Instrumentation Project :: Papers

Instrumentation Project The aim of this experiment was to calibrate a thermistor and having done this, to estimate my body temperature. [IMAGE]This is a diagram to show the circuit that we created in order to complete the aim. The thermistor that I used was a positive coefficient thermistor, meaning that the resistance increases as the temperature increases, this then leads to an increase in voltage. This circuit has created a potential divider. Two resistors in series divide the voltage across a circuit and form a potential divider. The output voltage is proportional to the input voltage, which is determined by the resistance. This is because of the following equation: V out = Vs x R1 (R1 + R2) This can be shown in the following examples, where the voltage supply is 5V and the fixed resistor (R2) is 100 ohms, R1 varies. In the first instance, it is 70 Ohms and in the second example R1 is 80 Ohms: 1) V out = 5 x 70 = 5 x 70 = 2.055 2) V out = 70 + 100 170 2) V out = 5 x 80 = 5 x 80 = 2.22 80 + 100 180 As you can see from these examples, the voltage has increased with an increase in resistance because the fractions were getting larger each time. The values that I used were realistic ones that appeared during my experiment. A resistor of 100 Ohms was one of the three that I chose to use and 70 Ohms was around the resistance of the thermistor at room temperature. When performing this experiment, I chose to put the voltmeter across the thermistor because it would give me an increasing voltage with an increasing temperature. If I had put the voltmeter across the fixed resistor, an inverse relationship would have been formed, which would have made analysis of data and finding my body temperature difficult. I also chose to use three different resistances on the sub box, which were 47, 100 and 200 Ohms.