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 Today, these gauges have largely been replaced by electronic vacuum gauges.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.

McLeod gages operate by taking in a sample volume of gas from a vacuum chamber, and then compressing it by tilting, and infilling with mercury. The volume of gas is thereby compressed. The pressure in this smaller volume is then measured by a mercury manometer, and, by knowing the compression ratio, the pressure of the original vacuum can be determined.

This method is fairly accurate for non-condensible gases, such as oxygen and nitrogen. However, with condensible gases, such as water vapour, the water may be in gaseous form in the low pressure of the vacuum chamber, but will condense when compressed by the McLeod gage. The result is an erroneous reading, showing a pressure much lower than actually present.

For this reason, and because modern vacuum gages are simpler and more accurate, McLeod gages are largely obsolete. They are also fragile devices that use substantial amounts of liquid mercury and are therefore hazardous instruments. If you find a McLeod gage it is best to take it to a hazardous waste disposal site for proper recycling.