RTD CHARACTERISTICS TRAINER - Chettinad College of · PDF file · 2016-05-30RTD CHARACTERISTICS TRAINER ... 4. FRONT PANEL DIAGRAM 3 5. ... with both bridge and "ohmmeter" methods

RTD CHARACTERISTICS TRAINER(Model No : ITB - 06CE)

User Manual

Technical Clarification /Suggestion : / Technical Support Division,Vi Microsystems Pvt. Ltd.,Plot No :75,Electronics Estate,Perungudi,Chennai - 600 096,INDIA.Ph: 91- 44-42048142, 91-44-24963142Mail : [email protected],Web : www.vimicrosystem

Version 1.0

CONTENTS

1. INTRODUCTION 1

2. TEMPERATURE MEASUREMENT 1

3. TECHNICAL SPECIFICATION 1

4. FRONT PANEL DIAGRAM 3

5. FRONT PANEL DESCRIPTION 4

6. RESISTANCE TEMPERATURE DETECTORS 4

7. ELECTRICAL-RESISTANCE SENSORS 7

8. CIRCUIT DESCRIPTION 14

9. DATA SHEET 15

10. EXPERIMENTAL SECTION 16

RTD CHARACTERISTICS TRAINER ITB-06CE

Vi Microsystems Pvt. Ltd., [ 1 ]

1. INTRODUCTION

Temperature measurement is one of the earliest areas of metrology and its use in control andinstrumentation is significant. This being one of the most important concept, we have releasedthis ITB-06CE card in our series of Instrumentation Trainer Boards, which would help thestudents to bring out their ideas in a very simple way.

2. TEMPERATURE MEASUREMENT

There are, in general, four types of sensors based on the following physical properties, which aretemperature dependent:

1. Expansion of a substance with temperature, which produces a change in length, volumeor pressure. In it's simplest form this is the common mercury-in-glass or alcohol-in-glassthermometer.

2. Changes in contact potential between dissimilar metals with temperature, thermocouple.

3. Changes in radiated energy with temperature, optical and radiation pyrometers.

4. Changes in electrical resistance with temperature, used in resistance thermometers andthermistors.

The fourth property is used in our design, to create a sensor. Resistance thermometry requiresa resistor properly mounted to create a sensor and by means of measuring the resistance of thesensor.

3. TECHNICAL SPECIFICATIONS

RTD SENSOR

Type : Pt100 (100 @ 0°C)

Material : Platinum (Protected by stainless steel sheath)

Resolution : 0.292 to 0.39/°C

Accuracy : ±0.6@100°C

Temperature Range : -200 to 850°C

Tube size : 120mm × 8mm



Connection through two core retractable lead (1 Meter Extended)

Thermowell material : Stainless steel

Coating : Nickel, chromium

ITB-006CE

Operating Temperature : 10°C - 55°C

Cabinet Material : Mild Steel with powder coating

Accuracy : 0.7% Full Scale Deflection

Linearity : 1.8% Full Scale Deflection

Dimension : 370mm × 280mm × 90mm

LED DISPLAY

Size : 50 × 20 mm

Type : Common Anode

Display : 3.5 Digit

Segment : Seven Segment

Colour : Red

POWER SUPPLY

Input : 230V AC / 50HzOutput : +5V / 1A

-5V / 500mA+12V / 500mA-12V / 500mA



4. FRONT PANEL DIAGRAM

RTD

CH

AR

AC

TER

ISTI

CS

TR

AIN

ER

( IT

B -

06C

E )

T1T3

T2T4

Sw1

EXT

INT

Sw

2

POW

ERO

N/O

FF

WH

EAT

STO

NE

BR

IDG

E

ZER

O

DIF

FER

ENTI

AL

AM

PLIF

IER

T6

T5C

GA

IN

AM

PLIF

IER



5. FRONT PANEL DESCRIPTION

Power ON/OFF Switch : To Switch ON / OFF the unit.

T1 & T2 : These are used to connect the two terminals of RTD.

SW1 : To select the resistance mode.

T3 & T4 : To measure the resistance value of RTD.

SW2 : This switch is used to select the internal / External mode ofoperation.

INT - place the switch SW2 towards downward.

EXT - place the switch SW2 towards upward.

T5 : To measure the differential amplifier output voltage.

T6 : To measure the signal conditioner output voltage.

T7 : This terminal is the common GND terminal.

3.5 LED Display : It displays the RTD sensor output interims of °C.

6. RESISTANCE TEMPERATURE DETECTORS

Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices thatmeasure temperature because of the physical principle of the positive temperature coefficient ofelectrical resistance of metals. The hotter they become, the larger their resistance.

