Vacuum measuring equipment

5.2 Application notes

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5.1 Fundamentals of total pressure measurement

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When selecting and installing vacuum gauges their properties and the particularities of vacuum measurement must be taken into account:

Appropriate selection of the installation location due to potential pressure gradients occurring in vacuum chambers.
Surfaces and sealing materials exhibit outgassing. The vacuum gauge could therefore indicate a higher pressure than that in the vacuum chamber. Connection flanges should therefore be a short as possible and the number of seals reduced to a minimum.
Ionization vacuum gauges can have a pumping effect and as a result indicate a lower pressure than the actual pressure in the vacuum chamber.
Cold cathode gauges have an inherent sputter effect which is particularly pronounced when operated with heavy gases (such as argon) in the medium vacuum range. This can result in inconsistent and inaccurate readings.
When hydrocarbons are present, ionization gauges become contaminated with decomposition products of the organic molecules. In the same way as the sputter effect, readings can be distorted or inconsistent.
Switching points for ionization gauges must be selected to avoid contamination due to the phenomena described above.
Strong magnetic fields and electrical fields can impair the function of vacuum gauges. This applies particularly for ionization gauges.
To enable ultra-high vacuum to be generated, the vacuum equipment, including the vacuum gauges, must be baked out. The maximum bakeout temperature and the conditions specified in the technical data must be adhered to.

Cold cathode gauges can be easily dismantled and cleaned in the event of contamination. With other measuring principles, it is usually possible to replace the sensor. It should always be borne in mind that vacuum gauges are subject to a certain degree of wear and contamination and therefore require to be replaced every so often. The wide range of operating conditions makes it impossible to make a general recommendation for the replacement interval.

Purely mechanical dial gauges (spring pressure or Bourdon tube manometers) are used in the pressure range from 1,000 – 1 hPa; however these gauges offer only limited accuracy and can only be read directly at the point of installation. Diaphragm vacuum gauges (piezo sensors or capacitive sensors) are used to obtain more accurate measurements and for remote display of readings.

Pirani thermal conductivity vacuum gauges are used between 10 hPa and 10-4 hPa. It is also possible to use special “high-pressure” hot cathode ionization vacuum gauges at pressures

< 10^-2 hPa. Either cold cathode ionization vacuum gauges or Bayard-Alpert ionization vacuum gauges are used for pressures of less than 10^-3 hPa, the latter in the case of clean conditions and rigorous accuracy requirements. It is also a good idea to use a combination of two or more sensors to cover the entire pressure range in use.

In the case of diaphragm vacuum gauges and Pirani vacuum gauges, pressure switch points are generated in order to activate ionization vacuum gauges only if the pressure is sufficiently low, thus protecting them against contamination or burn-out of the heated cathode. As a result we also supply combination sensors with automatic switchover, which are described below.