06presure Measurement

8/14/2019 06presure Measurement

1/30


2/30

Pressure Definition

Static Pressure.Pressure, P, is defined as

force, F, per unit area, A:

P = F/A
http://www.sensorsmag.com/articles/1198/index.htm


3/30

Pressure in open tankA container filled with a liquid has a pressure (due to the

weight of the liquid) at a point in the liquid of:

P = F/A

P = W/A

P = gV/A

P = ghA/A

P = gh

P = pressure

F = force

A = Area

W = weight of the liquid

V = volume above the Areag = gravitation

= mass density

h = distance from the surface

A

h


4/30

Static pressure of atmosphere

Gases differ from liquidsin two respects: they arevery compressible, andthey completely fill any

closed vessel in whichthey are placed.

The nonlinear airpressure variation withaltitude shown in the

figure is an example ofthe effect of thecompressibility of gases.

The compressibility of gases is

illustrated by nonlinear atmosphere

pressure as a function of altitude


5/30

Types of Pressure

measurements Absolute pressure is measured

relative to a perfect vacuum

(psia)

Gauge pressureis measured

relative to ambient pressure(psig)

Differential pressureis the

difference in pressure between

two points of measurement.

(psid).

Note that the same sensor may

be used for all three types; only

the reference is different.


6/30

Dynamic Effects.

Static pressure is measured under steady-state or equilibriumconditions, but most real-life applications deal with dynamic orchanging pressure.

For example, the measurement of blood pressure usually gives thetwo steady-state values of systolic and diastolic pressure.

There is much additional information in the shape of the bloodpressure signal, however, which is the reason for the monitors usedin critical-care situations.

To measure changing pressures, the frequency response of thesensor must be considered. As a rough approximation, the sensorfrequency responseshould be 5-10 the highest frequencycomponent in the pressure signal.

Another issue is the remote measurement of pressure where a liquidcoupling medium is used. Care must be taken to purge all airbecause its compressibility will corrupt the waveform.


7/30

Pressure in Moving Fluids

Bernoulli's theorem states that for horizontalflow the following relation holds:

PS = PO + PI

PS = stagnation (or total) pressure

PO = static pressure

PI = Impact pressure due to moving fluid

PI = Vo/2

Where V0= the velocity of the fluid

Hence we can measure the velocity if we knowthe pressure


8/30

Pressure Units

pascal

(Pa)

bar

(bar)

technicalatmosphere

(at)

atmosphere

(atm)

torr

(Torr)

pound-

force persquare inch

(psi)

1 Pa 1 N/m2 10

5 1.019710

5 9.869210

6 7.500610

3 145.0410

6

1 bar 100 106dyn/cm

2 1.0197 0.98692 750.06 14.5037744

1 at 98,066 0.980665 1 kgf/cm2

0.96784 735.56 14.2231 atm 101,325 1.01325 1.0332 1 atm 760 14.696

1 torr 133 1.3332103

1.3595103

1.3158103

1 Torr;

1 mmHg19.33710

3

1 psi 6,894 68.948103

70.307103

68.046103

51.715 1 lbf/in2


9/30

Pressure Sensing

Pressure is sensed by mechanicalelements such as plates, shells, andtubes that are designed andconstructed to deflect when pressureis applied.

This is the basic mechanismconverting pressure to physicalmovement.

Next, this movement must betransduced to obtain an electrical or

other output. Finally, signal conditioning may be

needed, depending on the type ofsensor and the application. Figure 8illustrates the three functional blocks.

Pressure

SignalConditioner

Sensing

Element

Transduction

element

displacement

electric

V or I output


10/30

Sensing Elements

The main types ofsensing elements areBourdon tubes,diaphragms, capsules,

and bellows All except diaphragms

provide a fairly largedisplacement that isuseful in mechanical

gauges and for electricalsensors that require asignificant movement


11/30

Potentiometric Pressure Sensors

Potentiometric pressuresensors use a Bourdon tube,capsule, or bellows to drive awiper arm on a resistiveelement.

For reliable operation the wipermust bear on the element withsome force, which leads torepeatability and hysteresiserrors.

These devices are very low

cost, however, and are used inlow-performance applicationssuch as dashboard oilpressure gauges


12/30

Inductive Pressure Sensors

Several configurations based onvarying inductance or inductivecoupling are used in pressuresensors. They all require ACexcitation of the coil(s) and, if a

DC output is desired,subsequent demodulation andfiltering. The LVDT types have afairly low frequency responsedue to the necessity of drivingthe moving core of the

differential transformer The LVDT uses the moving core

to vary the inductive couplingbetween the transformer primaryand secondary.


13/30

Capacitive Pressure Sensors.

Capacitive pressure sensors typically use a thindiaphragm as one plate of a capacitor.

Applied pressure causes the diaphragm to deflect andthe capacitance to change.

This change may or may not be linear and is typically onthe order of several picofarads out of a total capacitanceof 50-100 pF.

The change in capacitance may be used to control thefrequency of an oscillator or to vary the coupling of an

AC signal through a network.

The electronics for signal conditioning should be locatedclose to the sensing element to prevent errors due tostray capacitance.


14/30

Capacitive Pressure Sensors

Capacitive Pressure Sensors


15/30

Piezoelectric Pressure Sensors.

Piezoelectric elements are bi-directional transducerscapable of converting stress into an electric potentialand vice versa.

One important factor to remember is that this is adynamic effect, providing an output only when theinput is changing.

