Combination vacuum gauges are composed of multiple vacuum sensors each of which uses a distinct physical means for determining the vacuum within the gauge or the chamber connected to the gauge. Combination vacuum gauges allow users to measure a wider range of pressures than vacuum gauges having only a single sensor because combination gauges use different sensor types having different vacuum measurement ranges that overlap. The combination vacuum gauge outputs may be blended in associated control electronics that may be mounted to the gauge assembly or linked to the gauge assembly by a cable.
Since combination vacuum gauges have more than one physical means for measuring vacuum, different correction factors are required for each of the vacuum sensors to get accurate vacuum measurements. These correction factors depend on and vary with such things as pressure, gauge operation parameters, temperature, gas species and which sensors are operating.
It has been a common practice to calibrate sensors on the combination vacuum gauge with generic correction factors. However, such calibration practices produce inaccurate vacuum measurements because even sensors that are of the same type require different correction factors since all sensors are not identical. These measurement inaccuracies are particularly pronounced when measurements from two sensors overlap in a given pressure range and their measurement signals combine into a blended output signal. As a result, correction factors unique to each individual sensor of the combination vacuum gauge have been determined at the factory and provided through a memory module that can be plugged into the controller electronics. In this way the combination vacuum gauge is field replaceable with the memory module.