MacLeod gauge

A MacLeod gauge is a scientific instrument to measure very low pressures. They were once commonly found attached to equipment that operates under a vacuum, such as a lyophilizer. Today, these gauges have largely been replaced by electronic vacuum gauges.

The design of a MacLeod gauge is somewhat similar to a that of a manometer. Typically they are filled with mercury. If used incorrectly, this mercury can escape and contaminate the vacuum system attached to the gauge.