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TEMPERATURE MEASUREMENT

8.1 LEARNING OUTCOMES

At the end of this experiment, students should be able to:

i. Calibrate Type K, and Type J thermocouples.

ii. Analyze the principles of a thermocouple transmitter and calibration of a thermocouple

transmitter.

iii. Calibrate Platinum Resistance thermometers

8.2 INTRODUCTION

Thermocouples are based on the Seebeck effect, Peltier Effect and Thomson Effect. Seebeck

Effect states that if wire made of two different materials are joined together at their ends and if thetwo end or junctions are maintained at different temperature, a current will flow in the circuit.

Thermocouples are based on this effect. Two other effects known as Peltier Effect and Thomson

effect are also related to thermoelectric effects. The important pairs of materials used as

thermoelectric effects. The important pairs of materials used as thermocouples are shown in

Table 8.1.

Table 8.1 Important Thermocouple Pairs

Type Material Range, 0C Resolution V/0C B Platinum 30% Rhodium- 0 to 1820 11

Platinum 6% Rhodium

E Chromel-Constantan -270 to 1000 80

J Iron-Constantan -210 to 1200 60

K Chromel-Alumel -270 to 1370 45

R Platinum 13% Rhodium- -50 to 1760 15

Platinum

T Copper-Constantan -270 to 400 60

The Temperature-EMF relantionships of the important types of thermocouples are shown in

APPENDIX table A1-A6. These tables give the EMF for the cold junction at 0 0C and the hot

junction at the temperature T.

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8.2.1 Cold Junction Compensation

Though thermocouple calibration tables are given with the cold junction at 0 0C it is rarely possible

to maintain the cold junction at that temperature except in laboratories. In industrial applications

the cold junctions are maintained at the ambient conditions and suitable corrections are applied.

The EMF of a thermcouple maintained with the hot junction at T and cold junction at 0 , E T,O is

given by,

E T , O

E T , t

E t ,O

where,

E T,t = temperature of the thermocouple with hot junction at T and cold junction at t

E t,0 = temperature of the thermocouple with hot junction at T and cold junction at O

In industrial thermocouple transmitters cold junction compensation is performed electronically.

8.2.2 Thermocouple connection

Law of intermediate metals in a thermocouple made up of Metals A and B at temperatures t 1 and

t2 we can introduce one ore metallic wires between A and B without altering the EMF (Figure 8.2),

provided all the junctions with which the junctions at t 1 is replaced is maintained at t 1 and all the

junction with which the junctions t 2 is replaced is maintained at temperature t 2 .

Figure 8.2 Law of intermediate metals

Because of the law of intermediate metals we can use wire made of some other material to

connect a thermocouple with the secondary measuring instruments. For this purpose we use

compensating cables for e.g. Type K compensating cable.

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8.3 EXPERIMENTAL APPARATUS

The equipment required for performing the experiments are shown in Table 8.4

Table 8.4 Equipment Required

Description Unit name Model no

Experimental Units Temperature Measurement Unit Yokogawa Model YTC-01

Constant Temperature Bath Isotech Model Jupiter 650

Handheld Communicator Yokogawa Model BT200

Digital Indicator Yokogawa Model UM330

Units Under Test (UUT) Thermocouple Sets Types K, J, E,

T, B and R

Plantinum Resistance Thermometer

100 Ohms

Thermocouple Transmitter Yokogawa Model YTA110

ResistanceThermometer Yokogawa Model YTA110

Transmitter

Master Standard Units Wheatstone Bridge Yokogawa Model 275597

(MSU) Digital Thermometer Yokogawa Model 756301

8.4 PROCEDURE

8.4.1 Calibration of thermocouples

8.4.1.1 Experimental Setup and Procedure

Thermocouples can be calibrated up to 650 0C using the constant temperature bath (Figure 8.5)


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Figure 8.5 Calibration of Thermocouple

A platinum resistance thermometer together with Model 756301 digital thermometer is used as

the Master Standard Unit. A thermocouple together with UM330 digital indicator is used as the

Unit Under Test.

1. Connect the equipment as shown in Figure 8.5.Use a Type K thermocouple as the UUT.

2. Set the constant temperature bath temperature to 40 0C and allow the temperature to

stabilize. We can consider the temperature to be stabilized if the MSU reading does not

change for say 5 minutes. Note the MSU reading and the UUT reading.

3. Select a minimum of FIVE ( 5 ) bath temperatures between 40 0C and 300 0C to develop a

calibration curve for the type K thermocouple. After each change wait for about 15

minutes for the temperature to stabilize. Record all the relevant data.

4. Repeat the experiment for the type J thermocouples.


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8.4.1.2 Questions

1. Based on the data that you have collected, plot relevant graphs and compare with the literaturevalues given in Appendix table A1 to A6. Comment on your findings.

