Instrumentation Project

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.