The McLeod gauge tool is a scientific instrument used to measure very low pressures, up to 10 -6 Torr. It was discovered in 1874 by Herbert McLeod (1841-1923). McLeod gauges were once commonly found attached to appliances that operate under vacuum, such as lyophilizer. Today, however, these gauges have largely been replaced by electronic vacuum gauges.
The design of the McLeod gauge is somewhat similar to a mercury-column manometer. Usually filled with mercury. If used incorrectly, this mercury can escape and contaminate the vacuum system attached to the measuring instrument.
The McLeod meter operates by taking samples of gas volumes from a vacuum, then compressing them by tilting and pumping with mercury. The pressure in this smaller volume is then measured by a mercury manometer, and knowing the compression ratio (initial and final volume ratio), the pressure of the original vacuum can be determined by applying Boyle's law.
This method is quite accurate for non-condensable gases, such as oxygen and nitrogen. However, condensed gases, such as water vapor, ammonia, carbon dioxide, and steam pump oil can be gaseous in low pressure from the vacuum chamber, but will condense when compressed by the McLeod gauge. The result is a false reading, indicating a much lower pressure than the actual one. The cold trap can be used together with the McLeod gauge to condense this vapor before they enter the meter.
The McLeod meter has an easy-to-use and almost equal calibration for all non-condensable gases. The device can be operated manually and the scale is read visually, or the process can be automated in various ways. For example, a small electric motor can periodically rotate the assembly to collect gas samples. If a fine platinum wire is in a capillary tube, its durability indicates the height of the surrounding mercury column.
Modern electronic vacuum gauges are easier to use, less brittle, and do not present the dangers of mercury, but the readings are heavily dependent on the chemical properties of the gas being measured, and their calibrations are unstable. For this reason, the McLeod meter continues to be used as a calibration standard for electronic measuring devices.
Video McLeod gauge
See also
- Measuring pointer
- Pirani gauge
Maps McLeod gauge
References
External links
- http://physics.kenyon.edu/EarlyApparatus/Pneumatics/McLeod_Gauge/McLeod_Gauge.html
- https://web.archive.org/web/20060504124236/http://www.tau.ac.il/~phchlab/experiments/vacuum/Techniques_of_high_vacuum/Vacuum5.html
Source of the article : Wikipedia
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