How is thermistor resistance converted to temperature?

Last updated: 2026-08-19

The controller measures the thermistor resistance and converts it to temperature using the thermistor calibration coefficients. The conversion is not linear: an NTC thermistor normally has higher resistance at lower temperatures and lower resistance at higher temperatures.

Koheron digital temperature controllers use the Steinhart-Hart equation:

$$ \frac{1}{T} = A + B\ln(R) + C[\ln(R)]^3 $$

Here, T is the temperature in kelvin, R is the thermistor resistance, and A, B and C are coefficients for the particular thermistor.

The default coefficients are intended for a standard 10 kΩ NTC thermistor:

$$ A = 0.001125308852122 $$ $$ B = 0.000234711863267 $$ $$ C = 0.000000085663516 $$ $$

On controllers supporting the ntccal command, these coefficients can be changed to match another thermistor. Refer to the relevant user guide for the command syntax.

Using incorrect coefficients does not change the measured resistance, but it makes the displayed and programmed temperature values inaccurate. A different thermistor can therefore be used when supported by the controller, provided its resistance stays within the controller measurement range and its correct coefficients are configured.