Investigation into Temperature Sensors

Investigation Into Temperature Sensors

Introduction

In this project I will be investigating how it is possible to use a temperature sensor to keep a greenhouse from changing temperature too much. This is intended to help plants live and grow in their optimum temperature. This will create a perfect temperature for the enzymes in the plants to work in, and therefore resulting in a maximum growth/production rate. This could be useful for gardeners who wish to grow plants as quickly as possible or maximise profits by maximising the product yield from their plants (for example; fruit, vegetables etc.) The aim is to set up a circuit that will automatically react to a change in temperature by allowing a heater (or similar device) to operate until the temperature is restored to the original, optimum temperature.

Preliminary Work

First of all, I will test a few sensors and analyse the results to decide on which on one is best for this particular experiment. The best sensor to use will be the one with the largest range of resistance over the temperature range in consideration, as this will mean more accuracy. This is because a greater range in resistance will result in a greater sensitivity; a greater difference in voltage per degree temperature change. There are 3 different sensors available to me, and I will simply address these as Sensor 1, Sensor 2 and Sensor 3. I need to measure the range of resistance that each sensor offers, from roughly 0ÐŽÐ-C to 100ÐŽÐ-C. However, the equipment available to me is very basic, and will only include a kettle, a supply of ice and a thermometer. Therefore, it will not be possible to obtain a temperature of 0ÐŽÐ-C or 100ÐŽÐ-C, as the temperature will never quite be 100ÐŽÐ-C as the water will cool down very quickly in room temperature, not allowing enough time to place the sensor in a sample of it. Also, there is likely to be a delay in the time the sensor will take to change its resistance, as it will need some time to adjust from room temperature to the extreme temperatures (close to 0ÐŽÐ-C and 100ÐŽÐ-C). It will also be unlikely to obtain a water sample of 0ÐŽÐ-C as the method of simply adding ice-cubes will never decrease the temperature of water all the way to 0ÐŽÐ-C. Therefore the temperature will be slightly above 0ÐŽÐ-C and slightly below 100ÐŽÐ-C.

As I am finding the range of resistance in each sensor, it would make sense to take a reading from both extremes. This would be quicker than taking a range of readings for each sensor, but still allow me to analyse the results clearly and easily decide on the more suited sensor for this experiment. Therefore I will take a sample of water straight from the kettle, and take a reading from all 3 sensors. I will do the same for cold water; however I will allow time for the ice-cubes to decrease the temperature of the water as low as it will go. The exact temperature will not matter too much, as all 3 sensors will be giving a reading from it at the same temperature, and this makes the comparison fair.

To measure the resistance I will use the following circuit:

Results:-

Sensor 1 Sensor 2 Sensor 2

Temperature (ÐŽÐ-C) Resistance (§Ð©) Conductance (S) Resistance (§Ð©) Conductance (S) Resistance (§Ð©) Conductance (S)

4 8300 0.0001205 7700 0.000129 52500 0.0000191

83 600 0.00167 400 0.0025 4400 0.000227

This results table shows the temperature of the water sample and the resistance that each sensor gave. It also includes the conductance of each sensor for each value of temperature. The conductance is simply worked out using the following equation:

Conductance (G) =

As it is 1 divided by the resistance, it is therefore simply the inverse of the resistance, which is true because as the resistance of something increases, the conductivity of it increases proportionally. The reason for working out the conductivity in the table is so that I can plot a graph to show conductance against temperature. This is the best graph to plot as it will clearly show the range of each sensorЎЇs resistance, as the conductance changes more with every degree change in temperature. This therefore produced a clearer graph than that of resistance against temperature. The graph is displayed on the next page. The gradient of the lines represent , and on this graph conductance is represented by y, and temperature is represented by x. This means that the gradient represents how much the conductance changes per degree in temperature change. Therefore a steep line would represent a greater range in conductance than a shallow line. The graph shows that sensor 2 has the greatest range, as it has the steepest gradient. Therefore I will use sensor 2 in my main experiment, to maximise the accuracy of my main results.

Main Experiment

I am looking to build a potential divider:

I first of all need to find out what voltage to use throughout the experiment. As the voltage of the whole circuit will affect the voltage of the sensor at different temperatures, I will decide on a voltage that allows my readings to be the most accurate. The voltmeters I will be using are accurate up to 2kmV. Therefore I should try to keep the voltage across the sensor below 2V. However, I need to decide on which resistor to use, as the resistance of this will affect the voltage of the sensor. The resistor used should be roughly half the range of the resistance of the sensor being used. This will allow the voltage across the sensor to change more per degree change in temperature, therefore increasing the sensitivity of the sensor at the temperature range in consideration, and therefore increasing the overall accuracy. As I am using sensor 2, the best resistance to use will be:

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= 365§Ð©

The closest resistor available to