Thermistors are commonly used as temperature sensors, taking advantage of the fact that their resistance changes significantly depending on temperature. The characteristics of thermistors can also be used to measure physical quantities other than temperature.
When current is applied to a thermistor, the thermistor itself also generates heat (called self-heating).
When measuring temperatures, since self-heating makes it impossible to accurately measure the temperature of the measured object, a very small current is applied to the device so that the effect of self-heating is negligible.
When measuring physical quantities other than temperature, conversely, the thermistor is intentionally made to self-heat and the difference in the way this heat escapes is used for detection.
When the physical quantity to be measured changes around the detection thermistor sensor, the way heat escapes changes. In this case, a temperature difference arises between the self-heated compensating and detection thermistors, and the physical quantity can be determined from the difference in the two resistance values.
For example, when the same thermistor is placed in a dry air and a humid air room, the humid air has greater heat transfer (thermal conductivity), so it removes more heat from the thermistor than the dry air, resulting in a lower thermistor temperature (higher resistance value). The difference in this change gives us the humidity (a physical quantity other than temperature).