McLeod gauge

A 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). 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 Today, however, these gauges have largely been replaced by electronic vacuum gauges. A 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). 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 Today, however, these gauges have largely been replaced by electronic vacuum gauges.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 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. 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 pressure in this smaller volume is then measured by a mercury manometer, and knowing the compression ratio (the ratio of the initial and final volumes), the pressure of the original vacuum can be determined by applying Boyle's law. This method is fairly accurate for non-condensible gases, such as oxygen and nitrogen. However, condensible gases, such as water vapour, ammonia, carbon dioxide, and pump-oil vapors may be in gaseous form in the low pressure of the vacuum chamber, but will condense when compressed by the McLeod gauge. The result is an erroneous reading, showing a pressure much lower than actually present. A cold trap may be used in conjunction with a McLeod gauge to condense these vapors before they enter the gauge.