Calibrating a thermistor thermostat

Practical method

Aim

Calibrate a thermistor in a potential divider as a thermometer and use it to set a thermostat.

Variables

Independent variable: temperature
Dependent variable: output p.d. of the potential divider
Control variables: supply p.d.; fixed resistor value.

Apparatus

• NTC thermistor
• Fixed resistor (or variable resistor)
• 6.0 V supply
• Voltmeter
• Beaker of water and kettle/ice
• Thermometer
• Stirrer

Method

1. Build a potential divider: thermistor and fixed resistor in series across the supply, voltmeter across the fixed resistor.
2. Place the thermistor and a thermometer in water; heat or cool it and stir.
3. Record the output p.d. at temperatures from 0 °C to 80 °C.
4. Plot a calibration curve of output p.d. against temperature.
5. To make a thermostat, choose the fixed resistor so the output crosses a switching level (for example 3.0 V) at the required temperature.

Sample results

6.0 V supply, fixed resistor 2.2 kΩ, output across the fixed resistor.

Analysis

As temperature rises the thermistor's resistance falls, so a larger share of the 6.0 V appears across the fixed resistor. The calibration curve is non-linear; it is most sensitive near 40 °C, where the thermistor resistance equals the fixed resistor.

Uncertainty

The thermometer reads to ±0.5 °C and the thermistor lags behind the water temperature; stirring and waiting for a steady reading reduce the lag.

Errors and improvements

Risk assessment

Common mistakes

• Measuring across the wrong component.
• Assuming the calibration is linear.
• Not waiting for thermal equilibrium.

Worked analysis

At 40 °C the thermistor is 2.2 kΩ. \(V_{\text{out}}\) = \(\dfrac{6.0 \times 2.2}{2.2 + 2.2}\) = 3.0 V across the fixed resistor.

Common misconception

An NTC thermistor's resistance decreases as temperature increases; the output rises across the fixed resistor, not across the thermistor.