release of energy that can approach that of a

shot from a firearm.

Those failures due to corrosion dwarf the effect of

corrosion on accuracy. A change in the bourdon tube

stiffness effects gauge accuracy whether that change comes

from temperature, corrosion, or overpressure. The

likelihood that corrosion will destroy the bourdon tube

result in the ultimate certainty of "uncertainty" - total

non-accuracy due to failure to operate.

Whoever wrote this question missed the point entirely about

corrosion effects on pressure elements in piping.

The question should be, "How can corrosion alter the safety

of bourdon tube pressure gauge?"

Carl Ellis

Measure First

0 2014

-03-

24

14


Q #

1576

28

what is LAS in foundation fieldbus protocol?

Answ

er

What is Foundation Fieldbus LAS?

Link Active Scheduler (LAS)

A Link Active Scheduler (LAS) is a deterministic,

centralized bus scheduler that maintains a list of

transmission times for all data buffers in all devices that

need to be cyclically transmitted. Only one Link Master

(LM) device on an H1 fieldbus Link can be functioning as

that link's LAS.

The Master and Primary Backup Link Active Scheduler (LAS)

shall reside in the host control system in redundant H1

network interface cards.

An additional backup LM Link Master (B/U LAS Link Active

Scheduler) shall be configured for all networks and shall

reside in the network device with the lowest field node

address. The Backup LAS should be placed in a separate

device that does not have controlling functions.

Primary and secondary interfaces should sit in separate

backplanes with separate CPUs, preferably in separate

control panels. Similarly, the primary and secondary power

supplies should not share the same backplane. Having

redundant interface cards, CPUs, and power supplies in the

same backplane is self-defeating since the backplane is a

single point of failure.

Each network shall be configured for automatic fail-over to

the Back-up LAS in the event of Master LAS failure.

Carl Ellis

Measure First

0 2014

-01-

20

Q #

1577

91

Explain, and give a real-world example of MODULATING control

Answ

er

1) A light dimmer switch

2) the throttle on an automobile

0 2014

-05-

07

Q #

1580

50

What is require for PLC Programming?

Answ

er

You'd all fail my class for failure to mention these three

things:

- A project scope document

What is to be controlled, alarmed, displayed and

0 2013

-10-

15

15


communicated? What is the purpose and what are the

functional criteria for the project? What are requirements

for acceptance of the project? What limits the

dreaded 'scope creep' ("Oh, I thought it would

include . . ." or "we need to add this")

I tell my pupils that the homework problem description

constitutes the 'scope' for purposes of a class, but in the

real world it might be a 400 page scope document, but

whatever form it takes, they should never accept a verbal

description, but should insist on a written document,

because that's what defines whether their programming meets

the standard of having been done properly.

- An I/O list is absolutely essential. What are the

inputs, what are the outputs?

- The PLC's documentation

Nobody remembers all the little syntax or functional

details. It's up to the programmer to look this stuff up

because in the real world, there's no 'teacher' to answer

the question about the timer's minimum interval.

Those three things are essential. Lack of any of the 3

constitutes negligence.

Lack of mention of any of the three in the 5 answers already

posted tells me your instructors are book people, not real

world PLC programmers.

------------------------------


Q #

1590

31

I want to know if flowrate increases in 16' pipe line what will happen to upstream

pressure and why?

Answ

er

I want to know if flowrate increases in 16' pipe line what

will happen to upstream pressure and why?

There can be flow only if there is a differential pressure

to create the flow.

To increase flow rate, the differential pressure has to

increase.

To increase the differential pressure:

- the downstream pressure can decrease

- the upstream pressure can increase

- both upstream and downstream pressures can move in the

same direction, but vary in the magnitude of the change so

that the DP increases.

This principle applies to any flow, the size of the pipe

doesn't matter.

Carl Ellis

0 2014

-01-

06

16


Measure First

Q #

1601

56

Generally in which location ( JB or IO card or DCS) the thermocouple compensation

is done.Also where they measure the atmosphere temp ( for compensation) in JB OR

DCS panel? How they compensating that atmosphere temp? is any circuit for that (

like in rtd wheatstone bridge)?

Answ

er

Cold junction compensation (CJC) is done at the point where

the thermocouple wire or thermocouple extension wire

connects to the thermocouple input, either an I/O card, a

temperature transmitter, or a thermocouple input on a

recorder/controller/indicator/alarm. RTD's do not need

CJC. Thermistors do not need CJC.

The purpose of CJC is to measure the temperature of the

terminal connection block with respect to the freezing

point of water (because that is the 'cold junction' and the

national standard thermocouple tables are referenced to) a

cold junction at the freezing point of water) and add that

value to the temperature value determined by the EMF/mV

output of the thermocouple.

The temperature of the terminal connection block is affected

by ambient, room, or atmospheric temperature but is also

affected by the heat generated by the device's electronics,

other electronics in close proximity, and whatever cooling

the installation provides. Because all of those factors can

vary in temperature, the CJC involves measuring the

temperature of the (terminal) connections.

Temperature-sensing ICs, thermistors, and RTDs are used for

CJC measurements. Thermocouples cannot be used for CJ

measurement.

So the answer is that the CJ measurement is made at the

thermocouple input, presumably the I/O card or temperature

transmitter, never at a junction box.

The actual temperature calculation, measured T/C EMF plus

CJC, can be done on the I/O card or in the 'system', a PLC,

temperature transmitter, controller, DCS, whatever,

depending on design.

CJ should not measure room temperature or atmospheric

temperature (although some poorly designed thermocouple

devices do so); CJ should measure the terminal temperature

where the thermocouple wires connect to the I/O. A

temperature transmitter measures the CJC in order to do the

calculation for temperature.

There are special techniques for CJC for high accuracy

calibration involving an ice bath, but industrial devices

use integrated CJC.


0 2013

-07-

09

Q #

1603

what are the installation requirement is common to all flow meter when volumetric

flow rate is being measured

17


51

Answ

er

The (mostly unstated and assumed) common requirement for all

commercial flow meters is a full pipe.

The presence of air or gas in a liquid line not only causes

an inaccuracy in the measurement, in many cases it can cause

the measurement to fail altogether.

A close second for common requirements is to avoid vertical

downflows for liquids.

Carl Ellis

Measure First

0 2014

-06-

12

Q #

1604

40

why we use 24 vdc in instruments mostly?

Answ

er

There is no national or international standard for 24Vdc in

industrial instrumentation and control.

The use of 24Vdc has evolved out of common practice and has

seen global use and acceptance as a 'de facto' convention,

as opposed to a formal standard. The acceptance goes

beyond loop powered transmitters, it goes into discrete

control as well.

The reasons are many.

1) 24Vdc is a low enough voltage to be considered 'safe'.

It is accepted as a voltage level that does not shock human

beings through contact with dry skin.

2) 24VDc is a high enough voltage to do real work with

currents that are practical.

3) An enormous array and variety of field instruments and

controls can be designed and operated intrinsically safe at

24Vdc, not just loop powered transmitters.

4) Loop powered field instruments powered by 24Vdc can

drive 500-600 ohms of load resistance, enough for one valve

positioner (split ranging 2 valve positioners with a single

current output driven by 24Vdc is difficult).

5) Lead acid batteries are useable for battery back-up or

UPS supplies.

6) On the discrete side, 24Vdc is sufficient voltage to

break through oxide coating on switch and relay contacts

7) For discrete control, 24Vdc offers a tolerance band

around binary 'high's and binary 'low's of more than a

couple volts, which allows for operation in a moderate

level of electrical noise.

8) Statistically, 24Vdc devices have failure rates about

0 2013

-10-

07

18


half of that of 120/240Vac actuation devices (relays and

solenoids). This a serious consideration for emergency

shutdown or SIS systems.

Carl Ellis

Measure First

Q #

1621

15

generally transmitter power supply having some range(12v to 35v) but the out put(4-

20 mA) must same for given input.If we vary the power supply voltage then output

mA also to be change according to V=IR. Is there any voltage regulator in the

transmitter ?

Answ

er

Yes, V=IR, but that happens because the transmitter

functions as a transistor, varying its resistance in order

to regulate the loop current.

It is expected that the power supply voltage remains constant.

The transmitter acts as a transistor and regulates the loop

current as long as there is sufficient power supply voltage

to drive the current through the loop resistance (ohms law).

If the power supply voltage is not adequate, the loop

current will be limited to something less than a full 20mA

and in some cases the transmitter power-cycles itself and

starts through repeated initializations, trying to drive

current, but failing to do so.

Changing the power supply voltage will not change the loop

current as long as the power supply voltage change is above

the minimum needed, because that is the design

intent/purpose of 2 wire loop transmitters: to regulate the

loop current to match the process variable the transmitter

is measuring. Any excessive power supply voltage is

'dropped' in the transmitter in order to regulate the loop

current.

Every 2 wire loop powered transmitter has either an equation

for calculating the voltage required for a given loop load

resistance or a graph called a 'load line' that shows

voltage versus loop resistance. The minimum required

voltage to light up the transmitter is called "lift-off"

voltage, which is the minimum required when a power supply

is directly connected to the transmitter, like on the shop

bench, with no load; no analog input, no appreciable wire

resistance.

A typical 24Vdc power supply will drive 500 ohms of loop

resistance with most 2 wire transmitters.

Carl Ellis

Measure First

0 2014

-02-

08

Q #

1629

43

You are looking at the RS-485-side of an RS-232/RS-485 converter and the power

supply voltage for the converter is +5V. Under idle conditions (no transmission), and

assuming you have enabled the RS-485 output: what would you expect at: (a) Output

A measured against output B (b) Output A measured against ground (c) Output B

19


measured against ground

Answ

er

You are looking at the RS-485-side of an RS-232/RS-485

converter and the power supply voltage for the converter is

+5V. Under idle conditions (no transmission), and assuming

you have enabled the RS-485 output: what would you expect

at: (a) Output A measured against output B (b) Output A

measured against ground (c) Output B measured against ground

With a supply voltage of 5V, the sink driver is usually able

to pull the voltage on each conductor fairly close to signal

ground (0.5V), but the source driver usually has a voltage

drop of approximately 1-1.5V. The voltage swing on each

conductor is therefore typical 3-3.5V, but it may be as low

as approximately 2V.