They, in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTDtype, nearly linear over a wide range of temperatures and some small enough to have responsetimes of a fraction of a second. They are among the most precise temperature sensors availablewith resolution and measurement uncertainties or ±0.1 °C or better possible in special designs.Usually they are provided encapsulated in probes for temperature sensing and measurement withan external indicator, controller or transmitter, or enclosed inside other devices where theymeasure temperature as a part of the device's function, such as a temperature controller orprecision thermostat.

The advantages of RTDs include stable output for long period of time, ease of recalibration andaccurate readings over relatively narrow temperature spans. They are active devices requiring anelectrical current to produce a voltage drop across the sensor that can be then measured by acalibrated read-out device.



The lead wires used to connect the RTD to a readout can contribute to the measurement error,especially when there are long lead lengths involved, as often happens in remote temperaturemeasurement locations. Those calculations are straight forward and there exist 3-wire and 4-wiredesigns to help minimize or limit such errors, when needed.

Often the lead error can be minimized through use of a temperature transmitter mounted close tothe RTD. Transmitters convert the resistance measurement to an analog current or serial digitalsignal that can be sent long distances by wire or rf to a data acquisition or control system and/orindicator. RTDs, as mentioned above, work in a relatively small temperature domain, comparedto thermocouple, typically from about -200°C to a practical maximum of about 650°C to 700 °C.

RTDs can be made cheaply in Copper and Nickel, but the latter have restricted ranges becauseof non-linearities and wire oxidation problems in the case of Copper. Platinum is the preferredmaterial for precision measurement because in its pure form the Temperature Coefficient ofResistance is nearly linear; enough so that temperature measurements with precision of ±0.1 °Ccan be readily achieved with moderately priced devices. Better resolution is possible, butequipment costs escalate rapidly at smaller error levels.

All RTDs used in precise temperature measurements are made of Platinum and they aresometimes called PRTs to distinguish them. RTD works on the principle that electrical resistanceof the most metals increases linearly with temperature. If a metal wire has a Resistance R0 at 0°C,then the resistance at T°C will be given by:

The constant is called the temperature coefficient of resistance. Typical values are:

METALS Platinum

CopperNickel

0.00390.00430.0068

A temperature transducer using the above principle is called Resistance Temperature Detectors(RTD). These are simple to use, requiring no special wiring, are highly stable and very sensitive.RTD's commonly use Platinum, nickel and copper to form the sensor (see Figure-2), althoughiron, tungsten and alloys can be used. The former metals have the advantage that they can beobtained to high degrees of chemical purity.



COPPER SENSORS are an obvious candidate, as copper is readily available at high degree ofpurity and is quite linear. High temperature measurements are limited to about 100°C. Owingto the low resistivity of copper, long and hence bulky lengths of fine wire are required and theresulting relatively low resistance obtained makes accurate measurement difficult. Copper is alsovery susceptible to oxidation and corrosion.

Figure - 2

NICKEL SENSORS are appreciably non-linear, restraining measurements to less than 300°C,although passive networks can be added to give linear outputs with temperature. However,temperature cycling through the point at 358°C causes resistance instability. The resistivity ishigher allowing smaller devices to be reconstructed. As there is a little international agreementit's wider use is limited. Oxidation and corrosion is a problem.

PLATINUM SENSORS dominate resistance pyrometer. Platinum is stable, resists corrosion andoxidation, is malleable, has high melting point and a high degree of resistivity, and can be madeto a high degree of purity. All these are achieved at the expense of high manufacturing costs andcareful mounting to avoid strain gauge effects.

Figure - 3



Platinum-based resistance thermometers are probably the most widely used. Measurements canbe made from -220°C to 850°C readily to a stability of hundreds of a degree over several years.RTD is specified in terms of its resistance at 0°C, and change in resistance from 0°C to 100°C.This is known as the "fundamental interval".

Platinum RTDs are constructed with a resistance of 100 ohm at 0°C (and are often referred to asPt100 sensors) is used in our design. This gives a resistance of 138.5 ohm at 100°C, and hencea fundamental interval of 38.5 ohm. Pt100 sensors can be used over a temperature range of -200°C to 800°C.

RTD's are available in many shapes and sizes. Figure-4 shows various construction of RTDSs.

These are designed to protect the wire from mechanical shocks while not applying any stress onthe wire (which would cause resistance changes in a similar way to a strain gauge). Constructionof the wire to be in direct contact with the fluid gives a faster response, but little protectionagainst corrosion is necessary. The sensors in Figure-4 are totally enclosed, but the increasedmass gives longer time constants.

