Vacuum Gage
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Vacuum Gage
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An instrument for measuring pressure below atmospheric
pressure. Some of the more common types of vacuum gages listed in order of
descending pressure range of use are:
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(a) Manometer,
usually consists of a column of liquid supported by the pressure to be
measured, the determination of which is a matter of measuring the column
height.
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(b) Thermal
conductivity gage, consisting of a heated surface. The heat transported by
the gas molecules from the surface is related to gas pressure. The heat
transfer is reflected in changes in surface temperature (or in the heating
power required to maintain constant temperature).
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Various types of thermal conductivity gages are distinguished according
to the method of indicating the surface temperature. The most common types are
Pirani
gage and thermocouple
gage.
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(c) Knudsen
gage, which measures pressure in terms of the net rate of transfer of
momentum by molecules between two surface maintained at different temperatures
and separated by a distance smaller than the mean free path of the gas
molecule. Also called radiometer vacuum gage.
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(d) McLeod
gage, in which a known volume of gas, at the pressure to be measured, is
compressed by the movement of a liquid column to a much smaller known volume,
at which the resulting higher pressure is measured.
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(e) Ionization
gage, comprising a means of ionizing the gas molecules and a means of
correlating the number and type of ions produced with the pressure of the gas.
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Various types of ionization gages are distinguished according to the
method of producing the ionization. The common types are hot-cathode
ionization gage, cold-cathode
ionization gage, radioactive
ionization gage.
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References
This article is based on NASA's Dictionary of Technical Terms for Aerospace Use