This means that these sensors can be used only forvarying pressures.

The piezoelectric element has a high-impedanceoutput and care must be taken to avoid loading theoutput by the interface electronics. Somepiezoelectric pressure sensors include an internalamplifier to provide an easy electrical interface.


16/30

Piezoelectric Pressure Sensors.

Piezoelectric sensors convert stress into an electric potential and vice versa.

Sensors based on this technology are used to measure varying pressures.


17/30

Strain Gauge Pressure Sensors

Strain gauge sensors originally used a metaldiaphragm with strain gauges bonded to it.

the signal due to deformation of the material is

small, on the order of 0.1% of the base resistance Semiconductor strain gauges are widely used,both bonded and integrated into a silicondiaphragm, because the response to applied

stress is an order of magnitude larger than for ametallic strain gauge.


18/30


When the crystal lattice structure of silicon isdeformed by applied stress, the resistancechanges. This is called the piezoresistive effect.

Following are some of the types of strain gaugesused in pressure sensors.

Deposited strain gauge.Metallic strain gauges canbe formed on a diaphragm by means of thin film

deposition. This construction minimizes the effectsof repeatability and hysteresis that bonded straingauges exhibit. These sensors exhibit the relativelylow output of metallic strain gauges.


19/30


Bonded semiconductor strain gauge. A silicon bar may

be bonded to a diaphragm to form a sensor with

relatively high output. Making the diaphragm from a

chemically inert material allows this sensor to interface

with a wide variety of media


20/30

Piezoresis t ive In tegrated

Semiconductor IC processing is used to form

the piezoresistors on thesurface of a silicon wafer

There are four piezoresistorswithin the diaphragm area on

the sensor. Two are subjectedto tangential stress and two toradial stress when thediaphragm is deflected.

They are connected in a four-element bridge configuration

(see Figure 22) and providethe following output:

VOUT/VCC= R / R


21/30

Piezoresis t ive In teg rated

Semiconductor

IC processing is used to form the piezoresistors

on the surface of a silicon wafer to fabricate

an integrated piezoresistive pressure sensor.


22/30

Piezoresis t ive In teg rated

Semiconductor

Integrated silicon pressure sensor

measures 0.52 in. long by 0.44 in. wide

by 0.75 in. high, including the port.


23/30

Pressure Switches

Pressure switches, combining adiaphragm or other pressure measuringmeans with a precision snap switch, can

provide precise single-point pressuresensing.

Alternatively, simple electronic switchesmay be combined with electrical sensorsto construct a pressure switch with anadjustable set point and hysteresis.


24/30

Electrical Interfacing

Care must be taken to avoid corrupting the signal by noise of 60/50Hz AC pickup. If the signal must be run some distance to theinterface circuitry, twisted and/or shielded wire should beconsidered. A decoupling capacitor located at the sensor andconnected from the supply to ground will also filter noise, as will acapacitor from output to ground.

For long runs, a current output sensor should be considered. Thesedevices have a 2-wire interface and modulate the supply current inresponse to applied pressure. Obviously, wire resistance has noeffect and noise must change the loop current, not simply impress avoltage on the signal. The industry standard interface is:

PL = 4 mA PH = 20 mA

PL= low pressure range limitPH = high pressure range limit


25/30

Calibration

Dead-Weight Tester.A dead-weight tester uses calibratedweights that exert force on apiston which then acts on a

fluid to produce a testpressure.

For high pressures (>500 psi),oil is typically used for lowerpressures, pneumatic air

bearing testers are availableand are much moreconvenient as well as lessmessy to use.


26/30

Manometer

A mercury manometer is

a simple pressure

standard and may be

used for gauge,

differential, and absolutemeasurements with a

suitable reference. It is

useful mainly for lower

pressure work becausethe height


27/30

Selection Considerations

Selection of a pressure sensor involves consideration ofthe medium for compatibility with the materials used inthe sensor, the type (gauge, absolute, differential) ofmeasurement, the range, the type of electrical output,and the accuracy required.

Manufacturer's specifications usually apply to aparticular temperature range. If the range of operation ina given application is smaller, for example, the errorsshould ratio down.

Total error can be computed by adding the individual

errors (worst-case) or by computing the geometric sumor root sum of the squares (RSS). The latter is morerealistic since it treats them as independent errors thattypically vary randomly.


28/30

Selection Considerations Following is a comparison of the two methods.

Given the following error terms:

Linearity = 1% F.S.

Null calibration = 1% F.S.

Sensitivity calibration = 1% F.S. Temperature errorsare sometimes given as coefficients per Creferenced to 25C. Simply multiply the coefficientby the temperature range of the application to obtainthe total error.

Temperature error = 0.5% F.S. Repeatability and hysteresis = 0.1% F.S.


29/30

Selection Considerations

Worst case error is equal to the sum of all

the maximum errors:

Worst case error = 1 + 1 + 1 + 0.5 + 0.1 = 3.6%


30/30

Industrial ApplicationsFluid level in a tank:A gauge pressure sensor located

to measure the pressure at the bottom of a tank canbe used for a remote indication of fluid level usingthe relation:

h = P/g

Fluid flow:An orifice plate placed in a pipe sectioncreates a pressure drop. This approach is widelyused to measure flow because the pressure dropmay be kept small in comparison to some othertypes of flowmeters and because it is impervious to

clogging, which may otherwise be a problem whenmeasuring flow of a viscous medium or onecontaining particulate matter. The relation is:

Documents

06presure Measurement