Thermistors
A Thermistors is a small electrical component that measures temperature. It is a semi-conductor so when the Temperature increases the resistance drops. Thermistors can be used in a potential divider circuit to create different voltage outputs.
Ohm’s Law
Ohm’s Law is a law that connects V (voltage), I (current) and R (resistance). This means that if you have any two of these you can calculate the 3rd.
Prediction
I think that there will be an inversely proportional connection between the Temperature and the Voltage. So as the water temperature increases the voltage recorded will be lower. I believe this will happen because as the water temperature rises the resistance of the thermistor drops. If the supply voltage stays the same, and the current stays the same in the series circuit then the voltage must change as the resistance changes.
Preliminary Experiments
I conducted a preliminary experiment so that I could make adjustments and changes to the final experiment to give better results, creating a more accurate measurement.
Below are my preliminary results:
Supply voltage = 3.03 volts
Resistance = 500 ohm’s
Things to change
- I will use the higher supply voltage of 5 volts to make my data more accurate
- I will increase the resistance of the resistor in series with the thermister to 10’000 ohm’s (10 K ohm’s) this should increase the resolution of my results.
- I will make sure that the thermistor is placed at the top of the beaker containing the water so that the highest temp of the water is recorded.
- I will repeat each temperature 5 times so that I will have a clearer average and any strange results can be noticed.
-
I will measure between 30 and 65 oc and will increase the temperature in multiples of 5, i.e. 30,35,40,45…etc
Circuit Diagram
Safety
I will follow the following safety point so that I do not injure others or myself.
- I will make sure that my circuit is safe before I connect the power to it.
- I will be careful when using the hot water, so that I do not scald my skin
- I will keep the voltage of the power pack low so that I can not be electrocuted
- Make sure that the water I am using does not connect with the electrics
Fair test
It will be very important during my experiment that I keep the conditions of the experiment the same so that my results are correct.
Below is a list of points that helped my test to be fair:
- I used the same voltage throughout the experiment
- I made sure that the thermistor was at the same depth in the water for each reading.
- I made sure that the resistance of the resistor in the series circuit remained the same.
- I will use the same power amount of water for each reading
Method
To conduct the experiment I shall follow the sequence of events below:
- Create the circuit as shown in my diagram, check that the voltage output is roughly 5 volts and make sure that the resistor in series with the thermistor is 10’000 ohm’s.
- Make sure that the multimeter's I am using are correctly connected and on the correct setting.
- To check that the circuit is working correctly use some cold and hot water and check for a voltage change
- If the circuit is working correctly, will begin my experiment
-
I will heat the water in a kettle to above 70 oc then pour 500ml of the water into a beaker
- I will place the thermistor into the hot water and make sure it is fully submerged at the top of the water.
- I will use a glass thermometer to measure the water temperature and record the results at the 8 different temperatures.
- I will repeat this 5 times as required.
- Measure the response time resolution and sensitivity of my readings by timing the time taken for the resistance to change and how accurate it will measure.
Results
- Resistance of the resistor in series= 10’000 Ohm’s
- Amount of water= 500ml
- Supply voltage= 5.05 volts
Graphs
This is a computer-generated graph, see attached hand drawn graphs.
Sensitivity
The sensitivity of the measurement is the ratio of change in temp to the change in voltage. So for every degree change there should be an average voltage change
The average Change (Sensitivity) is = 0.023371 per degree
Resolution
Resolution is the smallest change that the circuit can detect. This determines how accurate the reading is.
50oC = 0.60 v 52 oC = 0.56
Response time
Response time is the time it takes for my circuit to respond to the change in temperature then give the output. I had a lot of difficulty calculating the response time of my circuit. This was because the response time of my circuit was much quicker than any thermometer I had. So it was detecting the change almost instantly, where as the glass thermometer was taking up to a few seconds. This meant that I had nothing to compare my circuit’s readings to.
Conclusion
Now that I have finished my experiment and collected and analysed my results I will now sum up what I have recorded. My results have shown that I predicted correctly and that the temperature is inversely proportional to the voltage across the thermistor.
This proves the theory that a thermistor is a semi-conductor, and that it behaves different from conductors. When the temperature of the thermistor increases, the current increases and the resistance drops.
