The 120HD analyzer functions as a katharometer. It operates by having two parallel tubes both of which contain 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 contains moisture-saturated air and the mixture to be tested is passed through the other channel. Hydrogen has a much higher thermal conductivity than air and so the Model 120 HD is very sensitive to hydrogen, but it will also respond to any gas whose thermal conductivity is different from that of air.