McLeod gauge
Encyclopedia
A McLeod gauge is a scientific instrument used to measure very low pressure
s, 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 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 Today, however, these gauges have largely been replaced by electronic vacuum gauges.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 the same nearly for all non-condensable gases.
The device can be manually operated and the scale read visually, or the process can be automated in various ways. For example, a small electric motor can periodically rotate the assembly to collect a gas sample. If a fine platinum wire is in the capillary tube, its resistance indicates the height of the mercury column around it.
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. For this reason McLeod gauges continue to be used as a calibration standard for electronic gauges.
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
s, down to 10-6 Torr
Torr
The torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
. It was invented in 1874 by Herbert McLeod
Herbert McLeod
Herbert McLeod was a British chemist, noted for the invention of the McLeod gauge and for the invention of a sunshine recorder.-Biography:McLeod was born in Stoke Newington on 9 Feb 1841 and died 3 October 1923,...
(1841–1923). McLeod gauges were once commonly found attached to equipment that operates under a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
, such as a lyophilizer
Freeze drying
Freeze-drying is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport...
. 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
Mercury (element)
Mercury is a chemical element with the symbol Hg and atomic number 80. It is also known as quicksilver or hydrargyrum...
. If used incorrectly, this mercury can escape and contaminate the vacuum system attached to the gauge.
Boyle's law
Boyle's law is one of many gas laws and a special case of the ideal gas law. Boyle's law describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant within a closed system...
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 the same nearly for all non-condensable gases.
The device can be manually operated and the scale read visually, or the process can be automated in various ways. For example, a small electric motor can periodically rotate the assembly to collect a gas sample. If a fine platinum wire is in the capillary tube, its resistance indicates the height of the mercury column around it.
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. For this reason McLeod gauges continue to be used as a calibration standard for electronic gauges.
External links
- http://physics.kenyon.edu/EarlyApparatus/Pneumatics/McLeod_Gauge/McLeod_Gauge.html
- http://www.tau.ac.il/~phchlab/experiments/vacuum/Techniques_of_high_vacuum/Vacuum5.html