Answer a)

Hence, output A (-) measured with respect to output B (+)

would read approx -3V to -3.5V ( a negative voltage) in the

idle state. (The logic is inverted: a negative voltage is a

mark or logic 1)

Answers b and c)

Output A (-) measured with respect to ground: ~3.5V to 4V

Output B (+) measured with respect to ground: ~0.5V to 1V

This is a 3V differential, with 2.5V Vcm common mode, where

common mode is the mean of the differential plus the Vcm

common mode above the reference ground.

Carl Ellis

Measure First

0 2014

-04-

19

Q #

1629

44

Profibus DP is a multi-master system. Would it work on RS-485 4W or not, and

justify your answer

Answ

er

The Profibus DP specification defines the hardware transport

physical layer as 2 wire RS-485. The communications

protocol and the transport layer are both part of the

Profibus DP specification (the separate 2 wire cable for

24Vdc power supply is not involved with the communications).

Any device that passes Profibus DP certification will

conform to the Profibus DP specification.

Therefore, a 4 wire RS-485 device will not work on a

Profibus DP network because it won't be a Profibus DP

device, it does not meet the Profibus DP spec, which is 2

wire RS-485.

Attempting to operate a 4 wire RS-485 device on a Profibus

DP network will only disrupt Profibus DP communications on

the network.

Carl Ellis

Measure First

0 2013

-12-

26

20


Q #

1650

86

how to connect the HART protocal with control valves

Answ

er

HART can be used to write setpoints to or to read position

or diagnostic information from HART enabled positioners on

control valves.

But it takes either a very high end PLC with HART I/O or a

DCS with HART I/O and the accompanying software application

to do so.

On the other hand, HART is routinely used to configure valve

positioners, for instance, either basic setup or more

sophisticated tasks, like entering a set of values for a

valve position characteristic table.

Carl Ellis

Measure First

0 2014

-03-

23

Q #

1656

39

How a rtd work, and what volts/current goes to a RTD to measure the resistence?

Answ

er

The source current, a fixed constant current, supplied to

an RTD resistive element is called the excitation current.

The excitation current through the RTD's resistive element

the copper lead wire develops a voltage drop.

The voltage drop across the third wire in a 3 wire RTD is

subtracted from the combined voltage drop of the RTD

element and the 2nd wire, which eliminates the voltage drop

of the copper wire and leaving only the voltage drop across

the RTD element.


0 2013

-10-

21

Q #

1660

92

what is the tempareture conpensation?what is the use of this?

Answ

er

What is temperature compensation? How is temperature used in

instrumentation?

Answer:

Temperature compensation is an adjustment to instrument

readings to account for known physical changes due to

temperature.

Temperature and pressure compensation to a differential

pressure measurement can produce an inferred mass flow rate

measurement, rather than a volumetric flow rate.

0 2014

-01-

09

21


Most high end instruments have internal temperature

compensation that affects the measurement that is reported,

for instance, DP transmitters measure pressure body

temperature and electronic body temperature.

Fuel storage tanks will measure temperature gradient from

the top to the bottom of a tank and calculate volume and

mass adjustments accordingly, for inventory purposes.

Thermal dispersion flow meters adjust their flow rate

calculations according to the flow medium's temperature.

Carl Ellis

Measure First

Q #

1673

01

Q #

167301

If i want use the 6 kg/cm2 pressure gauge then how to

calculate its pressure gauge range?

Answer The general rule of thumb is to put the working range in at the mid-range of the scale

of the pressure gauge.

Remember, a bourdon tube is damaged beyond

repair when overpressurized.

Alternative rules are

If the working Range is

- Steady: the working range can be 3/4 of full

scale value

- Fluctuating: the working range should be 2/3

of full scale value

- Full scale value: the gauge is working near

its limit.

Short duration at full scale value is advised.

Replacement with a higher range gauge is

advised.

Typical standard dual scale gauge dial ranges

are shown below:

kg/cm2 psi

0-1.06 0-15

0-2.1 0-30

0-4.2 0-60

0-7 0-100

0-10.6 0-150

0-11.0 0-160

0-14 0-200

0-21 0-300

0-28 0-400

The 0-11 bar-Kg/cm2 range puts a 6 kg/cm2

working value just

0 2014-01-

05

22


above mid range.

Yeah, yeah, I know. 1 bar is not exactly 1

Kg/cm2 but we're

talking mechanical gauges here.

Carl Ellis

Measure First

Q #

167364

Why ues 250 ohms Resistor during hart type

instruments calibration with hart protocol in details ?

Answer My friend Dan has an article on why HART needs a minimum resistance in the loop to work.

http://blog.lesman.com/2011/10/06/add250-ohm-

resistors-to-make-hart-work/#more-68

Anzal, above, is on the right path. The loop

power supply is a low pass filter. The 1200Hz

HART signal needs a

minimum resistance to develop a signal that the

HART receiver can 'see'.

Tell Dan Carl sent you.

0 2013-08-

17

Q #

167794

what do you mean by Loop checking ? What are the

cold loop check and hot loop check? what are the

difference between these two loops?

Answer Answer: A Cold Loop check is cold because it is done

without powering the loop.

A cold check consists of checking wire

continuity from the field instrument through

junction boxes and through DCS/PLC marshalling

panel terminals to the control panel. Once

continuity has been confirmed, then a Hot Loop

Check is

performed on the Loop's I/O.

A Hot loop check is 'hot' because it is done

with the loop

powered up.

A Hot loop check consists of

- simulating a process input at the field

instrument and

then checking the instrument's output and the

response to

the process signal input at DCS/PLC.

- simulating outputs at the control end and

checking for

correct response at the field end.

Carl Ellis

Measure First

0 2014-04-

23

23


Q #

167937

in my plant 100% feedwater control v/v is air to close

but due to instrument air fail,the valve opens but

instead of drum level high its get tripped in low low

drum level.the urv of level tx is 0 and lrv is -800.pls tell

me the reason. for such behaviour.

Answer Air-to-close means that the valve fails open.

A boiler feedwater valve should 'fail open' so

that the drum level never drops too low, which

can be an unsafe condition

and/or a damaging condition.

For that same reason, a boiler's feedwater

valve should be driven open on a low-low drum

water alarm to prevent the

drum level from dropping too low and creating

an unsafe condition and/or a damaging

condition.

Carl Ellis

Measure First

0 2014-01-

06

Q #

170066

if a dp transmitter has range 100 in H2O and flow is

20mmcf and if the flow is changed to 25 mmcf then

what is the formula to determine new range of

transmitter

Answer design max flow rate = 20 mmcf design max DP = 100 in H2O

new max flow rate = 25 mmcf

new max flow rate = ?

The max flow rate increases by 125% over the

design max flow

rate:

25/20 = 125% increase

DP calculations are done in normalized

percentages. 125% = 1.25

To calculate the corresponding DP at a max flow

rate 125%

higher than the original, where X = DP

sq rt of X = 1.25 (square root of normalized

percentage

of DP)

X = 1.25^2 = 1.562

1.562 = 156.2% (work in normalized

percentage)

0 2014-05-

22

24


156.2% of 100 inH2O = 156.2 in H2O at a max

flow rate of 25 mmcf

Carl Ellis

Measure First

Q #

170852

i am confusing about differential pressure transmitter

as i seen so many DPTs in different ranges how can we

calculate level and flow with mmh20 plz tel me

Answer When the primary flow element (orifice plate, venturi, averaging pitot tube) is sized to

design conditions, the result is a DP value

corresponding to 100% flow rate. That value is

used to configure the DP transmitter.

For level, measurements are made from the

transmitter datum or installed position to the

maximum level, with compensation for the

medium's specific gravity. When remote seals

are used, the specific gravity of the fill

fluid is accounted for in a compensating

calculation.

Carl Ellis

Measure First

0 2013-12-

21

Q #

172451

What is the procedure of changing a Faulty Module

of Allen Bradley PlC without stopping the Process

Answer The situation is not unique to Allen Bradley; many controller brands have I/O modules that

can be replaced without powering down the

controller which might require stopping the

process.

The marketing term is the acronym RIUP, Removal

and Insertion Under Power

The guys in the shop call it, "hot swap", hot

meaning "while powered", swap meaning to change

out.

RIUP (Removal and Insertion Under) means that

while the rack is powered, any of the I/O

Modules can be removed or inserted:

• With no physical damage to the module, to the

rack, or to other modules in the rack

• Without disturbing the functions of other I/O

modules in the rack or in the system.

It does not mean that the process will not be

disturbed.

If an I/O card needs to be changed out under

power, it means that something is wrong with

the I/O card and the process

has already been disturbed.

0 2014-01-

01

25


Under carefully controlled circumstances, this

feature enables the user to remove and insert

an I/O module without completely shutting down

a running system. However, it must be

recognized that removing or inserting an I/O

module under power is potentially hazardous

to property and to personnel.

Circumstances that dictate prudent actions

depend on conditions and specific process

applications at each user facility. It is the

responsibility of site personnel to know all

potential consequences of RIUP, and to take

actions to prevent all adverse consequences

before removing or inserting an I/O module

under power.

Each plant or installation needs to develop

guidelines for establishing appropriate 'hot

swap' procedures because

a) there are potentially lethal voltages on

Terminal Boards associated with I/O Modules.

b) each signal at each of the terminals for an

I/O module has a specific function. Any or all

of the signals may be vital for safely

controlling a process.

Carl Ellis

Measure First

Q #

172761

What is the use of +/- 20ma in analog

measurement,and which instrument gives +/-20ma

output

Answer 192 views of this thread up to this point, yet no one's answered by Jan 2014.

I have read a reference to motor speed and

direction controller that use or used a bipolar

+/- 20mA as an input

demand signal, but that's the only use I've

ever heard of.

Bipolar +/- current signals are rare in the

process industries. Even the TTY

teletypewriters of yesteryear used a unipolar

current loop, not a bipolar current.

That's not to say that it can't be done, and

many industrial analog inputs have bipolar +/-

DC voltage functionality,

like -50 to +50mV, -1.0V to +1.0V, or -12 to

+12V dc, and Phoenix Contact has an

instrumentation signal isolator that

has bipolar current output at +/- 20mA, but I

0 2014-01-

04

26


haven't a clue what one would use the bipolar

function for.

Of course, pumping a bipolar current through a

precision dropping resistor would provide a

bipolar +/- dc voltage

signal.

Carl Ellis

Measure First

Q #

173430

how to calculate the temp. of RTD&Thermocouple

when we got some mV r Ohms?if anybody has d chart

pls mail the chatrs to me...

