Definitions for mcleod gauge
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a device, rather like a manometer, used to measure low pressures close to that of a vacuum, by using an application of Boyle's law
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. McLeod gauges were once commonly found attached to equipment that operates under a 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 Today, however, these gauges have largely been replaced by electronic vacuum gauges.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 a 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, and then compressing it by tilting, and infilling with mercury. 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. Boyle's law is then used to find the initial pressure from the final pressure, and the initial and final volumes. 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. It has the advantage that it is simple to use and that its calibration is nearly the same for all non-condensable gases.
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"mcleod gauge." Definitions.net. STANDS4 LLC, 2015. Web. 25 Nov. 2015. <http://www.definitions.net/definition/mcleod gauge>.