Why?

All manufacturers of ionisation vacuum gauges develop different products even if belonging to the same type of gauge. They differ in the selection of materials, potentials and, most important, geometry. For this reason, they also differ significantly in their relative sensitivity factors. In addition, all available ionisation gauge types are lacking long-term and transport stability, the instability being about 5 % over one year. And worst of all, even products of the same type from the same manufacturer show a significant spread of sensitivities.

This means

• for high accuracy (measurement uncertainty < 20%) each individual ionisation vacuum gauge has to be calibrated
• relative gas sensitivity factors (sensitivity in relation to nitrogen gas) may spread widely between gauges (> 10 % up to factor of 10)
• a cathode exchange requires recalibration
• replacing an ionisation vacuum gauge in a vacuum process usually requires re-adjustment of the process parameters
• the value as reference gauge for calibrations is rather limited
• recalibrations (yearly) are needed for high accuracy

What is the solution?

A novel ionisation vacuum gauge type developed in the EMPIR project 16NRM05 overcomes this situation. This gauge with a measurement range from 10^-6 Pa to 10^-2 Pa showed predictable sensitivities with a very small spread (< 1.5%), very good short-term repeatability (<  0.05%) and reproducibility (< 1%), even after changing the emission cathode and drop-down tests. The transport for reasons of comparison between institutes in Germany, Czech Republik, Slovenia and France showed a change of sensitivity after 7 months of -0.25 % only.

Measured sensitivities for N2 of six gauges from manufacturer X and five gauges from manufacturer Y as well as three calculated sensitivities from different simulation software packages

This was achieved by a completely new design, simulated by different software packages before manufacturing. The electrons take a straight path from the emitting cathode through the ionisation space into a Faraday cup. Compared to existing ionisation vacuum gauges, this has the advantage that the electron path length is well defined. It is independent of the point and angle of emission and is not affected by space charge around the collector. In addition, the electrons do not hit the anode where they can be reflected, generate secondary electrons or cause desorption of neutrals or ions. Predicted sensitvities obtained from the simulations and measured sensitivities agreed within 2 %. For further details see the publication list on this website.

How did we proceed?

We wanted to standardise this novel ionisation vacuum gauge type so that all experienced vacuum gauge manufacturers world wide can produce this gauge in their own way. Due to the standardisation, all these gauges show the same characteristics, in particular the same sensivity for nitrogen and the same relative gas sensitivity factors, together with excellent long-term and transport stability.

The ISO Technical Committee (TC) 112 "Vacuum Technology" is responsible to develop written standards and technical specifications in the field of vacuum technology. The Working Group 2 (WG 2) within ISO TC 112 prepares standards for vacuum instrumentation. Our project worked closely together with WG2. ISO TS 6737 "Characteristics for a stable ionisation vacuum gauge" was published in November 2023.

We want to encourage as many manufacturers as possible to produce this gauge according to the new ISO TS 6737. Each manufacturer can find its best solution to realise the required design dimensions and a competition can develop of how to manufacture in the most economical and efficient way. Manufacturers can save the costs for the individual gauge calibration.

Photographs of two model gauge types realising our design, each from a different manufacturer (INFICON and VACOM)

What does it mean for customers and users?

Buyers of such new gauges will certainly have to pay a higher price but will also get a higher value and save other costs. From the results of the former project, we expect that they

• get a high accuracy vacuum gauge, as good as a spinning rotor gauge, suitable as reference standard for other calibration, e.g. a QMS (RGA)
• can reliably use the gauge for all gas species where data were generated by accredited labs or National Metrological Institutes
• can exchange the gauge in a process without readjustment of the process parameters
• can exchange the cathode without need of recalibration