Answer 1) You can't calculate temperature from millivolts from a thermocouple without cold

junction compensation. The thermocouple tables

only apply when the reading is cold junction

compensated.

2) To determine temperature from a resistance

value, several data are required

- the type of RTD (Pt, nickel, copper)

- the temperature coefficient (the alpha) in

Ohms/ohm/Deg C must be given - 0.00385?

0.00392?

- the RTD Standard, IEC, US DoD, DIN 43760

With the base and the alpha tables are

available on the web.

- the Ro, the nominal resistance at 0 deg C

The table you request is specific to the RTD in

question. No data, no table.

Carl Ellis

Measure First

0 2013-12-

23

Q #

173723

What is Burnout feature in a recorder?

Answer Burn-out is a fail-safe setting, normally associated with control equipment, but also

used on monitoring devices like recorders to

indicate the loss of a valid input signal.

For controllers, burn-out drives the calculated

output to a condition that is considered safe

for the process, typically to a state where

energy is not put into the process.

Burn-out is configured to go up scale (full

scale), or down scale (full scale).

For control purposes, when a signal that is a

process variable in a control loop goes full

upscale, the controller's output will drop to

zero in a reverse action

0 2013-12-

19

27


controller.

Similarly, when a signal goes full scale down

scale, a controller's output will drop to zero

in a direct action controller.

Carl Ellis

Measure First

Q #

173747

What kind of feedback is used in DP cells.

Answer What kind of feedback is used in DP cells?

In electronic DP transmitters there is no

feedback. Field sensors measure a physical

phenomenon but do not use feedback.

Closed loop controllers use feedback (the

process variable reflecting the change in the

controller's/final control element output) and

valve positioners use position feedback.

Carl Ellis

Measure First

0 2014-05-

15

Q #

174057

What is the load line in control valve?

Answer A Load line shows the valve's pressure drop at different flow rates. When graphed, the flow

rate is typically on the y axis, the pressure

drop on the x axis.

Typically the load line starts high at low flow

(high DP)and decreases as the flow increases to

a low DP when the valve is wide open.

Carl Ellis

Measure First

0 2014-02-

08

Q #

174060

IF TX SHOWS SLOW RESPONSE, WHILE A

CLOSE LOOP AND WHEN WE PUT THE

CONTROLLER ON MANUAL THE PROBLEM

RECTIFIED, SO WE CAN SAY THE PROBLEM IS

IN

Answer The problem is in the controller's PID tuning, which is tuned to be too sluggish.

All of the loop's control components work

properly:

- the transmitter reports the process variable

- the controller responds to an error (albeit

too sluggishly)

- the control valve responds to the

0 2014-03-

16

28


controller's output, as evidenced by switching

to manual mode

The PV response as reported by the transmitter

depends on the controller's appropriate

response to error. If the

controller's response to error is sluggish,

then the loop's PV response will be sluggish.

Manual mode offers a means of overriding the

PID's calculated output value and driving the

output to where it needs to be.

This question qualifies as one of the best

interview questions I've read on this site.

I'll be sure to use this question when I

interview candidates.

Carl Ellis

Measure First

Q #

175260

1)why we are using mostly Ultrasonic Flow meters to

measure flow, rather than Radar waves? 2)why we are

using Ultrasonic waves and Radar waves to measure

the Level?

Answer Most fluids will 'conduct' ultrasonic energy. Conduction is necessary because the energy

needs to originate in a

transmitter transducer, pass through the liquid

and be received by a receiver transducer. The

movement of the

fluid changes the sonic velocity of the energy,

which is measured to then calculate a flow

rate.

Many fluids either reflect the radar frequency

signal or absorb the signal so that there is

not enough energy left to

measure.

Radar frequencies reflect when a radar signal

encounters a dielectric difference. So radar

easily passes through air, with a low

dielectric constant, but reflects back from

water, which has a high dielectric.

So radar is useful for level detection with

liquids or solids that have a dielectric higher

than the minimum required to reflect energy

back to the transducer.

Ultrasonic energy reflects from most liquids

and solids so it can be used for level

measurement.

Carl Ellis

Measure First

0 2014-06-

09

29


Q #

175636

Is it possible to convert 4 wire transmitter to convert

to 2 wire with additional relay.Please send detial with

drawing,i will appreciate.

Answer No, it is not possible to convert a 4 wire device (power on 2 wires, output signal on 2

wires) to a 2 wire, loop

powered transmitter.

A 2 wire loop powered transmitter uses 3.7mA of

the current loop to runs its electronics.

That's why the 'live zero'

starts at 4.0mA, so that there's some power

available to the transmitter's electronics.

There's lots of technologies that cannot run on

the limited power of approximately 3.7mA at

24Vdc. Thermal dispersion flow meters, most

wet cell gas detectors, thermal ionization

detectors, to name a few, which have to be 3 or

4 wire devices in order to have sufficient

power to operate.

Carl Ellis

Measure First

0 2013-09-

15

Q #

177555

I have a LT of 10000mmwc range. can i use this LT in

1000mmwc range of tank?? if yes than what

happened???

Answer I have a LT of 10000mmwc range. can i use this LT in 1000mmwc range of tank?? if yes than what

happened???

A change from a range of 10000 mmwc to 1000

mmwc is a turn down of 10:1

If your transmitter supports a minimum range of

1000 mmwc within the accuracy spec you require,

then re-range the

transmitter and use it.

Most smart DP transmitters from major

manufacturers will support this.

Carl Ellis

Measure First

0 2014-06-

10

Q #

177698

I want condensate pots vent position erection of flow

transmitter ?

Answer A 'vent' or 'fill port' is installed at the top of a condensate pot, facing upwards, so the

water can be poured

through the vent's opening to fill the impulse

tubing/piping and the condensate pot.

0 2013-12-

19

30


Carl Ellis

Measure First

Q #

177775

What is i/e or o/p transmitter(ma/v)?

Answer An i/e converter is a current to voltage converter.

A current to voltage converter is a resistor.

Electrical current through a resistance

develops a voltage. Ohms Law

determines the voltage drop, E = I/R.

An i/e resistor is typically used in the

process world in 4-20mA current loops as the

component installed across the

(+) and (-) terminals of an analog input

because analog inputs expect a voltage input.

The majority of DCS, PLC, recorder, controller

and indicators use 250 ohms as an input but

other values are used, 1 ohm, 10 ohms, 62.5

ohms, 50 ohms, 100 ohms. Do not assume an

analog input is 250 ohms.

Carl Ellis

Measure First

0 2014-03-

20

Q #

178766

what is meaning of static pressure

Answer Static pressure is the line pressure (gauge or absolute) at the high port when measuring a

differential pressure.

Static pressure affects the DP measurement

(with current technologies) at pressures

greater than 1,000 psig (69barg).

Some commercial DP transmitters (Honeywell and

Yokogawa)use an absolute pressure sensor

sensing the high side port pressure which is

used to correct for static pressure error when

static pressure is greater than 1,000 psig.

A separate calculation for static pressure

error is provided for DP transmitters (by the

major vendors) as part of a total uncertainty

calculation.

Multivariable transmitters (a DP transmitter

with a temperature sensor and an absolute

static pressure sensor)

use the same technique of measuring (absolute)

static line pressure for calculating inferred

0 2013-06-

19

31


mass flow for gases.

Carl Ellis

Measure First

Q #

179067

if field commnictor 475 does not communicate with

transmitter what will you do ?

Answer 1) Check for 24Vdc? Yes, but assuming the power supply is 24Vdc, don't expect the

voltmeter to read 24v, unless you're

back in the control panel where the power

supply is.

If you're out in the field where the

transmitter is, you are not close to the

(-) side of the power supply, so where do

you put the black voltmeter probe? The only

place for it on the transmitter's

(-) terminal, but measuring voltage

across the transmitter's (+) and (-) terminals

only shows what the transmitter's voltage drop

is, not what the source voltage is.

There is the voltage drop across all parts of

the loop, the transmitter, the load and the

wiring. So checking voltage across the

transmitter's (+) and (-) terminals will never

show a full 24Vdc because of the voltage drops

in the loop.

With short wiring runs and a 250 ohm load

resistor, the voltage across the (+) and (-)

transmitter terminals might be as high as 23V @

4.0mA output, but could be as low as 10V with a

20mA output across 500 ohms.

2) Checking to confirm that the transmitter is

a smart transmitter is a good idea. There are

protocols other than HART or Foundation

Fieldbus (Honeywell's DE), and there are lots

of 'dumb' transmitters, too.

Some brands, like Siemens, allow HART to be

enabled or disabled, so the transmitter may be

HART capable but have the HART comm disabled.

3) Connecting a resistor in series is a good

idea if the load resistor is an unknown.

The article at the link below tells why a HART

loop needs at least 230 ohms of resistance:

http://blog.lesman.com/2011/10/06/add250-ohm-

resistors-to-make-hart-work/#more-68

The temporary use of a 250 ohm resistor is

typical because they're so common in a process

0 2013-07-

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plant.

4) Is the HART device at address 0 (zero)?

The communicator can talk to any address, but

when connecting point-to-point on home run

wired transmitters, the expectation is that the

transmitter will be at HART address 0.

5) Experience will show that there are the

inevitable problems like

- broken or corroded communicator leads or

clips that can go 'open' or be intermittent.

- power supply ripple or EMF induced noise on

the loop circuit can conflict with HART's 1200

baud FSk digital

communications

- some loop device, like a loop isolator or

HART-unfriendly loop indicator has stripped off

the HART signal with a low pass filter, leaving

only the 4-20mA current signal

- wrong version DD so the communicator will

connect but only perform the so-called "common

commands".


Q #

179567

what is input and output cards of plc needs to be hot

swappable mean

Answer To 'swap' is to 'exchange'.

'Hot' means electrically live and powered.

The term "hot swappable" means that an I/O card

can be replaced while the PLC/PAC/controller

rack/chassis is powered and running.

The assumption is that the controller will

recognize or 'see' the replacement card and do

whatever initialization is required to bring

the I/O card back on-

line.

An I/O card that is not 'hot-swappable'

requires a cold start, power-up, initialization

sequence for the controller to recognize the

card.


5 2013-08-

17

Q #

179619

How does milli volt is created in themocouple?

Answer The difference in temperature between the hot end and the cold end, call a thermal gradient,

of a pair of wires made from dissimilar metals

produces a voltage.

