Here's what Ametek says:
"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 the 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 120HD is very sensitive to it, but it will also respond to any gas whose thermal conductivity is different from that of air."
Used, but very clean and in good cosmetic condition. Powers up (requires a screw-terminal 6VDC battery or equivalent) and I was easily able to set the CURRENT ADJUST to 120.0.
It was impossible to get consistent readings through numerous tests. It appears there may be a fault or short in the main board or selector switch. To be honest, this type of tech is above my pay grade so it's being offered 'AS-IS' at a greatly reduced price.