A vacuum exists when pressure in a given space is below atmospheric pressure meaning that it is effectively devoid of matter. Pressure is measured as force per unit area or in mbars but when the mbar is below 1 it is not so easy to measure pressure using that equation. However, the gas in a vacuum will have thermal conductivity and this can be measured likewise ions moving between electrodes. Measuring pressure in a vacuum is undertaken in a variety of ways depending on the required information. There are different gauges available for different levels of pressure which can go as low as less than 10-12 mbar. Some gauges measure absolute pressure while others measure differential or relative pressure.
Relative pressure gauges: Bourdon and Diaphragm gauges
The Bourdon gauge is a simple relative measuring gauge which measures the difference between the normal atmospheric pressure and the pressure inside the vacuum. The difference is indicated by a calibrated needle. A diaphragm gauge is a similar instrument which is placed between the two pressures, the diaphragm is deformed on one side as a result of the pressure difference which is then measured.
Absolute pressure gauges: Heat loss and Ionization vacuum gauges
Heat loss vacuum gauges are mostly used for medium vacuums by measuring the thermal conductivity of the gas with the assumption that it is a result of the density of the gas particles and therefore the pressure. However they need to be calibrated for different gases.
Ionization vacuum gauges measure high and ultra-high vacuums through the use of a cathode which neutralises them. The current is then measured as an indication of absolute pressure. Vacuum equipment exists in many types of industries and the tools and equipment used to measure them is continuing to evolve in design and efficiency.
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