0 2013-08-

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33



Q #

179776

what is loop folder?

Answer A loop folder is a manila folder or an electronic folder that contains an assortment

of documents related to the

instrumentation for a specific process control

loop.

The exact contents of a loop folder are defined

in a project's standards/requirements.

A loop folder typically includes

- instrument manufacturer's spec/data sheet

- P&ID

- loop diagram

- loop check test sheet/checklist

- instrument calibration cert

- plot plan/location diagram

It can include

- loop test procedure

- hook-up diagram/instructions

- control/function narrative

- alarm setpoint list

- construction completion certificate

- interlock diagram

- flow sizing sheet

- CoC Certificate of Conformance

- COE Certificate of Origin

- MRT Material Test Report

- NACE certificate

- instrument configuration

(Compliments to Iris)

Carl Ellis

Measure First

0 2014-02-

08

Q #

179846

What is difference between magnetic flow transmitter

& Electromagnetic flow transmitter?

Answer Nothing. One is a shortened form of the other term.

Magmeter is an even shorter form that means the

same thing.

Carl Ellis

Measure First

0 2013-08-

19

Q #

179853

please explain clearly in PID controller if We increase

P only what will be action of the contrlere.

Answer Any given amount of error will produce less output change than before the proportional band

0 2013-08-

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was increased.

Increasing the proportional band will make the

controller less sensitive to the deviation of

the process variable

(PV) from the setpoint (SP).


17

Q #

179893

yokogawa Temperature transmitter showing Error

15 which is Reverse Cal Fail (confirmation calculation

result is bad) in manual they mentioned reasion That

the output goes to the value set by hardware failure

mode jumper. How can solve this problem ? anybody

can say solution

Answer In the manual (from which you cited) the user action for error 15 is 'contact service

personnel'.

You can try that, but I suspect that it means,

for all practical purposes, replace the

transmitter. I don't know if Yoko offers a

spare 'puck' that can be replaced in order to

salvage the housing and display, or not.

You'll have to ask.

You might try cycling power to it to see if an

electrical spike on the loop wiring adversely

affected the processor, and cycling the power

might re-initialize it, but the likelihood of

success is very small.

When a transmitter goes to fail-safe mode, it's

usually the last act of the last performance,

the actors are clasping

hands, and taking a final bow before the show

closes.

tell 'em Carl sent you

0 2013-08-

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Q #

180004

Why drum level range is given -500 to 0 mmwc pls

give me that answer

Answer How does any DP cell make a measurement?

Ans: High side pressure minus low side

pressure

What does a drum level DP cell measure?

A DP transmitter measures the difference in

pressure between

- the water level in the drum (and steam

pressure) and

0 2013-08-

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35


- the water in a static column (and steam

pressure), also called the “Reference Leg”.

The DP's low side is connected to the reference

leg which extends to the top and is filled with

water.

The DP's high side is connected to a low point

in the boiler drum.

What happens (algebraically) when a large

number is subtracted from a small number?

For instance, 5 - 10 = -5

What are the relative values that are being

subtracted from each other in the DP cell?

Carl Ellis

Measure First

Q #

180065

How many problems can be occurring in solenoid

valve? And how it can be rectified?

Answer The answer warrants an entire discussion. See Plant Engineering's article on

troubleshooting solenoid valves

here: http://tinyurl.com/kvmefcm

Carl Ellis

Measure First

0 2013-08-

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Q #

180071

Why we use the double acting control valve in flow

line?

Answer Part of your problem is your terminology.

Valves are chunks of machined metal and

elastomers, valves are not single action or

double action, the actuator that moves the

valve stem is.

So your Google search should be for double

action actuator, which will provide you an

answer.

Carl Ellis

Measure First

0 2013-08-

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Q #

180079

how to calibrate temprature switch and flow switch ?

Answer The key to 'calibrating' a temperature switch is it is a switch, it is not a continuous

analog sensor. So there's

no need to mess around with a zero and span

value, like you would on temperature

transmitter.

36


All that matters is setting the switch's trip

point, the setpoint, and in some cases, the

reset (deadband/hysteresis) point. There is no

need to mess around anything but the trip

point, it's time consuming enough to get a

precise setpoint on switch.

You need to know

- the setpoint

- in some cases, the reset

(deadband/hysteresis) point values

- whether the setpoint (SP) is for a rising or

falling temperature. Because they're

mechanical switches, a SP for falling is NOT

same as a SP for rising.

- switch functionality - Does the switch close

or open on rise or fall. There are

applications for all the variations: close on

rise, close on fall, open on rise, open on

fall.

Check how to change the setpoint on the switch.

Some switches have a setpoint lock (nut). Some

are blind adjustments, some are adjustments

with indicated temperature values. Some have

a separate deadband/hysteresis adjustment. The

latest temperature switches are digital, and

the setpoint and hysteresis reset point are

entered as values.

Many procedures require an actual test with a

calibrating heat/cold (dry block calibrator or

oil bath) source and a

temperature "standard" is needed for a

reference temperature, to prove the mechanical

setting is correct.

Sometimes the standard is a certified dry block

with a readout, sometimes it's a mercury

thermometer, sometimes its certified RTD with a

certified readout. Older cal equipment has

stand-alone electrical equipment, like an

indicator lamp or an ohmmeter that is used to

indicate switch closure on the temp switch,

more modern cal equipment has built-in

connections and an indicator for that.

Make the setpoint adjustment, run the

temperature source, heat or cool, and note the

trip and reset points. If needed, make

adjustments and re-run the test until the

setpoint (and reset point) are within the spec

needed.

Record the results, put a cal sticker or tag on

the beast, and you're on to the next one.


37


Q #

1801

32

Furnace draft transmitter range is -50 to +50 mmwc. why the impulse

line connected in hp side?

Answ

er

A draft range measurement is typically a gauge pressure

measurement, referenced to atmosphere.

Draft range is very, very low pressure, so DP transmitters

are typically used because they're readily available in a

'draft' range with the large diaphragms necessary for low

pressure measurements.

The high side is connected to the process with an impulse

tube. The low side is exposed to atmospheric pressure.

Good practice puts a filter, like a scintered pneumatics

filter on the low side, so that dirt doesn't fill the low

side cavity and air movement (wind, passing vehicles) is

buffered and damped to prevent the forces of air movement on

the low side from affecting the draft measurement.

If the impulse line were connected to the low side, the DP

would be negative, a DP subtracts the low side pressure from

the high side pressure (atmosphere minus some positive

pressure = negative pressure).

Convention says that draft range is positive so the impulse

connection is to the high side, to get a positive value.


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Q #

1801

89

Good Evening all.My question is - For a feed water line to an

equipment if i need to measure the flow we will use orifice

/magnetic/vortex depend on cost/spec.If i need to measure a

''Alarm''signal and ''Interlock'' from one point i think we can use

orifice type in one location with 2 different tapping points - one for

alarm and one for an interlock to flow valve..If in case i need to have

both these signals(Alarm & interlock) from magnetic flow meter,is it

possible??will 2 seperate sensors will be located inbuilt r we can

program it ??..i thk we can have one transmitter with these 2

signals...need to know any technical issues are there?pls clarify

Answ

er

If this is not SIS (safety instrumented), the 4-20mA from

a single flow transmitter can feed a separate I/O modules

(4-20in, relay out) an alarm module and a separate interlock

module.

Or, depending on the converter/transmitter model, most

magmeters have at least one assignable relay output, some

have two relay outputs.

I've always used orifice flange unions that have only one

set of taps. There might be a double tap flange unions, or

I suppose you can drill and weld the pipe for taps. But

again, unless this is SIS, why would you? Decisions made on

a 4-20mA flow signal on a PLC/DCS/Controller/I?O modules can

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do multiple functions.

If your situation is SIS, you should state that.

Carl Ellis

Measure First

Q #

1801

89

Good Evening all.My question is - For a feed water line to an

equipment if i need to measure the flow we will use orifice

/magnetic/vortex depend on cost/spec.If i need to measure a

''Alarm''signal and ''Interlock'' from one point i think we can use

orifice type in one location with 2 different tapping points - one for

alarm and one for an interlock to flow valve..If in case i need to have

both these signals(Alarm & interlock) from magnetic flow meter,is it

possible??will 2 seperate sensors will be located inbuilt r we can

program it ??..i thk we can have one transmitter with these 2

signals...need to know any technical issues are there?pls clarify

Answ

er

It seems you are using a flow meter's relay(s) to generate

alarm signals at various amplitudes: low, low/low, hi,

hi/hi.

The flow meter doesn't really use these alarm signals

itself, rather it generates the alarm signals and provides

the alarm signals to other devices, for whatever purpose,

alarm, annunciation or an interlock.

There might be a flow meter with 3 alarm relays. I don't

know. That's a shopping issue.

Most places will use a PLC, a controller or a DCS to

generate alarms from the analog flow signal.

But if your flowmeter doesn't have enough relays, then use a

"DC alarm trip" I/O module with the flow meter's 4-20mA

analog output.

The modules have an analog input and one or two relay

outputs with adjustable setpoints.

There's probably a dozen models on this page:

http://www.api-usa.com/signalconditioners_alarms.php

These I/O modules are not used "in a magmeter", they're

external I/O modules.

They place a small burden resistance in the 4-20mA circuit,

need an enclosure and AC or DC power to operate, but they

accomplish the task of providing multiple alarms at various

setpoints.

The 4-20mA signal is wired in series with the I/O module and

any other receiver devices.

Carl Ellis

Measure First

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Q #

1801

96

How to calibrate electronic type level switch low?

Answ

er

Calibrating a point level switch

Contact level switches detect the presence or absence of the

medium, whether solid or liquid, by direct contact with the

medium. The switch output for most devices is an on/off

binary signal that reflects the presence or absence of the

medium. Some level switches use a 4-20mA output that does

not modulate, the current signal is either 4mA or 20mA,

depending its detection state.

Most level switches offer some form of sensitivity

adjustment (depends on the technology) and some have an on-

delay or off-delay adjustment of the output relay trip, used

to avoid nuisance trips.

Calibration consists of

- testing whether the switch output changes state when the

presence or absence of the medium is changed and adjusting

the sensitivity setting for the specific medium used with

the switch.

