5.2.1 Measuring ranges
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 $p$ < 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.
Figure 5.8: Pressure measurement ranges and measurement principles
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.