A katharometer is an instrument for determining the composition of a gas mixture.

It functions by having two parallel tubes both containing gas and heating coils. The gases are examined by comparing the rate of loss of heat from the heating coils into the gas, the coils are arranged in a bridge circuit so that resistance changes due to unequal cooling can be measured. One channel normally holds a reference gas and the mixture to be tested is passed through the other channel.

The thermal conductivity of a gas is inversely related to its molecular weight. Hydrogen has approximately six times the conductivity of nitrogen for example.

Katharometers are used medically in lung function testing equipment and in gas chromatography. The speed of results is slow compared with a mass spectrometer, but the device cost is low and accuracy good.