- whether the on-delay and off-delay settings comply with

the requirements of the installed application

Depending on the technology, there might be sensitivity

adjustments or on-delay or off-delay adjustments, which are

specific to the installed application, and a calibration

would cover.

A non-contact level switch, like Siemen ultrasonic ULS200,

can be tested in place by comparing the reading with a

physical distance measurement (tape measurement) or out-of-

process by shooting against a solid target and measuring the

distance to check the trip and reset setpoints.

Carl Ellis

Measure First

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06

Q #

1802

04

1 How to calculate dp transmitter steam and water? 2 impulse line for

liquid steam and gas

Answ

er

The DP pressure/flow transmitter is mounted above the pipe

for gas applications (empty impulse tubing).

The DP pressure/flow transmitter is mounted below the pipe

for liquid applications (liquid filled impulse tubing)

Steam flow applications using DP pressure transmitters were

historically mounted below-the-pipe (water filled impulse

tubing, frequently with condensate/seal pots), until

Emerson/Rosemount recently discovered that within certain

temperature limits, a DP transmitter close coupled on a

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coplanar manifold to an Annubar averaging pitot tube can

survive steam temperatures mounted above-the-pipe.

See this link for Emerson's white paper on steam service

installation recommendations:

http://www2.emersonprocess.com/siteadmincenter/PM%20Rosemoun

t%20Documents/00870-0200-4809.pdf

Carl Ellis

Measure First

Q #

1803

37

please tell me how to convert double acting ytc positioner yt-1200 into

single acting ytc positioner ?

Answ

er

Many manufacturers make all positioners a double acting

positioner and install a plug in the 2nd port and call the

model with the plugged 2nd port a single acting positioner.

No guarantee that all manufacturers do so, but it's true for

some of them.

You have a double action positioner - install a plug in the

2nd output port, plumb it to air and an actuator, give it

try as a single acting positioner and see if it works.

What do you have to lose?

Carl Ellis

Measure First

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Q #

1803

45

On drum level, why we are using air to close Control valve?

Answ

er

If the valve's actuator is Air-to-Close, what state does

it fail-safe state?

Check out the article, "How to destroy a boiler - Part 1"

at this link:

http://www.nationalboard.org/index.aspx?pageID=164&ID=238

and scroll down a couple pages to the section entitled "Low

Water Conditions".

The description will provide details that help you

understand what the fail-safe state due to the loss of a

control signal on a boiler feedwater control valve should

be.

Carl Ellis

Measure First

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24

Q #

1803

62

if temperature transmitter is showing wran. sensor 2 fail how can you

remove from the transmitter display m/n.3144p rosemount

41


Answ

er

It is very likely that the original question has not

correctly copied the error code.

"wran sensor 2" is most likely "WARN SENSOR 2"

It is impossible to search electronic pdf documents if the

error message is spelled incorrectly.

This is why employers look for things like misspellings in

documents submitted by job candidates, it shows whether the

candidate pays attention to details.

Here is what the 3144P manual says about the WARN error

message:

"The transmitter is still operating, but something is not

correct. Possible causes of this condition

include, but are not limited to, an out-of-range sensor, a

fixed loop, or an open sensor condition. In the

case of a Sensor 2 failure with Hot Backup enabled, the

Process Variable line displays “WARN,” and the

Process Variable Unit line alternates between “SNSR 2”

and “RANGE.” "

Carl Ellis

Measure First

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23

Q #

1804

07

To calculate temperature, why we multiply 25 with mV?

Answ

er

Apparently, your temperature signal is linear across a mV

range, coming from some type of temperature transmitter, and

the mV value represents temperature/25.

If your source is a raw thermocouple signal, then

multiplying by 25 does NOT result in a true temperature

value because

- cold junction compensation has not be applied

- thermocouples are not linear

Temperature transmitters perform both of those tasks, cold

junction compensation and linearization of the output

signal.

Carl Ellis

Measure First

3201

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25

Q #

1804

30

what are the possible reasons if a control valve fails to open/close?

Answ

er

I seen 10 to 20 causes for each of these categories, but

I've only listed one as an example.

Reasons for valve not closing or opening:

intentional or unintentional

5201

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42


- lack of energy - air/electric/hydraulic (Fuse blown)

- insufficient energy - (supply air regulator turned down)

- control system issue (control signal interlocked)

- positioner or I/P issue (locked in manual)

- actuator crippled (punctured diaphragm)

- positioner-to-actuator linkage/plumbing issue (tubing

failed)

- actuator-to-valve linkage issue (sheared roll pin)

- valve trim damage

- foreign object blocking movement (how does all that junk

get in the piping in the first place?)

- change in design process conditions (actuator can't close

against increased line pressure)

Not all failures to actuate/move are faults - a valve could

be intentionally locked in place for safety reasons – access

to the piping/tank.


Q #

1804

83

why temperature transmitter is showing ambienttemp.but 21.5ma.

alarm AO showing at the display &how to sloves this problem.

Answ

er

When you need troubleshooting assistance you need to

supply certain critical information:

1) Has the device ever worked correctly before for some

time period and then failed?

Or is this a first time commissioning issue?

2) What is the brand name and model number of the device?

3) Because all devices have an input and an output, exactly

what each is (is your sensor one thermocouple, two

thermocouples, or one RTD?)

- You do tell us the output is 21.5mA.

4) What are the error codes or what the error symptoms?

- You mention the display shows ambient temperature

5) What you've done to troubleshoot the problem, like

substitute a known source (simulator) and the result

was . . . . ?

Evaluation:

ambient temperature: A short circuit (a wire) across the

terminal inputs of a thermocouple input will display near-

to-ambient temperature, specifically the temperature of the

terminal block connectors. The value is the cold junction

compensation (CJC) temperature.

Some high end transmitters will show the CJC temperature.

Is the display stuck on the CJC temperature, rather than

its input?

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43


Is there a short across the (+) and (-) input terminals?

21.5mA is a very common fail-safe alarm level, outside the

normal 4-20mA range, but indicating that there is a serious

fault in the transmitter setup or operation.

If the fail-safe is configured for fail high, an open input

(among many other things) could cause the fault.

Smart transmitters generate an error code and save it.

If the transmitter has HART, the error code is in one of the

diagnostics registers, but a HART communicator is needed to

access the error code.

Carl Ellis

Measure First

Q #

1804

83

why temperature transmitter is showing ambienttemp.but 21.5ma.

alarm AO showing at the display &how to sloves this problem.

Answ

er

A short circuit (both thermocouple wires touching each

other) in the thermocouple extension wire leads will create

an ambient or near-ambient temperature reading, but I am

unaware of any temp transmitter that can detect that problem

and go to fault mode at 21.5mA.

Carl Ellis

Measure First

0201

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30

Q #

1805

41

what does mean by analog output trim and where this one used for?

Answ

er

Analog output trim is done on all process field

transmitters that do not communicate over a digital

protocol like Profibus or Foundation Fieldbus.

Modern electronics use an A/D to digitize a value from a

sensor, a microprocessor to process the value and a D/A to

output the process value as an analog signal.

Adjusting the zero and span of the D/A is commonly

called 'trim'. It trims the D/A output a small amount,

generally about ±1% at the zero and at the span

(separately), generally to match a variation or deviation of

an analog input's resistance value.

Trim is NOT setting the URV and LRV values to correspond to

process inputs, as was once done with analog electronic

transmitters.

Carl Ellis

Measure First

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44


Q #

1805

57

what sud u do if u dont get feedback from control valve?

Answ

er

There's two kinds of feedback from a control valve:

a) the change of the process variable because the valve

position changes

b) position feedback directly from the positioner to the

controller/control system.

We'll assume that the question is directed at the loss of

position feedback signal from the loop's positioner.

1) Position feedback cards are options on smart

transmitters and need to be initialized after installation.

Has it been initialized?

2) DC power is typically loop power for position feedback.

Is the loop power supply still functioning?

3) Has the position feedback card or its wiring been

disabled or removed?

4) If the position feedback loop is intrinsically safe, is

the loop's I/S barrier open circuited?

5) Has the loop wiring been changed in some manner so as to

affect the position feedback signal integrity?

Carl Ellis

Measure First

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06

Q #

1806

87

Explain , how timer based pneumatic control valve will work ?

Answ

er

The timer forces the on-off pneumatic control to drive

open or closed based on the timer's output state.

The timer's output relay activates or deactivates a 3 way

solenoid valve that supplies air to the valve's actuator.

In one timer state, the solenoid valve passes supply air to

the actuator driving the valve to its 'not-failed' state.

In the other timer state, the solenoid stops the air supply

and dumps the existing actuator pressure to atmosphere,

letting the valve assume its 'failed' state, whether open or

closed.

Carl Ellis

Measure First

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15

Q #

1806

what are the documentation and instrument identification system

used to identify the correct device to be calibrated? name atleast three.

45


92

Answ

er

- P&ID: tag name/number for the instrument in question,

but not the location

- Loop diagram shows tag name/number, has a location

description (field, panel, I/O rack, control room) and

wiring terminals ID (are wires 1578 and 1579 connected to

the instrument in question?)

- ISA's instrument spec sheet has a section for calibration

with input/output, but more importantly, the following

section has Component Identification by manufacturer and

model number. The serial number isn't there because this

document defines the functionality, not a specific device.

- The company's "Instrument Specification Data Form" should

have a field for a location and instrument serial number

- Calibration data sheet should have a field for "Location",

tag and serial number.

- There should be a stainless tag on a field instrument

- The handheld communicator displays the tag name when

connected. The instrument's tag name should match the work

order's tag.

- The local "Calibration status label" on or inside the

instrument always has a tag ID.

Carl Ellis

Measure First

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15

Q #

1807

81

what will happen if transmitter wire get shoted?

Answ

er

Part I.

Let's assume the question involves a 2 wire, loop powered

transmitter powered by a 24Vdc power supply in series with

the transmitter and a load resistor across the analog input

of the receiver device.

The loop current is nominally 4-20mA.

The power supply negative is wired to the analog input

negative.

The short puts a positive 24Vdc on the (+) side of the

analog input.

The load resistor for the analog input is across the (+)

and (-) terminals, so the resistor now 'sees' 24Vdc across

its 250 ohms.

Part II. extra credit. Calculate the wattage spec needed

for a 250 ohm resistor to survive the short circuit in the 2

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wire, loop powered cable.

Carl Ellis

Measure First

Q #

1807

92

what is maximum temprature in instrumentation

Answ

er

Whatever the manufacturer says the maximum process or

ambient temperature is on the relevant specification sheet.