Figure - 4

7. ELECTRICAL-RESISTANCE SENSORS

The electrical resistance of various materials changes in a reproducible manner with temperature,thus forming the basis of a temperature-sensing method. Materials in actual use fall into two mainclasses: conductors (metals) and semiconductors. Conducting materials historically came first andtraditionally have been called resistance.

Semiconductor types have been given the generic name thermistor. Any of the various establishedtechniques of resistance measurement may be employed to measure the resistance of thesedevices, with both bridge and "ohmmeter" methods being common.



7.1 Conductive Sensors (Resistance Thermometers) The variation of resistance R with temperature T for most metallic materials can be representedby an equation of the form

R R a T a T a Tn n 0 1 221( ... )

where R0 is the resistance at temperature T = 0. The number of terms necessary depends on thematerial, the accuracy required, and the temperature range to be covered. Platinum, nickel, andcopper are the most commonly used.

Sensing elements are made in a number of different forms. For measurement of fluidtemperatures, the winding of resistance wire may be encased in a stainless steel bulb to protectit from corrosive liquids or gases. Open-type pickups expose the resistance winding directly tothe fluid (which must be non corrosive) and give faster response.

Bridge circuits used with resistance temperature sensors may employ either the deflection modeor operation or the null model. While the resistance/temperature variation of the sensing elementmay be quite linear, the output voltage signal of a bridge used in the deflection mode is notnecessarily linear for large percentage changes in resistance.

Resistance-thermometer bridges may be excited with either AC or DC voltages. The direct or rmsalternating current through the thermometer is usually in the range 2 to 20mA. This currentcauses an I2R heating which raises the temperature of the thermometer above its surroundings,causing the so-called self-heating error. The magnitude of this error depends also on heat-transferconditions and usually is quite small.

Platinum can withstand high temperatures while maintaining excellent stability. As a nobel metal,it shows limited susceptibility to contamination. All metals produce a positive change inresistance with temperature. This, of course, is the main function of an RTD. The system erroris minimized when the nominal value of RTD is large. This implies a metal with a high value ofresistivity should be used for RTDs. The lower is the resistivity of the metal, the more materialwe will have to use.

The requirements of a conductor material to be used in RTDs are:

1. The change in resistance of material per unit change in temperature should be as large aspossible.

2. The material should have a high value of resistivity so that minimum volume of material isused for the construction of RTD.

3. The resistance of materials should have a continuous and stable relationship with temperature.

The most common RTDs are made of either platinum nickel or nickel allows. The economicalnickel wires are used over a limited temperature range. They are quite non-linear and tend to driftwith time. For measurement integrity, platinum is the obvious choice.



An examination of the resistance versus temperature curves of Fig.1 shows that the curves arenearly linear. In fact, when only short temperature spans are considered, the linearity is moreevident. This fact is employed to develop approximate analytical equations for resistance versustemperature for a particular metal.

7.2 Linear Approximation

A linear approximation means that we may develop an equation for a straight line whichapproximates the resistance versus temperature curve over a specified span.

A straight line has been drawn between the points of the curve which represent 1°C and °C with0°C representing the mid point temperature. The equation of this straight line is the linearapproximation of the curve from 1°C to 2°C. The equation of the straight line is written as:

where R = R0 (1 + 0 ) with 1 < 0 < 2R = approximate resistance at °C ; R0 = approximate resistance at 0°C ; = - 0 = change in temperature ;°C

7.3 Quadratic Approximation

A quadratic approximation of the resistance temperature curve is more accurate representationof the curve over a limited range of temperature. The quadratic approximation relationshipincludes both a linear term as in Equation and an addition has a term which varies as the squareof the difference in temperature. The approximation is written as:

where 1 = linear fractional change in resistance, /°Cand 2 = quadratic fractional change in resistance, /(°C)2

RTD exhibits change in resistance with temperature. Before it can be used for measurement orcontrol, this change in resistance must be converted to a change in voltage or current. Theelectrical power dissipated in the RTD for this conversion must be strictly limited to avoid errorsdue to I2R heating of the sensor. Typically 10mW dissipation will cause the temperature rise of0.3°C, which implies low values current (less than 10mA) and voltage (below 1V). Thecommonest circuits, however are based on Wheatstone bridge of Figure-5. If the measuringcircuit has high impedance (so that it does not load the bridge), simple circuit analysis shows that:



Figure - 5

Unfortunately, because Rt appears in both the numerator and denominator of the left hand term,V does not change linearly with changes in Rt. There are three common ways of overcoming thisnon linearity ( Shown in Figure-6 )

This non-linearity can be reduced to acceptable levels by making R3 >> Rt and R1 >> R2(typically by a factor of 100). This has the side effect of reducing the bridge voltage by a factorof 100 as well, but this can easily be re-established by a means of a DC amplifier.

