5.2 Application notes

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.