Carl Ellis

Measure First

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Q #

1808

21

What is live zero and dead zero

Answ

er

No, no, no.

A live zero is where the signal's value of zero engineering

units is an electrical signal value larger than zero.

For example, 4-20 mADC, 1-5 VDC, 10-50mV, etc., are all

examples of live zero. 4mA = 0 eng units. 1Vdc = 0 eng

units.

On the other hand, a dead zero is where the signal's value

of zero engineering units is also zero electrical units.

0-20 mADC, 0-100 mV, 0-1Vdc, 0-5 Vdc, 0-5Aac are examples of

zero engineering units equaling zero electrical units.

0mA = 0 eng units; 0Vdc = 0 eng units

The value of a live zero is that zero electrical units

indicates a failed circuit; which could be lack of power, an

open circuit, a short circuit or failed signal source.

Neither live nor dead zero has an bearing on the linearity

or non-linearity of what the electrical signal represents.

Linearities are application specific.

Carl Ellis

Measure First

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21

Q #

1811

07

I HAVE PRESSURE TRANSMITTER WITH TWO WIRES 24

VOLTS SUPPLY.IF TRANSMITTER WILL GIVE SIGNAL TO

THE DCS WHAT HAPPEN TO THE 24 VOLTS POWER SUPPLY

IT WILL DROP OR REMAIN TO 24 VOLTS POWER SUPPLY.

Answ

er

The measured voltage across a regulated power supply will

remain at 24Vdc. If the voltage across the power supply

drops below 24Vdc, then either the power supply is too small

to carry the combined load or it is faulty.

0201

3-

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47


The measured voltage across the transmitter will vary

according to the loop current.

Carl Ellis

Measure First

26

Q #

1811

07

I HAVE PRESSURE TRANSMITTER WITH TWO WIRES 24

VOLTS SUPPLY.IF TRANSMITTER WILL GIVE SIGNAL TO

THE DCS WHAT HAPPEN TO THE 24 VOLTS POWER SUPPLY

IT WILL DROP OR REMAIN TO 24 VOLTS POWER SUPPLY.

Answ

er

You describe a situation with a 24Vdc power supply, a 250

ohm dropping resistor and a 2 wire loop powered transmitter

at 4.0mA

To get the voltage to vary across the transmitter, you have

to change whatever the transmitter is measuring to get its

output to change, or force an output with a HART

communicator. The loop current does not vary on its own, it

varies in response to whatever the transmitter is measuring.

Carl's Loop Rules

The voltage across the power supply remains constant: 24Vdc.

The voltage drops across the transmitter and analog input

vary with the current in the loop.

Assuming a bench setup with very short wire runs and a very

low wire resistance, then if the analog input dropping

resistor is 250 ohms, then

at 4.0mA the transmitter drops 23.0V, the AI resistor drops

1.0V

at 12.0mA the transmitter drops 21.0V, the AI resistor

drops 3.0V

at 20.0mA the transmitter drops 19.0V, the AI resistor

drops 5.0V

The sum of the voltage drops must equal the power supply

voltage.

You can bank on it.

Carl Ellis

Measure First

0201

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12-

03

Q #

1812

03

Dear all... I have 1 0-400 duplex type RTD, I connected the A side to

the temperature indicator and B side to the PLC panel, but when I

have observed then found that The PLC side temp has the variation.

and it was distub to other RTD temp also. so what can I do.

Answ

er

The RTD element is ALWAYS isolated from the sheath. Dual

element RTDs should be isolated from each other. 0201

4-

48


Therefore, there should be no common electrical connection

between any wire in a dual element RTD.

The fact that a connection to one receiver (the PLC)

disturbs the other receiver's RTD reading means either

a) the RTD elements or lead wires are shorting to either

the sheath, the other RTD element or a lead wire; meaning

the RTD assembly is damaged internally.

b) the field wiring for one RTD is shorting to the field

wiring of the other RTD

Carl Ellis

Measure First

06-

10

Q #

1812

12

Dear all... I have 1 0-400 duplex type RTD, I connected the A side to

the temperature indicator and B side to the PLC panel, but when I

have observed then found that The PLC side temp has the variation.

and it was distub to other RTD temp also. so what can I do.pls help me

Answ

er

An offset in readings from separate RTD's can indicate

- a difference in calibration of the analog input devices

- a bad RTD element, one that has drifted out of spec

- a common ground issue with ground loops between the analog

input devices

An RTD is supposed to be ungrounded and the two RTD's in a

duplex element are supposed to be electrically isolated from

one another, so it is unlikely the ground loop is unlikely

unless poor wiring of the extension wire or head wiring has

created a connection to ground.

If you're complaining about a small difference of the

indicated temperatures, then get out your cal gear and cal

the inputs.

To check for a bad RTD, I'd take an ohmmeter and see if any

leg on the A side has any connection to the B side by

disconnecting the wiring from each analog input and check

the resistance. If there's a low resistance, it indicates

some connection from A to B. If the extension wires are

then disconnected and the resistance is checked on the RTD

lead wires and the resistance remains, then it's a bad RTD.

If the resistance goes high (open circuit) then the

extension wire is shorting or making a low resistance

connection somewhere.

Carl Ellis

Measure First

0201

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12-

09

Q #

1812

15

Can we used Condensate Pot on flow transmitter which is using on

liquid having sluge and DP transmitter is in pit and below the process

line,Can we used Condensate pot for accamulate the Sluge?

Answ Yes, you can use a condensate pot to accumulate the 0201

49


er sludge.

It's just a version of a "drip leg" that extends down

vertically below the tap or port for the impulse line (at

the datum elevation) to the DP transmitter. Other methods

are just constructing a drip leg from pipe fittings that are

rated for the temperature and pressure.

A consideration is whether the drip leg or pot can be

drained to remove the sludge, because it will fill up over

time.

Carl Ellis

Measure First

3-

12-

06

Q #

1812

55

Can we use a 10000 Lph Flow Transmitter in 100 LPH flow line to get

20mA O/P by adjusting SPAN ?

Answ

er

You can do whatever you want to with the span adjustment,

but what do you think the results are going to be?

10000:100 is a 100:1 turndown.

Which flow meter technology that uses an analog adjustable

20mA span has 100:1 turndown?

Which flow meter technology has a HART configurable URV with

a 100:1 turndown?

Flow meter accuracy curves ALWAYS bend exponentially upwards

towards increasing uncertainty as the flow approaches zero

low flow. The lower the flow, the greater uncertainty.

Where do you suppose 100 Lph is on the accuracy curve of a

meter designed for 10000 Lph?

Carl Ellis

Measure First

0201

3-

12-

10

Q #

1812

67

What is the relationship between GAIN and PROPORTIONAL

BAND.

Answ

er

Proportional Band * gain = 100

Proportional Band = 100/gain

Gain = 100/proportional band

Carl Ellis

Measure First

0201

3-

12-

19

Q #

1812

68

What is the purpose of valve positioner. Explain 4 uses.

50


Answ

er

What is the purpose of valve positioner. Explain 4 uses.

1) Use supply air pressure to ensure the correct valve

position through feedback control to overcome stiction or

forces on the valve plug.

2) Characterized the non-linear valve movement with respect

to the input signal, for instance, equal percentage or quick

opening

3) Provide position feedback to the control system via

conditioned process signal (4-20ma)

4) Provide adequate seat loading force to ensure tight

shut-off.

Carl Ellis

Measure First

0201

4-

02-

08

Q #

1812

75

What will the new flow factor if D/P Transmitter is RE-RANGED

from 25" WC to 50" WC

Answ

er

Answer:

The flow rate at 50.0"wc will be 141.4% of the flow rate at

25.5"wc.

Reason:

The design maximum flow rate at 25.0" is 100.0% flow rate.

Flow rate is the square root of the percentage of flow at a

given DP.

The flow rate at 25.0"w.c. DP is the square root of 1.00 (or

100%)

If the DP doubles from 25.0 to 50.0, then the DP increases

from 100% at 25.0"wc to 200% at 50.0"wc

The flow rate at 200% DP increases by 141.4% because the

square root of 2.00 (or 200%) is 1.414.

Carl Ellis

Measure First

0201

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12-

19

Q #

1814

21

What is open loop and close loop?

Answ

er

An open loop has no feedback, like a timed operation. For

instance, an irrigation system operates for 45 minutes per

day, starting at 10:15 AM. It does this whether the water

is needed or not, because the controller, the timer, has no

feedback about the need for water.

A closed loop uses feedback. In the irrigation example, a

soil moisture sensor provides feedback to a controller to

let the controller make a decision as to whether irrigation

0201

4-

01-

08

51


is needed and for how long.

Carl Ellis

Measure First

Q #

1815

48

Where we cant instal mass flow meter.

Answ

er

These remarks refer to the use or non-use of a coriolis

mass flow meter.

A coriolis mass flow meter should not be used

- where tight shutoff valve(s) cannot be installed upstream

and downstream for the purpose of stopping all flow so that

zeroing the coriolis flow meter can be accomplished (turning

off power to the pump is NOT the same as shutting off the

flow with a ball valve). Otherwise zero shift will plague

the operation.

- in a batch operation where the meter starts empty, then

fills, and at the end might run partially filled, unless

appropriate piping is implemented to keep the Coriolis flow

meter filled with fluid all the time.

- with suspended solids in a liquid medium that are abrasive

[abrasives erode the thin tube(s)]. Reducing the flow

velocity tends to reduce abrasion and erosion. A Coriolis

tube will quickly fail when it carries suspended solids like

sand. Unless of course, frequent replacement is just the

cost of doing business.

- with a corrosive medium that will corrode the flow tube

- with medium that is above the manufacturer's stated

process medium temperature (burns up the electronics)

- with a two phase media (liquids and gas or liquids that

will flash to vapor at the operating pressures and

temperatures)

- with a medium that tends to solidify (Coriolis tubes are

very thin, very light and don't like to be brush

cleaned)unless periodic solvent cleaning is feasible.

- with a medium that tends to coat the tube, unless the

required solvent or mechanical cleaning is feasible.

- where the flow tube is subject to mechanical vibration

- when the meter is mounted where the flow is vertical down

- when the meter is mounted in horizontal piping at a high

5201

4-

01-

24

52


point where gas/air can be trapped at a high point

- where the pressure drop across the flowmeter is higher

than the process can tolerate

- when the location is too close to another coriolis meter;

where each meter can be affected by crosstalk vibrations

from the other coriolis meter.

