An Investigation into the Resistance of a Thermistor, its Application as a Sensor and Within a Potential Divider

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An Investigation into the Resistance of a Thermistor, its Application as a Sensor and Within a Potential Divider

In a Brewery, the conditions need to be controlled to maintain an efficient rate of reaction. If the temperature rises above 40?C, enzymes involved in the reaction become denatured. Therefore, within the reactor, there needs to be some sort of temperature sensor which signals a warning when the temperature is greater than this.

Resistance simply means the "opposition to the flow of electric current." Resistance is important because it has a controlling effect on the amount of current which flows with an applied voltage. Materials that have few free electrons and so do not allow current to flow through them have a very high resistance are called "Insulators". Materials which have free electrons are lower in resistance and are called "Conductors"...current flows freely through them...and there are of course materials in between the two extremes. Different materials have their own respective resistance e.g. Copper has a very low resistance whereas perspex or pure water has quite a high resistance.

First of all, we need a component which changes its conductance as the temperature changes. Almost every common conducting material does change its conductance somewhat as its temperature varies. However, some make greater changes for a smaller degree of change. (ie. they have a high resolution)

Most thermistors are made of semiconductor metal oxides whose resistance decreases very rapidly with temperature. These are called negative temperature coefficient (ntc) thermistors. As the temperature rises, more charge carriers are available and the conductance increases. Due to the larger resistance change with temperature of an ntc thermistor, they are usually more suitable than positive temperature coefficient thermistors. However as mentioned above, the resistance change is non-linear which is a drawback as it means that calibration is required. Standard thermistors operate from -55 °C to 150 °C although some have been shown to operate up to 300 °C.

I have decided to use a thermistor in this investigation due to its stability, resolution/sensitivity, suitable range, consistency and low cost. As a medium in which to test the thermistor's behaviour, I have chosen distilled water - It is accessible, has a low viscosity, and a high specific heat capacity. The specific heat capacity of the medium needs to be a lot larger than thermistor's so that the temperature of the medium remains constant when the thermistor is immersed.

So now in order to make use of our thermistor, we need to calibrate it i.e. we need to investigate exactly how the resistance alters as the temperature changes...

A 250ml beaker was filled with distilled water and placed on a tripod and gauze. A thermistor (stock code 256-095) was insulated with duck tape and taped to a (0-100?C, mercury) thermometer so that the bulb of the thermometer was about one centimetre away from the component. This was connected to a multi-meter (set to record resistance) and suspended in the water so that the thermometer bulb and thermistor were below the surface of the water. A Bunsen burner was used to heat the water and for every 5°C increase in temperature (as observed on the thermometer),, the resistance of the thermistor was recorded (between 25°C and 80°C). At 80°C, the Bunsen burner was switched off and the water was allowed to cool, again measuring the resistance at the same temperatures as before (but this time as the temperature decreased).
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Safety Issues

Goggles were worn for safety. Another safety precaution was the use of distilled water. Because electrical components were being submerged in the water, pure water needed to be used as any impurities in water lower its resistance and create a possibility of water shorting and/or electrocution. Extreme care needed to be taken with hot water and mercury thermometers.

A couple of important things which were not necessarily obvious were ensured in order to keep the results fair and accurate. One was to keep the thermistor as close to the thermometer's bulb as possible. This ...

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