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: </dd>
(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. </dd>
(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). </dd>
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. </dd>
(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. </dd>
(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. </dd>
(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. </dd>
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. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use