- hazardous or classified zones where the Coriolis' power

requirements exclude it from meeting hazardous area

approval.

- for flow measurement requiring pipe sizes larger than

commercial coriolis meters (10 or 12" in 2014)

Carl Ellis

Measure First

Q #

1815

60

Why we use 3-15 psi air for control valve input?

Answ

er

Because it's a standard, widely used conventional

pneumatic

control transmission range.

The 3-15 psi signal is a standard (ISA SP7) that came from

widespread use even before that standard came into being.

As early as the 1920's pneumatic regulators, pressure and

temperature controllers were using 15 psi at the high end.

Various low end 'live' or elevated zero values were used

before standardization.

Any pneumatic control or transmission signal starts with an

'elevated' or 'live' zero' at 3 psi because there is

internal friction and a fair amount of hysteresis in the

movement of pneumatic components, the flapper, nozzle,

diaphragms, spring preloads, pivots and bellows that make up

the transmitters and receivers. This friction and

hysteresis manifests as non-linearity below 3 psi.

A 3 psi minimum pressure puts the transmission signal into a

linear response range. Obviously, control system

measurements, signal transmission and the receivers need a

linear response to effect accurate and precise control.

Not all pneumatic control signals are in the 3-15 psi range.

The ISA SP7 standard (1996) that addresses instrument air

quality standards also lists 5-25 psi, 6-30 psi, 3-27 psi as

standard pneumatic transmission ranges, along with the

minimum supply pressure associated with each range. These

ranges all include an elevated zero.

Carl Ellis

Measure First

53


Q #

181571

Why we use 250 ohm resister in series hart

communicator?

Answer HART communications requires a minimum of 230 ohms to develop a sufficient voltage drop

for the 1200 baud FSK HART signal to be 'seen'

by a HART communicator/modem.

The capacitance in a DC power supply acts as a

low pass filter which filters/strips off the

FSK HART data. Additional loop resistance is

needed for HART communications to take place.

If there is insufficient loop resistance, the

HART communicator/modem will fail to establish

HART communications.

In normal field work, it is assumed that the

analog input to which the field instrument is

wired will provide the

minimum resistance necessary. Most DCS analog

inputs have an input resistance of 250 ohms.

But not all devices do.

And when bench testing/configuring HART

instruments it is not uncommon to connect a DC

power supply, but not an analog

input. A HART communicator/modem will not

work with just a power supply powering a HART

field instrument. The loop requires a minimum

of 230 ohms.

A 250 ohm resistor is commonly used for a bench

test/configuration because they're commonly

available in the instrument shop.

Carl Ellis

Measure First

5 2014-01-

17

Q #

181571

Why we use 250 ohm resister in series hart

communicator?

Answer Answer #2 is not correct because the HART communicator is not an analog input, it does

not use a 1-5Vdc signal. A HART communicator

gets and display a transmitter's primary

variable (or other variables) as a digital

value; NOT as an

analog value, not as a 1-5Vdc signal developed

across an analog input's resistor. I repeat,

HART communications is NOT analog in nature, it

is digital.

The HART Foundation specifies a minimum

resistance of 230 ohms, as described in answer

#1, in order to develop the 1200 baud HART

signal, that rides on top of the DC loop

current signal.

0 2014-02-

08

54


It's easy to prove that 250 ohms (for 1-5Vdc)

is not a requirement for HART communications as

a bench test.

Take a 2 wire loop powered HART transmitter,

connect a DC power supply and use a 330 ohm

resistor in series in the

loop. The communicator will work and the

primary process variable will be correct.

Then try a 470 ohm resistor. The 470 ohm

resistor will work, as well, and the primary

variable will be correct.

Q #

181778

while stroke checking of a linear control valve ,found

its travel is in non linear ,what can be possible reason ?

( please answer send in [email protected]) pls

pls pls if any body know

Answer The positioner is configured for quick opening, equal percentage, or some custom non-

linear characterization.

Carl Ellis

Measure First

0 2014-02-

06

Q #

181946

My rtd working well at the same time r/i converter is

not working properly.when power is given to rtd..it is

giving 30mA and getting 2.4 mA if i remove the

compensate leads No matter how many ohms.why it is

happening like so?what is the solution?rtd range-0-

200deg c& 4-20 mA o/p.please give me solutiin

anyone......

Answer Any 2 wire, loop powered 4-20mA transmitter that puts out 30mA is broken. The only

transmitter that puts out 30mA is

a legacy 10-50mA transmitter from 20-30 years

ago.

While the upper end of the 4-20mA signal range

is 20.0mA, most 2 wire, loop powered

transmitters allow for some slight

signal over-range, typically 20.5mA.

NAMUR standards use 21.0 mA to signal a fault

or failure mode. Other vendors use other

values, 20.8mA, 21.2mA or some value in the

range of 21mA. But no vendor uses 30mA as a

fault mode.

The transmitter is broken or dysfunctional.

Carl Ellis

Measure First

5 2014-03-

24

55


Q #

182000

what is control valve and how is work

Answer What is a control valve and how does it work?

A control valve is a mechanical device that

removes energy from a hydraulic (liquid, gas,

or steam) flow stream.

It works by partially or fully blocking the

flow stream.

Carl Ellis

Measure First

0 2014-03-

06

Q #

182257

How to convert mA into Percentage?

Answer formula to convert 4-20mA to 0-100%

(mA - 4.00)/16*100 = %

Carl Ellis

Measure First

5 2014-03-

28

Q #

182438

can i calibrate DP transmitter online (while Tx in

Service) if yes ,how?

Answer In order to 'wet' calibrate a transmitter (by applying reference pressure), the transmitter

needs to be powered, but if the transmitter's

output signal is an input to a control loop or

alarm, then those functions need to be disabled

(loop put in manual, alarm disabled) because

the

transmitter's output is not representative of

the process during calibration.

The transmitter has to be isolated from the

process, so that cal pressure can be applied.

This done through isolation

valving; typically a 3 or 5 valve manifold for

a DP transmitter.

So, is a transmitter 'in service' if it's

powered, but its signal is not representative

of the process and is, in fact, isolated from

the process?

My answer is no, a transmitter being calibrated

is not 'in service'.

Carl Ellis

Measure First

0 2014-04-

15

56


Q #

182440

I have a orifice installed in plant which is design for

0~100 inH2O for 0~80MMSCFD, now due to process

change the maximum flow is change to 120 MMSCFD

so for this new process value what will be the DP

across orifice.

Answer 100 in H2O = 80 MMSCFD x = 120 MMSCFD

Maximum flow rate is the square root of the

normalized percentage of the maximum DP (at the

maximum flow rate)

The maximum flow rate increases 150% of the

original max flow rate:

120/80 = 1.50, so 120 MMSCFD is 150% of 80

MMSCFD, or a 50% increase in maximum flow rate

To calculate the corresponding DP at a max flow

rate 150% higher than the original, where X =

DP

sq rt of X = 1.50 (square root of normalized

percentage of DP)

X = 1.50^2 = 2.25

2.25 = 225% (work in normalized percentage)

225% of 100 inH2O = 225 in H2O at a max flow

rate of 120 MMSCFD

Carl Ellis

Measure First

0 2014-04-

16

Q #

182481

without color coding, without nameplate, without

multimeter how can you find the type of

thermocouple???

Answer Let's assume base metal elements only, since noble metal elements are so expensive that they

are under lock and key;

adequate protection from loss of

identification.

For types T, J and K, (and by exclusion, E) the

options are

-a spectropscopic assay which will positively

reveal the alloys' composition, but given that

the cost of an assay is

probably 20x times the value of a base metal

element, it's not likely to happen.

- visual identification of a type T by the

copper color of its positive leg which is

copper. The alloy leg is a shiny 'metallic'

0 2014-04-

23

57


color.

- testing Types J or K with a magnet.

The Type J positive leg is made of iron which

has a very strong magnetic attraction.

The Type K negative leg is made of a nickel

alloy which is mildly magnetic.

Type E is non magnetic, but so are other types.

It can be tricky to distinguish mild attraction

on thin gauge wire.

Testing with a multimeter can be very

misleading, because of the difficulty in

determining both the hot junction AND the

cold junction temperatures. Without accurate

temperature values, of what use is a mV

reading?

Carl Ellis

Measure First

Q #

182543

i want the equation of this question. Tranmitter range

is 120 mmwc and dcs flow range is 1700 nm3/hr. if i

want to change dcs range from 1700 to 2400 nm3/hr

then how much dp i have to enter in transmitter

Answer Answer: 239.2 mmH2O

Present max flow rate = 1700 nm3/hr

Present max DP (at max flow rate) = 120 mm H2O

New max flow rate = 2400 nm3/hr

New max DP (at new max flow rate) = x

2400/1700 = 1.412 or an increase of 141.2% in

flow rate.

The DP and the flow rate relationship is a

square root relationship, but the calculation

is based on normalized percentage.

X = 1.412^2*120

X = 1.993*120

X = 239.2 mmH2O

The increase in max DP from 120 to 239.2 mmH2O

is an increase of 239.2/120 = 1.993 or 199.3%

increase in max DP at max flow rate.

Sq root of 1.993 = 1.412 or 141.2% increase in

max flow rate (2400/1700)

0 2014-06-

02

58


Carl Ellis

Measure First

Q #

182624

Difference between ultrasonic flow meter and radar

type flow meter

Answer Difference between ultrasonic flow meter and radar type flow meter

That's easy. Radar (radio frequency) energy is

not widely used, if at all, in commercial

industrial flow meters.

Commercial ultrasonic flow meters use acoustic

energy above the range of human hearing to

measure flow velocity of

liquids and gases. The measurement is

inherently a velocity measurement.

Transit time technology for liquids with no

suspended solids or bubbles, or doppler

technology can be used for liquids with

suspended solids or bubbles. In-Line transit

time ultrasonic is accurate enough to be used

for custody transfer. Doppler has limited

accuracy.

Carl Ellis

Measure First

0 2014-06-

12

Q #

182647

WHAT IS A SYMPTOM OF FAULT WITH THE

SPAN OF AN INSTRUMENT? 1)ZERO-POINT AND

100% ARE CORRECT BUT NOT A MID-RANGE.

