THERMAL CONDUCTIVITY GAUGES
by Phil Danielson
Thermal conductivity pressure gauges are extremely common in vacuum technology, but an understanding of their principles of measurement and operation can help in their proper application.
Measuring pressure is a necessity for virtually all vacuum processes and vacuum practitioners, but there are so many variations in pressure ranges and the degree of required accuracy and precision that there is no possibility of identifying a single type of gauge that will be of use to all. Applications might vary from simply monitoring all or part of a pumpdown cycle, carefully measuring a stringent ultimate pressure, or measuring and controlling a critical process gas. There are, however, two types of gauges that are arguably the most commonly used out of the many gauge types available: ionization gauges and thermal conductivity gauges.
A high vacuum process will need to be provided with gauging that follows the pumpdown cycle from atmospheric pressure through the volume zone and into the drydown zone. A thermal conductivity gauge can follow the pressure all the way through the volume zone, but when the system goes into the drydown zone below about 10-3 torr where water vapor becomes the predominant residual gas, an ionization gauge is required. In general, with the exception of some extended range gauge modifications, these two gauges together can be used to cover the full pumpdown cycle. This is why so many electronic gauge controllers combine both types of gauges in the same unit.