Katharometer

A katharometer is a thermal conductivity device for determining one gas in a binary or pseudo-binary 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 results are slower to obtain compared to a mass spectrometer, but the device is inexpensive, and has good accuracy when the gases in question are known, and it is only the proportion that must be determined.

Katharometers have been used for a long time for hydrocarbon detection in the oil industry, but they have a long history of awkward and unstable calibrations. The initial comment in this article, concerning their use in binary gas mixtures is relevant ; in normal drilling practice, 5 hydrocarbon gases, plus a couple of non-hydrocarbon gases, are expected in normal samples. The use of katharometers is strongly deprecated in the industry, though they are still used two working generations after their obsolescence.