Aim To build and test a temperature sensor and analysing its suitability as a bath water thermometer.

* The negative temperature coefficient thermistor or NTC thermistor has a decreased resistance as temperature increases.

Deciding on which of these to use in my circuit isn't a problem because both will change with temperature change just one has its resistance changed opposite to the other in temperature change. However there are some thermistors which are not suitable for this type of temperature measurement. Many PTC thermistors are of switching type, which means that their resistance rises suddenly at a certain critical temperature. The barium titanate thermistor does this but also changes between PTC and NTC thermistors at high and low temperatures.

For this circuit I will use a simple NTC thermistor which changes variably, not suddenly, with change of temperature. This works by having the thermistor made of a semiconductor and works because raising the temperature of a semiconductor increases the number of electrons able to move about and carry charge, the more charge carriers that are available, the more current a material can conduct and so resistance decreases.

Another component I could use for this is a thermocouple. This is made by twisting two wires of differing metals (best is an antimony-bismuth thermocouple) together and having junctions, where the wires are twisted more, placed in a high heat, the following junction cold and this produces a voltage output. However a big limitation of the thermocouple is that it is very difficult to make one with an error of less than one degree, which could be important in bath water, especially for the elderly or babies. For this reason I will still use an NTC thermistor.

For my system a specific circuit allowing for me to measure the voltage output and for the thermistor to affect this voltage output is needed. Two appropriate circuits are the potential divider or the Wheatstone bridge.

The first one, the potential divider, would be set up as follows;

This circuit will mean that as temperature increases the resistance of the thermistor decreases and so Vout decreases. To measure the voltage a digital multimeter could be used and the other resister will be of similar resistance of what the range of the thermistor is.

The Wheatstone bridge cicuit would look like this;

In this circuit all resistors are of equal resistance and so as in the potential divider as the temperature changes and the thermistor resistance changes so does the voltage output.

The main difference between the two is that the Wheatstone bridge helps to eliminate noise as fluctuations from the power supply affect both sidesof the bridge. Having done some preliminary testing I found the school power packs to have few fluctuations that shouldn't be worried over. Because the potential divider has less wires and is much simpler and the power pack doesn't affect the results I will use the potential divider circuit in my system.

Appartus

Water bath

Wires

2V power pack

Digital multimeter

NTC thermistor (about 47k?)

47k? resistor

Temperature probe

Data logger

Diagram