A McLeod gauge is a scientific instrument to measure very low pressures, down to 10-6 mbar. It was invented in 1874 by Herbert G. McLeod (1841-1923). 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 Today, these gauges have largely been replaced by electronic vacuum gauges.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 range of this vacuum gauge is 0.1- 0.00005 mm/Hg