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I have been assigned the task to test and evaluate a sensor. The sensor I have chosen is a temperature sensor.

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Introduction

Alex Willcocks

Sensing Coursework

        I have been assigned the task to test and evaluate a sensor. The sensor I have chosen is a temperature sensor. The sensor is a potential divider circuit. It consists of a power supply of e.m.f 2V, a variable resistor and a 15KΩ (ptc) thermistor. I will vary the resistance of the variable resistor to see if this will affect the sensitivity of my sensor.

To test my sensor I will measure the potential difference across the thermistor at varying temperatures. I will do this using a multimeter, connecting it in parallel across the thermistor. As it is a positive temperature coefficient (p.t.c.) thermistor I will expect the potential difference to increase as the temperature decreases.

Experiment 1

Potential difference across thermistor (V)

Temperature of thermistor (ºC)

1st attempt

2nd attempt

3rd attempt

Mean, μ

Standard deviation, σ

0

1.96

1.97

1.96

1.96

0.01

5

1.93

1.94

1.95

1.94

0.01

10

1.93

1.93

1.94

1.93

0.01

15

1.93

1.92

1.90

1.92

0.02

20

1.90

1.91

1.91

1.91

0.01

25

1.89

1.87

1.88

1.88

0.01

30

1.85

1.86

1.86

1.86

0.01

35

1.80

1.83

1.83

1.82

0.02

40

1.78

1.79

1.80

1.79

0.01

...read more.

Middle

90.36

0.06

5

74.80

74.70

74.90

74.80

0.10

10

56.70

56.60

56.70

56.67

0.06

15

45.30

45.20

42.50

45.23

0.06

20

36.50

36.50

36.50

36.50

0.00

25

29.60

29.80

29.70

29.70

0.10

30

23.70

24.00

24.10

23.93

0.21

35

19.80

19.70

19.90

19.80

0.10

40

16.10

16.30

16.30

16.23

0.12

These results were taken using a power supply of e.m.f. 2.00V and the variable resistor at 1MΩ

Experiment 3

Potential difference across thermistor (mV)

Temperature of thermistor (ºC)

1

...read more.

Conclusion

The lines of best fit for the graphs of experiments 2 and 3 are both similar shaped curves whereas the line of best fit for the graph of experiment 1 looks like a reflection. I feel that this is due to human error and multimeter used not being sensitive enough. I would like to experiment further with the value of the variable resistor in experiment 1 with a more sensitive device to measure voltage. By taken more results with a more sensitive device I could be able to determine if the 1st experiment produced odd results.

The response time for my sensor was quicker than the mercury thermometer. However, the greater the value of the variable resistor the greater value the sensor can resolve to. So the more accurate sensor will take longer to respond.  

For this circuit to produce an output that displays the temperature in ˚C or Kelvin the potential difference recorded across the thermistor would have to be processed and shown on a display (could be analogue or digital). To do this the potential difference across the thermistor would have to be sensed by another sensor and then this information would be processed using the appropriate equation given above. This data would then be shown on a display unit given an actual value for the temperature in its units.  

...read more.

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