Vacuum measurement

The term "vacuum measurement" is no longer a paradox when you consider the definition given in ISO 3529-1, which states that vacuum is the state of a gas characterized by a pressure or density lower than atmospheric pressure. As far as its measurement is concerned, vacuum is a low absolute pressure. The result of a vacuum measurement is therefore expressed in Pascals. However, the ability to measure pressure does not necessarily mean the ability to measure vacuum, if only because of the way in which these fields are approached. In fact, in the field of pressure measurement, instruments deliver a signal proportional to the pressure between zero and full scale, which is visualized on a linear scale. Seen in this way, on an axis that would represent the entire range of pressure traceable to the SI, between zero (a conceptual absolute vacuum) and 1.6·10 ⁺⁹ Pa (maximum pressure whose traceability is recognized according to the information available on the website of the BIPM, Bureau International des Poids et Mesures, the area covered by the vacuum would not even be visible to the naked eye.

Understanding the vacuum domain can therefore only be envisaged on a logarithmic scale. Considered in this way, the measurable vacuum declines over fourteen decades, between 10 ^–9 Pa and 10 ⁺⁵ Pa. On the other hand, whereas pressure measurement relies on the existence of a macroscopic mechanical force exerted by the fluid (gas or liquid) to which the measuring instruments are sensitive, vacuum measurement relies instead on physical effects linked to the molecular density of the gas. One of the difficulties, specific to vacuum measurement, is the non-neutrality of gas, in the metrological sense, in the implementation of reference methods and measuring instruments. In addition, vacuum generation usually requires a pumped volume, which means that gas pressure is not identical at all points within the volume.

This article begins with the definitions and theory required to understand the topics covered. The reference methods and traceability of vacuum measurement, from conventional pressure standards, are then presented. Emerging reference standards aimed at supplanting/consolidating conventional pressure standards are also discussed, as their principle is based on the physical properties of gases. Vacuum measurement instrumentation is extensively detailed, with specifications for use: mechanical pressure gauges common to both vacuum and pressure fields, thermal convection pressure gauges, rotating ball pressure gauges and ionization pressure gauges. An estimate of the uncertainty of measurement associated with the implementation of reference methods...