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

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Introduction

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. ...read more.

Middle

However this method is not very practical. In the given situation it would be better to use a potential divider. A potential divider is an electronic arrangement which can be used to convert the physical stimulus (temperature) and convert it into an electrical signal (the voltage output). The output signal can then be used to power an output device such as a cooler (producing a feedback system) or an alarm to let an employee know if the temperature is too high. I have investigated such a system, using a potential divider... Introduction to Potential dividers If two resistors of equal value are placed in series within a circuit, the potential difference across one of them will be half the potential difference across both. If the value of one resistor increases so that it is twice the size as the other, the potential difference across it will be two thirds that of the total voltage input. This setup is shown below: Thus a formula can be deduced: Vout = Vin � R2/(R1 + R2) As the temperature increases, the resistance of R2 decreases and, in turn, the voltage across it decreases. However, if we measure the potential difference across the base resistor rather than the thermistor, this will change our results to show a positive correlation - As the temperature increases, the measured voltage also increases. Potential Dividers in More Detail Although seemingly obvious, it should be mentioned that the circuit will be most sensitive if the base resistor's value is the same as the thermistor in its working conditions. This will give the greatest increase in Vout for the smallest difference in resistance/temperature. ...read more.

Conclusion

I also made sure that the thermometer and thermistor were very close to each other. However there still may have been some slight differences in temperature between the point which was being measured by the thermometer and the thermistor. Therefore, next time it would be a good idea to stir the water constantly and perhaps to use a more thermally conductive medium. * Self heating effects The thermistor may have been releasing some energy also which it will have then sensed, and will have affected the thermistor's resistance. * Apparatus errors The thermometer has a ? 0.03% error margin and the thermistor has a ? 10% error margin. This gives a total maximum % error of 10.03% which is very high. The accuracy and stability of the multimeter is an additional consideration to add to this figure. Such a high % error margin is likely to have been the most influential source of inaccuracies in the results. * The Resistance of wires will have had an affect on the readings but it is so minimal that this is negligible Random error was evident, this is when the results are never the 'true' value but fluctuate around a value. This is why an average was calculated of the results. Conclusion We have seen how resistors can be important when used within potential dividers which are used for sensing. There are many special resistors whose conductance varies when a physical stimulus changes. One such example is a thermistor. This can be used within a potential divider to convert the physical stimulus into an electrical signal. This output voltage can be used, for example, to power a warning alarm if the temperature in a brewery exceeds 40?C. Chris Schulz 12CW 02/05/2007 Page 1 of 8 ...read more.

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