Vacuum gauge
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A vacuum gauge is used to measure the pressure in a vacuum, which is broadly divided into two categories: high and low vacuum (and sometimes ultra-high vacuum). Many of the different techniques used to measure these categories have an overlap at some point in the pressure range. By combining several different types of gauge it is possible measure system pressure from 10 mbar down to 10e-11 mbar. [www.vacgen.com] Introduction. Accessed 15 April 2006.
Pumps used as gauges
Turbopump
Make the output a dead end and measure current for a given rotations/s.
Membrane pump
Measure membrane displacement to measure the pressure difference between two spaces.
Ion getter pump
This gauge is named the Penning gauge. It measures the current.
Vacuum gauges not related to pumps
Liquid-column manometer
A simple mercury or other liquid manometer can be used to measure pressures from atmospheric down to a few mmHg. Care must be taken to ensure that vapour from the working liquid does not contaminate the vacuum system.- Useful range: above about 1 torr (roughly 100 Pa)
McLeod gauge
A McLeod gauge isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few mmHg. The gas must be well-behaved during its compression (it must not condense, for example). The technique is slow and unsuited to continual monitoring, but is capable of good accuracy.- Useful range: above 10-4 torr [Techniques of high vacuum] (roughly 10-2 Pa)
Diaphragm gauge
The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere or part of an evacuated unit. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.Capacitance Manometer
Pirani gauge
A Pirani gauge consists of a metal wire open to the pressure being measured. The wire is heated by a current flowing through it and cooled by the gas surrounding it. If the gas pressure is reduced, the cooling effect will decrease, hence the equilibrium temperature of the wire will increase. The resistance of the wire is a function of its temperature: by measuring the voltage across the wire and the current flowing through it, the resistance (and so the gas pressure) can be determined.Thermocouple gauges and thermistor gauges work in a similar manner, except a thermocouple or thermistor is used to measure the temperature of the wire.
- Useful range: 10-3 - 10 torr [VG Scienta] (roughly 10-1 - 1000 Pa)
Ionization gauges
In ionization gauges, collisions between gas atoms and electrons generated by thermionic emission generate positive ions. The ions are attracted to a suitably biased electrode known as the collector. The current in the collector is proportional to the rate of ionization, which is a function of the pressure in the system. Hence, measuring the collector current gives the gas pressure. There are several sub-types of ionization gauge.- Useful range: 10-10 - 10-3 torr (roughly 10-8 - 10-1 Pa)
See also
- pressure gauge, includes vacuum gauges
External links
- [www.vacuum-guide.com] - vacuum gauge manufacturers
References
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