Grades 6–8 · Everyday Life
How does an electronic thermometer turn warmth into a number?
The number on an electronic thermometer begins with a physical change in its probe. One common sensor is an NTC thermistor: a material whose electrical resistance decreases as its temperature rises. The probe exchanges heat with its surroundings, so its sensor temperature changes over time.
In the diagramA thermistor turns warmth into a resistance changeInvented calibration points • NTC resistance falls30 °CProbe exchanges heat3 V supply10 kΩ1.24 V7 kΩResistance → electrical reading → calibrated numberOnly the shown example uses these calibration values.
Electronics measure the thermistor using a small electrical signal. In a simple voltage-divider example, it shares a supply voltage with a fixed resistor. A change in the thermistor’s resistance changes the measured voltage. A converter turns that voltage into a digital reading, and a calibrated relationship translates it into temperature.
The invented table gives resistances of 10, 7 and 5 kilohms at 20, 30 and 40 degrees Celsius. With the shown 3 V divider, these correspond to 1.5, about 1.24 and 1.0 V. Real sensors have their own nonlinear calibration curves and need time to exchange heat. Some thermometers use other sensing principles. The display reports an interpreted sensor measurement, rather than measuring temperature independently of its probe.
About this explanation
Starting knowledge: Recognize resistance and that objects exchange heat.
Sources
- Texas Instruments — Temperature sensing with NTC thermistor circuit
- Texas Instruments — Temperature Sensing with Thermistors