Gas sensitivity of vacuum gauges

The discipline of vacuum is almost unique in having a dynamic range of 15 or more orders in its application.

As with pumps, there is not one gauge mechanism that can span this range. There are many additional aspects and variables to consider when choosing a gauge.

These include:

• Technical parameters such as accuracy/uncertainty (the range of systematic error)
• Precision (the level of random error)
• Resolution
• Measurement range
• Linearity of the output
• Reproducibility
• Robustness (process compatibility, radiation, and magnetic field resistance)
• Response time. 

Additionally, size, communication protocols, and cost all have a bearing on the choice.

Direct reading or Mechanical gauges are so called because they measure pressure ‘directly’ as the force per unit area (of a solid or liquid area).

These are ‘Total’ (Dalton’s law: The total pressure of a mixture of gases = ∑ partial pressures of each gas) pressure gauges and include U-tube and capacitance diaphragm manometers and bourdon gauges.

Indirect gauges are often the most practical and employed gauges and measure a pressure dependent physical property of the gas, for example thermal conduction or viscosity, or a physical quantity which is proportional to number density.

These Total Pressure gauges include Pirani, Bayard-Alpert Hot Cathode Ionization (BAG), Penning and Spinning Rotor Gauges. Partial pressure gauges (Residual Gas Analysers) with reliance on ionization can be used to analyse the relative constituents of a vacuum.

For a more in-depth study you can read the paper ‘Gas correction factors for vacuum pressure gauges’ from the Wiley online library.