McLeod gauge is a scientific instrument used to measure very low pressures, down to 10−6 Torr. It was invented in 1874 by Herbert McLeod (1841–1923).[1] McLeod gauges were once commonly found attached to equipment that operates under vacuum, such as a lyophilizer. Today, however, these gauges have largely been replaced by electronic vacuum gauges.
The design of a McLeod gauge is somewhat similar to a that of a mercury-column manometer. Typically it is filled with mercury. If used incorrectly, this mercury can escape and contaminate the vacuum system attached to the gauge.