Test how the sensor I have chosen reacts to an external change and compare the properties of three different sensors.

Sensor Project

Preliminary Work

My aim is to test how the sensor I have chosen reacts to an external change and compare the properties of three different sensors. The sensors I have chosen to investigate are thermistors named sensor A, sensor B and sensor C. To compare the 3 sensors I shall be primarily looking at their sensitivity and what purpose each sensor would be best suited to after establishing this.

To do this I will have to set up a circuit shown above to produce a set of results calculating the resistance of the given sensor using the formula R = V/I as it varies with temperature.

My circuit consists of two 1.5V batteries in series with an ammeter (which will give me my current reading), thermistor and a variable resistor, which I have placed in the circuit to act as a potential divider. The use of a potential divider is to tap off a certain voltage so that the voltage I currently use does not over heat and potentially damage the sensors. This also maintains a better degree of accuracy in my results as preventing over heating in the circuit would not affect the thermistors resistance. I have also positioned a voltmeter is parallel with the batteries to show the voltage the batteries are producing and also a voltmeter in parallel with the thermistor to test the voltage over the sensor.

For my method I firstly set up the circuit as shown above and also set up a beaker, and a thermometer. I will boil 200ml of distilled water in a kettle and poured it into the beaker. I will then place the thermistors into the boiled water making sure that the positive and negative wires do not touch as this will cause a short circuit and my readings will be completely inaccurate. The distilled water was essential in obtaining accurate results as unlike distilled water, normal tap water contains ions, which conduct electricity and could create errors in my results. As I did not know the thermal capacity of my sensors I did not want to overload them, therefore I decided not to expose them to any temperatures greater than 100°C. Any temperatures over 100°C would create more of a safety hazard and danger from the water bubbling over the beaker. I found that after boiling the water in the kettle and pouring it into the beaker, the temperature of the water from 100°C had dropped considerably. It became very hard to sustain a temperature of 100°C as the water cooled so quickly so I started my results at 85°C, as this would give me a good range of temperatures to test and also give my sensor time to adjust to the temperature and for the readings to stop fluctuating on the meters. I also measured the temperature of the water a room temperature and found it to be around 35°C; this then gave me my full range of temperatures. As I am reading the temperatures by eye this gives my results a certain degree of inaccuracy, also the thermometer was clearly marked every 5°C so because of these factors I established that it would be best to take my readings every 5°C. After this I did a few mock experiments to determine the resistance I should set my variable resistor at so that it suited all my sensors well. The value of the variable resistor is crucial in determining the sensitivity of this circuit in sensing temperature. This is because it is a potential divider and the potential difference has been divided up between the two resistors in the ratio of their resistances. I found that by setting it at 1Kohm it gave all three sensors a readable current and voltage. By keeping the potential divider the same for all three sensors this meant that the sensitivity of the sensors could still be compared fairly and accurately. During my mock experiment I began by using a 10amp ammeter but found that sensor B and C needed a more sensitive ammeter as only a very small current flowed through the circuit. I thus used a multimeter that measured my current in milliamps. This meant that my results should be much accurate but I would have to convert my voltage readings into millivolts to get the correct resistance. To gain the best and most accurate results possible I intend on repeating my experiment three times for each thermistors. I will then average these out in the hope that the amount of systematic errors will reduce.