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Calibration of a Thermocouple.

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Introduction

SHEEMA PANKHANIA                                                INSTUMENTATION TASK

Calibration of a Thermocouple

Aim

My aim of this investigation is to build and investigate a thermocouple.  I have chosen to do this task because it looks like a very challenging task to accomplish and I also hope to learn more about thermoelectric e.m.f.

Introduction

A thermocouple is a pair of different metals, which produces a thermoelectric effect (e.m.f.), which is used to sense and measure a difference in temperature.

A thermocouple consists of two different metal wires joined together.  When there is a temperature difference between the two junctions, a thermoelectric emf is produced.  A thermocouple is a source of a temperature – dependent emf that can be used to monitor or measure temperatures.

When two different metals are connected together, electrons leave one metal and transfer to the other metal, causing a potential difference across the two junctions.   This is known as the Seebeck effect.  This potential difference occurs because electrons can leave one of the metals more easily than the other so the first metal looses the electrons to become positive and the second metal gains electrons to become negative.

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Middle

Micro voltmeter

12v power supply – for power

First of all I will need to make my thermocouple. I will simply do this by cutting two equal lengths of some wire, the copper wire and one length of the other iron wire.  I will then assemble them together by first of all cleaning the ends using sandpaper and then twisting the ends together of the different metals to make the two junctions. I will then secure the ends together using insulating tape around the wires to prevents them disconnecting when in use.  I could alternatively solder the ends together, however this can be difficult to do to get a secure joint, so ill do it this way which is just as effective.

I will then need to connect the micro-voltmeter to an ac power pack supply and then connect the micro-voltmeter to both ends of the thermocouple.  I must make sure that my results are in milli-volts so I must change my range on the micro-voltmeter to milli-volts.

Next I will cool some cold water with ice in a beaker, getting it near enough 0’C as I can, and stick both my junctions into it.

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Conclusion

s table 2:

‘ C

3

10

26

36

44

57

62

70

83

mV

0

0.04

0.12

0.22

0.26

0.36

0.39

0.48

0.57

Results table 3:

‘ C

3

10

25

37

43

52

62

71

83

mV

0

0.1

0.15

0.25

0.28

0.35

0.40

0.45

0.58

From these results you can see that the results that I have obtained are quite similar but are not exact each time.  To over come this I have worked out the average of my results by adding up all the values for each temperature range from tables 1, 2 and 3, and dividing the value by three.  The average results are shown in table 4.

Results table 4:

‘ C

3

10.3

25.6

36.0

44.0

55.0

61.6

72.0

82.6

mV

0

0.06

0.16

0.23

0.27

0.36

0.40

0.47

0.56

 See graph 1 for graphical representation of my results.

ANALYSIS

        From the graph you can see that the relationship is linear.  The straight line represents this relationship, which means that E, e.m.f., is proportional to T, temperature.  This can be written in the form y= mx + c, where y is the emf, x is the temperature, m is the gradient of the line and c, is the intercept on the y-axis.  This gives us a general formula to approximate the output for a given temperature

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