Figure - 6



Figure - 7

In Figure-7, the non-linear output from the bridge is processed by a suitable line arising circuit togive an output voltage which is linearly related to the temperature. The line arising can beperformed by an op-amp circuit.

In most industrial applications, the RTD will be situated remote from it's measurement electronics.If the connecting leads are more than a few meters in length, they will introduce an unknownresistance, "r", into each lead as shown in Figure-8.

Figure - 8



Figure - 9

This unknown resistance is itself subject to change caused by temperature and strain gauge effectsand is a possible source of error. This is overcome by using a four-wire connection to the RTDas shown in Figure-9. Each lead will experience the same conditions, so the changes introducedinto the RTD will be matched by the changes in temperature of the RTD, not the lead. Figure-9shows the commonest industrial circuit for RTD.

Figure - 10



Figure - 11



8. CIRCUIT DESCRIPTION

In ITB-006CE unit the Pt100 type of RTD is used to measure temperature in terms of resistance.Platinum sensor is preferred here, because it highly stable, resists corrosion and oxidation, it ismalleable and has high melting point and also has high degree of sensitivity compared to otherRTD sensors.

As said earlier the output of the RTD is in the form of resistance. The resistance of the RTDvaries with temperature. This is then converted in to voltage or current.

The output of RTD is interms of resistance. The resistance of RTD varies with temperature. Thisresistance is then converted to voltage by means of a wheat stone bridge. The wheat stone bridgegibes the output voltage interms of MV. Since the output is interms of mV, it should be amplifiedto a comfortable range of 0-5V by using the signal conditioner amplifier. The output of thisamplifier is given to the LED display, which display the output in the form of temperature with°C. The block diagram is given below.

Figure - 12 Block Diagram of ITB-006CE Unit

POWERSUPPLY

RTDSENSOR

Wheat stone Bridge

SignalConditioner

mV Display 0 - 5V DC



9. DATA SHEET

Sensor Type 1.100 ohm Platinum RTD Celsius

Temp Resis Temp Resis Temp Resis Temp Resis Temp Resis

-40 84.27 1 100.39 42 116.31 83 132.04 124 147.57-39 84.67 2 100.78 43 116.70 84 132.42 125 147.94-38 85.06 3 101.17 44 117.08 85 132.80 126 148.32-37 85.46 4 101.56 45 117.47 86 133.18 127 148.70-36 85.85 5 101.95 46 117.85 87 133.56 128 149.07-35 86.25 6 102.34 47 118.24 88 133.94 129 149.45-34 86.64 7 102.73 48 118.62 89 134.32 130 149.82-33 87.04 8 103.12 49 119.01 90 134.70 131 150.20-32 87.43 9 103.51 50 119.40 91 135.08 132 150.57-31 87.83 10 103.90 51 119.78 92 135.46 133 150.95-30 88.22 11 104.29 52 120.16 93 135.84 134 151.33-29 88.62 12 104.68 53 120.55 94 136.22 135 151.70-28 89.01 13 105.07 54 120.93 95 136.60 136 152.08-27 89.40 14 105.46 55 121.32 96 136.98 137 152.45-26 89.80 15 105.85 56 121.70 97 137.36 138 152.83-25 90.19 16 106.24 57 122.09 98 137.74 139 153.20-24 90.59 17 106.63 58 122.47 99 138.12 140 153.58-23 90.98 18 107.02 59 122.86 100 138.50 141 153.95-22 91.37 19 107.40 60 123.24 101 138.88 142 154.32-21 91.77 20 107.79 61 123.62 102 139.26 143 154.70-20 92.16 21 108.18 62 124.01 103 139.64 144 155.07-19 92.55 22 108.57 63 124.39 104 140.02 145 155.45-18 92.95 23 108.96 64 124.77 105 140.39 146 155.82-17 93.34 24 109.35 65 125.16 106 140.77 147 156.19-16 93.73 25 109.73 66 125.54 107 141.15 148 156.57-15 94.12 26 110.12 67 125.92 108 141.53 149 156.94-14 94.52 27 110.51 68 126.31 109 141.91 150 157.31-13 94.91 28 110.90 69 126.69 110 142.29-12 95.30 29 111.28 70 127.07 111 142.66-11 95.69 30 111.67 71 127.45 112 143.04-10 96.09 31 112.06 72 127.84 113 143.42-9 96.48 32 112.45 73 128.22 114 143.80-8 97.87 33 112.83 74 128.60 115 144.17-7 97.26 34 113.22 75 128.98 116 144.55-6 97.65 35 113.61 76 129.37 117 144.93-5 98.04 36 113.99 77 129.75 118 145.31-4 98.44 37 114.38 78 130.13 119 145.68-3 98.83 38 114.77 79 130.51 120 146.06-2 99.22 39 115.15 80 130.89 121 146.44-1 99.61 40 115.54 81 131.27 122 146.810 100.00 41 115.93 82 131.66 123 147.19