2)ZERO-POINT IS CORRECT ,BUT 1005 INPUT IS

NOT GIVING 100% OUTPUT. 3)LINEARITY PRBS.

4)1005 O/p IS CORRECT BUT ZEO POINT IS

WRONG.

Answer The correct answer is 2, but number 4 also requires a span correction.

1) The "Zero-point and 100% are correct, but

not a mid-range value" problem is a linearity

problem.

2) The "Zero-point is correct, but 100% input

is not giving 100% output" is a span problem.

4) The "100% output is correct but zero point

is wrong" is a zero offset AND a span problem

because when the zero point is corrected with

the addition of a bias/offset, the 100% point

will shift up or down the same amount as the

zero

shifted with the bias/offset correction.

So the 100% point will no longer be correct

5 2014-05-

08

59


once the zero is corrected.

Hence, the zero AND the span will need to be

corrected.

Carl Ellis

Measure First

Q #

182728

Can a pneumatic control valve work without

positioner?

Answer Yes, a pneumatic actuator can drive a valve without a positioner.

In the HVAC world it's done all the time

because the service conditions

(temperature/pressure/wear factor) for, say, a

chilled water valve does not necessarily demand

the precision control that a positioner offers.

An I/P (a current to pneumatic transducer) is

used to convert a 4-20mA control signal to a

pressure control signal.

The service required in heavy process

industries is usually sever enough that a

positioner is used on a control valve.

Company and industry practice will determine

the use of a positioner.

0 2014-05-

18

Q #

182751

WHAT IS SELF CLEAN ELCTRODE IN

MAGNETIC FLOWMETER?? WHAT DO YOU

MEAN BY THIS TERM??

Answer A magmeter cleaning unit can be used for applications where deposits accumulate on the

liner material and electrodes.

Non-conductive deposits will reduce the

electrode signal, while conductive deposits

will partially short circuit the electrode

signal, in both cases deteriorating the

accuracy of the meter (depending on the

character and thickness of the deposits).

The cleaning unit is used for electrochemical

cleaning of the sensor (electrodes) by applying

voltage for approx. 60 seconds. Metering is

resumed after another 60 seconds when cleaning

is complete.

During cleaning (120 seconds) the

transmitter/converter typically maintains the

last flow reading in the display and

in the signal outputs until metering is

resumed.

5 2014-05-

21

60


AC cleaning is used to remove fatty deposits

from the electrodes. These fatty deposits are

seen in waste water

applications, in abattoirs and water

applications with oil residuals.

During the cleaning process, the surface of the

electrodes get warmer, which tends to soften

grease particles and the gas bubbles generated

mechanically lift deposits away from the

surface of the electrodes.

DC cleaning is used to eliminate electrically

conductive deposits in the measuring pipe

influencing the measuring accuracy.

Particularly in district heating applications

an electrically conductive deposit

(magnetite)may occur and short-circuit the

electrode signal. In this case the accuracy of

the meter decreases and the signal/noise

conditions of the meter become inferior.

The problem only arises if the conductivity of

the water is less than approx. 250 uS/cm.

During DC cleaning electrolysis

takes place where the flow of electrons removes

the particle deposits from the electrode area.

Setup parameters determine the frequency of a

cleaning process.

It is mandatory that liquid fills the pipe

during the cleaning process. The “empty pipe

detection” should be used to detect when the

cleaning process should be interlocked.

Carl Ellis

Measure First

Q #

182811

why to use 3 wire at RTD ? what's the reason for

common wire ?

Answer It is correct that an RTD indicates a change in temperature by a change in resistance of an

element.

The change in resistance must be monitored

electronically and any circuit connected to an

RTD will include copper lead wires which will

add resistance to the circuit.

The problem is not that the lead wires will

have different lengths, the problem is that any

copper lead wire has some resistance which will

contribute to a voltage drop from the source

current and appear to be the resistance of the

RTD element. Any contribution of resistance to

the measurement is an error.

0 2014-06-

03

61


Two lead wires connect to the RTD element and a

3rd lead wire is used to compensate for the

added lead wire resistance so that the lead

wire resistance is eliminated

and only the RTD resistance is measured and

used to calculate temperature.

Carl Ellis

Measure First

Q #

182888

How to convert gpm into inches of eater.I nerd

formula with example so pls help me.

Answer The pressure differential (h) [inches of water column]developed by a differential pressure

flow element is measured, and the volumetric

flow rate (Q) [gpm] can be

calculated using the following generalized

formula:

Q = kA(h/D)^0.5

k is the discharge coefficient of the element

(which also reflects the units of measurement),

A is the cross-sectional

area of the pipe's opening, and D is the

density of the flowing fluid.

The discharge coefficient k is influenced by

the Reynolds number (see Figure 1-5) and by the

"beta ratio," the ratio between the bore

diameter of the flow restriction and the inside

diameter of the pipe.

Nothing can be determined from only 2 data,

volume in gpm and DP in inches w.c. There is

insufficient data and probably insufficient

attention in the classroom to think that one

converts to the other.

Carl Ellis

Measure First

0 2014-06-

12

Q #

182911

Pls.could any one Clarify the following with an

example. 1.At What condition Close on Rise and Close

on Fall Pressure Switch contacts are used. 2.If

Pressure is abnormal as per set pressure,if connecting

Terminal of pressure switch breaks,how the voltage

extends to Relay base. 3.Are these contacts come under

fail safe philasaphy.

Answer 1. >At What condition Close on Rise and Close on Fall Pressure Switch contacts are used.

Close on Rise: the goal is to detect

overpressure in normal operation.

0 2014-06-

09

62


Common to NO is 'made' in the alarm state,

logic high state.

Common to NC is open in the alarm state, a

logic low state

Common to NC is open in the alarm state, the

fail-safe

mode that detects a logic low state.

Close on Fall: the goal is to detect and signal

loss of pressure in normal operation

Common to NO is 'made' in the alarm state.

Common to NC is open in the alarm state

Common to NC is open in the alarm state, the

fail-safe mode that detects a logic low state

2. >If Pressure is abnormal as per set

pressure, if connecting Terminal of pressure

switch breaks, how the

voltage extends to Relay base.

A pressure switch is wired directly to the

wiring terminals on the microswitch inside the

pressure switch. There is no 'relay base'

that I'm aware of.

For normal operation (the Common and the NO

contact), either broken wire will leave the

electrical circuit in a logic low state (no

voltage).

In normal operation an alarm state is a logic

high condition (the presence of voltage).

That means that a broken wire or open circuit

can not conduct a voltage, which means that the

circuit will have a logic low state and will

fail to detect an alarm condition.

If the circuit is wired fail-safe (the Common

and the NC contact) a broken wire creates an

open circuit that cannot

conduct voltage so the circuit will be in a

logic low state (the absence of voltage) which

is the same as an alarm condition, a logic low

state.

3. >Are these contacts come under fail safe

philasaphy.

Fail Safe is accomplished by

a) wiring to the Normally Open (NO) contacts

(Common to NO is 'made' when there is an

absence of pressure) and

b) having the logic solver look for an alarm

condition or a fail condition in the logic low

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state.

Either the absence of pressure or an alarm

state will produce a logic low, whether the

setpoint is set for close-on-rise or close-on-

fall.

Fail-safe pressure switch wiring can not detect

failure conditions like a plugged impulse line

or a valve-blocked impulse line, with the

exception of United Electric's ONE series

electronic pressure switch which has plugged

port detection and an I-AM-WORKING signal mode.

Carl Ellis

Measure First

Q #

182917

how to set calibration range in a DP transmitter to

measure level for a closed tank and for an open

tank,and also please explain me how to set calibration

range for a DP flow transmitter in a line ?

Answer Configuring the range for a DP transmitter

Open tank:

- Flanged DP transmitter

- high side at lower flange

- no remote seals:

- units: water column units = uwc

- LRV = 0.00 uwc

- URV = physical upper level elevation (uwc) *

specific

gravity of fluid

Closed tank:

- flanged DP transmitter

- high side at lower flange

- no remote seals

- dry leg from vapor space above liquid is

piped/connected

to the low side

- LRV = 0.00 uwc

- URV = physical upper level elevation (uwc) *

specific

gravity of fluid

When the receiver (DCS/PLC/indicator/recorder)

receives the

level signal, it converts to physical level

elevation by dividing the process level signal

by the specific gravity and scaling to physical

level units.

Volumetric flow rate with DP transmitter on DP

flow element:

- units: water column units = uwc

- LRV = 0.00

- URV = maximum DP value at the maximum flow

0 2014-06-

10

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rate stated

on the sizing sheet of the DP flow element

Whether the signal is square rooted in the

transmitter or in the receiver for conversion

to flow rate is up to the

plant convention/standards.

Q #

182923

While giving Power Supply of 24 VDC to a

Transmitter,why Shield Terminal is connected either

at Transmitter or at ISB.Why not Shield Terminal

connected at both ends.If the Reason is to avoid Noise

interference,which type of noice that is interfering.Is

the Noice from the 24 VDC supply terminals or from

any external source.

Answer 1) A shield (screen) connected at both ends will carry ground loop current due to the

difference in ground potentials at each end. A

shield (screen) carrying current

will induce noise into the wires it is

shielding.

Historically, shields (screens) have been

connected at one end only, typically at the

receiver (DCS/PLC/PAC/RTU/controller) end.

Current thinking as outlined in

IEC 61000-5-2 Electromagnetic compatibility

(EMC) - Part 5 Installation and mitigation

guidelines - Section 2: Earthing and cabling,

recommends grounding the shield at each

end for point-to-point and at every point for

multi-drop.

Two point grounding provides protection against

EMC and for safety as well. However, this

method assumes that all

points are on the same ground (i.e. use an

equipotential grounding grid).

Typically, Siemens has adopted this

recommendation for Profibus DP, an RS-485

multi-drop network. Of course, the

gotcha is "all points at the same ground

potential". Not in the real world, Mr.

Siemens.

2) Although 24Vdc power supplies can be noisy,

major manufacturers produce DC power supplies

with very little ripple and excellent

regulation.

DC power supplies used for HART need to meet

the HART spec:

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Maximum ripple (47 to 125 Hz) = 0.2V p-p

Maximum noise (500 Hz to 10 kHz) = 1.2 mV rms

Maximum series impedance (500 Hz to 10 kHz) =

10 ohm

24Vdc power supplies are not typically the

source of noise

Carl Ellis

Measure First