McLeod gauge

A McLeod gauge is a scientific instrument used to measure very low pressures, down to 10−6 Torr (0.133 mPa).

[1] McLeod gauges were once commonly found attached to equipment that operates under vacuum, such as a lyophilizer.

McLeod gauges operate by taking in a sample volume of gas from a vacuum chamber, then compressing it by tilting and infilling with mercury.

The device can be manually operated and the scale read visually, or the process can be automated in various ways.

Modern electronic vacuum gauges are simpler to use, less fragile, and do not present a mercury hazard, but their reading is highly dependent on the chemical nature of the gas being measured, and their calibration is unstable.