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For my sensor project coursework I will be investigating a thermistor.

Extracts from this document...

Introduction

   For my sensor project coursework I will be investigating a thermistor.

   Gardeners in Britain use greenhouses so that they can grow their plants in a warmer environment than that outside the greenhouse. This is to try to protect the plants from frost and animals, whilst the seeds germinate and grow, and plants are stored. After this period, plants can be put into the garden. There can be problems with greenhouses though. One example of a problem would be in the height of summer when an unventilated greenhouse can reach internal temperatures that start to kill the plants. Another problem can arise in late autumn, winter and early spring when the internal temperature of the greenhouse will decrease, also causing the photosynthesis reaction undertaken by plants to slow down, and even stop causing plants to die.

I have been asked by a major British plant growing company to design a sensor that would be able to measure when the temperature is too cold inside the greenhouse for plants to survive and also when it becomes too hot for plants to survive.

The specification that they have given me is as follows:

  • The temperature sensor should be able to detect when the temperature drops bellow 20°C and when the temperature rises above 30°C.

   The optimum temperature for growing plants is about 25°C. The company have a series

...read more.

Middle

The results that I have collected are as follows:

Temperature in °C

Potential Difference in Volts

5

6.90

10

6.67

15

6.15

20

5.39

25

4.73

30

4.30

35

4.00

40

3.37

45

2.95

50

2.45

image00.png

 The specification that I was given by the plant growing company stated that the sensor should be able to tell when the temperature drops below 20°C and when the temperature rises above 30°C. So I have taken results every 1°C from 18°C to 32°C so that I can give a more accurate potential difference output at 20°C and 30°C:

Temperature in °C

Potential Difference in Volts

18

5.70

19

5.54

20

5.39

21

5.30

22

5.15

23

5.05

24

4.96

25

4.73

26

4.76

27

4.57

28

4.46

29

4.37

30

4.30

31

4.24

32

4.13

image01.png

This graph shows me what potential difference the potential divider will give out when the temperature output is 20°C and 30°C, information that could be used to calibrate a circuit that can open and close the windows, and turn the heaters on inside the greenhouse.

Although the potential difference output from the lab pack transformer fluctuated, I corrected this as best as I could before every reading was taken. The potential difference was correct to one hundred millivolts of the total output. The 1000 ohm fixed resistor that I used actually was 996 ohms when I measured it after the experiment. This shouldn’t affect my results drastically though, because I used the same fixed resistor throughout my experiment and the percentage error is only 0.4% which is quite small overall, especially given that the percentage error (tolerance) of the resistor that I used could have been up to 5%, according to the manufacturer.

...read more.

Conclusion

Rt in Ω

R2/Rt

Input voltage

Output Voltage at 30°C

500

800

1300

0.6154

10.0

6.154

600

800

1400

0.5714

10.0

5.714

700

800

1500

0.5333

10.0

5.333

800

800

1600

0.5000

10.0

5.000

900

800

1700

0.4706

10.0

4.706

1000

800

1800

0.4444

10.0

4.444

1100

800

1900

0.4211

10.0

4.211

1200

800

2000

0.4000

10.0

4.000

1300

800

2100

0.3810

10.0

3.810

1400

800

2200

0.3636

10.0

3.636

1500

800

2300

0.3478

10.0

3.478

Value of Fixed

Output Voltage

Output Voltage

Difference Between Output

Resistance in Ω

 at 20°C

at 30°C

 Voltages at 30°C and 200°C

500

7.059

6.154

0.905

600

6.667

5.714

0.952

700

6.316

5.333

0.982

800

6.000

5.000

1.000

900

5.714

4.706

1.008

1000

5.455

4.444

1.010

1100

5.217

4.211

1.007

1200

5.000

4.000

1.000

1300

4.800

3.810

0.990

1400

4.615

3.636

0.979

1500

4.444

3.478

0.966

Appendix 2. Safety whilst performing the experiment.

When I perform the experiment to find what the output of the potential divider across the thermistor is, these are some things that I should be aware of.

I must be careful when I use the hot water from the kettle, the water could be hot enough, if it is boiling to burn or scald me or another student. For this reason, when I use the kettle to heat the water I shall try to plug it in as close to my experiment as possible so that I don't have to carry the kettle around and risk spilling some of the hot water.

As with any experiment I should also be very careful given that I am using water and electricity. The sensor circuit will only run on 10v and therefore shouldn’t be an electrocution hazard if it comes into contact with water, nonetheless there will be a short circuit.  I will have to be careful that water doesn’t get into the bag that the thermistor will be in, if it does my results will not be accurate. The lab pack transformer is more of an electrocution risk because it has a 230v input, so this should be kept away from the beaker of water which will also be in a tray to try to stop spillages.

Appendix 3. This is a graph showing all of the results that I have obtained showing the more temperature range that I have explored further. image02.png

...read more.

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