eli5: How do electronic thermometers work?


I understand mercury and mechanical thermometers. How do electronic thermometers, like the one in my car, measure temperature?

In: 55

The electrical resistance of metal changes with temperature. So there’s a circuit that measures the resistance of the metal probe and converts it to a temperature.

There are many “electronic thermometers”, e.g.:

– Resistance based (RTDs, Thermistors): the resistance of effectively every material depends on the temperature. Measure it to deduce the temperature. Thermistors are usually semi-conductor based, while RTDs are classical metal wires.
– Thermocouples: interfacing two different materials sometimes causes a voltage. This is most pronounced with doped semi-conductors, but there are many variants. Measure this (pretty small) voltage with a carefully designed amplifier to deduce the temperature.
– Thermal emission: not directly electronic, but everything emits thermal light radiation, usually from radio waves and IR to visible light and UV. Focus on one “color” (wavelength), measure the amount of light of this color something emits, and you can maybe deduce the temperature. “Maybe”, because you still need to know the absorptive behaviour the object has in that color; you can either measure it as well using an IR diode, or use a table of known values.
– Several more. For example you can measure the drop-off voltage of a diode, which depends on temperature as well.

Resistance changes with temperature, most things get higher resistance the hotter they get, but this isn’t the case with the types of sensors used in cars.
They are almost always NTC, negative thermal co-efficient, so higher resistance is lower temperatures.
The electronics measure the resistance, and look up a temperature value to display.

A faulty or missing sensor will often show as infinite resistance / open circuit, and the scaling of the sensors often means it’ll be interpreted as about -40C / -40F. If you see a dash readout saying it’s -40 outside, your sensor might have fallen off. It’s a similar thing for the engine sensors, a fault one can think it is freezing outside, have a high idle and not run very well.

Material’s electric properties change with temperature. There are some which change in very steady and predictable way, we can measure these changes from which we can derive the temperature from (generally change in resistance, however you can use whatever property you choose and can measure).

Kinda like you can tell temperature by putting two dissimilar metals together. As it bends with thermal expansion you can figure the temperature. Put a magnet to end of it, measure the magnetism, and you have rudimentary magnet based thermometer. Or you can measure the current caused by that magnetic field moving as the rod bends.

As long as you know the min, max, some stable points in between, you can derive the temperatures from many properties