10. EXPERIMENTAL SECTION

EXPERIMENT-1

AIM

To study the temperature Vs resistance characteristics of RTD (Pt100)

APPARATUS REQUIRED

1. ITB-06CE2. RTD sensor3. Water Bath4. Thermometer5. Multimeter (optional)

PROCEDURE

1. Patch the wires of RTD to the T1 and T2 terminal of the RTD input block and switch ON theunit.

2. Place the RTD and thermometer into the holes provides in the waterbath.

3. Keep the SW1 in right direction.

4. Place the multimeter in the resistance mode across T3 and T4 terminals.

5. Switch ON the waterbath and note the temperature in thermometer and correspondingresistance value in multimeter.

6. Plot the temperature Vs resistance graph.

This gives the characteristic curve of the RTD. Refer to the model graph.

TABULAR COLUMN

Temperature (°C) Resistance ()

0°C.. ..

100 °C

100....

138.5



MODEL GRAPH

The graph between Temperature and Resistance are drawn.

TEMPERATURE (°C) VS RESISTANCE

SAMPLE READING

Temperature 0°C 100()Temperature 100°C 138.5()

RESULT

Thus the study of Temperature Vs Resistance characteristics was studied and graph is plotted.

PRECAUTIONS

1. Gradually heat the water and note the corresponding resistance simultaneously.

2. The multimeter / ohmmeter should be in the range (0-200) to measure for (0-100)°C.

RES

ISTA

NC

E (

)

TEMPERATURE ( C )

TEMPERATURE 0 C 100 ( )TEMPERATURE 100 C 138

o

o

TEMPERATURE ( C ) VS RESISTANCEo

o



EXPERIMENT - 2

AIM

To study the temperature Vs voltage characteristics and the accuracy of the signal conditioningboard.

APPARATUS REQUIRED

1. ITB-06CE2. RTD sensor3. Water Bath4. Thermometer5. Multimeter (optional)6. Power

FORMULA TO BE USED

% *ErrorActual Temperature Displayed Temperature

Full Scale Division

100

PROCEDURE

1. Patch the wires of RTD to the T1 and T2 terminal of the RTD input block.

2. Switch ON the ITB-06CE Unit.

3. Keep the switch SW1 in left direction and switch SW2 in external mode.

4. Now adjust the ‘Zero’ Potentiometer to read 0°C at the display. This is done for initial setupof the unit and this adjustment should be left undisturbed.

5. Place the multimeter in voltage mode across the T6 and T7 terminals.

6. Insert the RTD and thermometer into the waterbath and note the temperature without anyheating at ambient condition.

7. Switch ON the waterbath and note down the actual temperature in thermometer, outputvoltage of the unit and the displayed temperature simultaneously.

8. Plot the graph for Actual Temperature Vs Voltage.

9. Calculate the % error and plot the graph for Temperature Vs % Error.

The first graph measures the linearity of the signal conditioning unit and the second graph



measures the accuracy.

TABULAR COLUMN

ActualTemperature (°C)

Output Voltage (V) DisplayedTemperature (°C)

%Error

MODEL GRAPH

1. The graph between temperature Vs voltage are drawn.

2. The graph between temperature and % Error are drawn.

OU

TPU

T V

OLT

AG

E ( V

)

TEMPERATURE ( C )

TEMPERATURE : 100 COUTPUT : 5 V

o

o

TEMPERATURE ( C ) VS OUTPUT VOLTAGE ( V )o



RESULT

Thus the study of Temperature Vs Voltage and the accuracy of signal conditioning board wasstudied and the graph is drawn.

Documents

RTD CHARACTERISTICS TRAINER - Chettinad College of · PDF file · 2016-05-30RTD CHARACTERISTICS TRAINER ... 4. FRONT PANEL DIAGRAM 3 5. ... with both bridge and "ohmmeter" methods