2. Discuss about the sensitivity of thermocouples J and K. Calculate the sensitivity and compare

with the values reported in literature as given in Table 8.1. Discuss about the linearity of the

thermocouples over the temperature range.


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8.4.2 Step response of thermocouples

In this section the dynamic response of the thermocouple is determined by step testing.

The experimental setup for performing step response testing is shown in figure 8.8.

Figure 8.8 Step response of thermocouples


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8.4.2.1 Experimental Set-up and Procedure

1. Connect the equipment as shown in figure 8.8

2. Keep the thermocouple in the air outside the constant temperature bath.

3. Adjust the bath temperature at say 70 0C.

4. Suddenly dip the thermometer into the bath and keep it there. This way we are introducing

a step change

5. Note the change in temperature with respect to time.

6. After the temperature reading has become constant, do the reverse step by suddenly

taking out the thermometer from the bath and keeping it in the air. Wait till the temperature

again stabilizes.

8.4.2.2 Questions

1. Based on the data that you have collected for both heating and cooling, identify whether theresponse for each step is first order or higher order. Determine the corresponding timeconstant(s) and gain of each system. Discuss your results.

8.4.3 Thermocouple transmitter

The function of the temperature transmitter is to convert the mV output given by different types of

thermocouples to standard 4-20 mA output. Yokogawa YTA110 transmitter will be calibrated in

this experiment. In this experiment distributor is introduced to supply 24 VDC to the transmitter

and convert its 4-20 mA output to 1-5 V.


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Figure 8.11 Thermocouple Transmitter Calibration

1. Connect the equipment as shown in figure 8.11.

2. Adjust the bath temperature for 40 0C. After the temperature has stabilized read the value

given by the digital thermometer and the digital indicator.

3. Repeat the experiment by selecting a minimum of FIVE (5 ) bath temperatures between40 0C and 300 0C

4. Record all relevant data.


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8.4.3.2 Questions

1. Convert the output of the transmitter to corresponding temperature and analyze your datawith appropriate plots. Perform maximum error calculation and verify its consistency with

the UUT specifications. Discuss your results.

Specifications of YTA110 is as follows:

Total accuracy = {(A/D accuracy)/span + D/A accuracy} or 0.1% of span, whichever is greater.

For thermocouple inputs, add Cold Junction Compensation accuracy of 0.5 0C to the total

accuracy.

For type K TC,

Total accuracy = (0.25/500 + 0.02% of 500) + 0.5 0C or (0.1% of 500 + 0.5) = 0.85 or 1.0

= 1 0C


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8.4.4 Resistance thermometer transmitter


Two and three wire connections in resistance thermometers

Figure 8.13 Resistance thermometer connection

1. Make connections as shown in figure 8.13 for 3 wire connection

2. Disconnect the lead wires from the YTA110 transmitter. Measure the resistance of the

lead wire (terminal B and B) using the wheatstone bridge. The lead wire resistance for

terminal A and B is same as lead wire resistance for terminal B and B.

3. Reconnect the two lead wires to the transmitter YTA 110.

4. For the three wire connection read the output of the transmitter on UM330 Digital

Indicator.

5. Connect brain terminal to the transmitter. Change sensor type from 3 wire to 2 wire.

6. Read the output of the transmitter on UM330 Digital indicator.

7. Adjust the temperature bath for 50 0C.

8. Repeat step 1 to 6 with the lead wires in the temperature bath.

9. Record all relevant data.

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8.4.4.2 Questions

1. Based on your data, determine the resistance of the 2-wire and 3-wire systems at both roomtemperature and 50 oC and calculate the corresponding error %. Discuss your results.


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8.4.5 Resistance thermometers


Figure 8.15 Calibration of resistance thermometer up to 300 0C

1. Connect the equipment as shown in figure 8.15. Use a 2 wire resistance thermometer as

the UUT. Short circuit terminal 2 and 3 at the back of UM330.

2. Set the constant temperature bath to 40 0C and allow the temperature to stabilize. We can

consider the temperature to be stabilized if the MSU reading does not change for say 5

minutes. Note the MSU reading and the UUT reading

3. Select a minimum of FIVE ( 5 ) bath temperatures between 40 0C and 300 0C to develop a

calibration curve for the 2 wire resistance thermometer. After each change wait for about

15 minutes for the temperature to stabilize. Record all the relevant data

4. Repeat the experiments for the 3 wire resistance thermometer. Record all relevant data.


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8.4.5.2 Questions

1. Based on your data, plot relevant graphs and comment on your findings. Compare thesensitivity of a resistance thermometer with a thermocouple.


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8.6 REFERENCES

1. Coughanowr, D. R, Process System Analysis and Control, 2 nd edition McGraw Hill New

York 1991.


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temperature measurement version 1

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