How fast do vacuum gauges measure?

In many areas of industry, vacuum gauges must detect rapid pressure changes with a high time resolution. This is the case, for example, for the coating of PET bottles in high vacuum, the plating of CDs or DVDs, leakage tests using vacuum technology or various processes in the lighting industry, all of which require cycle times of only 1.5 s to 3 s. Whereas the manufacturers of vacuum gauges can demonstrate rather well to their customers that the electronic unit in their measuring instruments is fast enough for the controlling of such processes, the response time of the realized measuring principle to rapid pressure changes cannot be tested for the measuring heads that are exposed to vacuum.

Within the scope of the European Research Project EMRP IND12, PTB has, thus, set up a dynamic vacuum standard. Hereby, the pressure is reduced by 3 decades by letting the gas expand out of a very small volume of approximately 0.1 L into a very large evacuated volume of approximately 180 L. The vacuum gauges to be tested are flange-mounted onto the small volume. A large gate valve, which was specially developed for this purpose and can release a cross section of 12.5 cm² within 4.6 ms, lets the gas move from one volume into the other, so that the pressure inside the small volume can be reduced from 100 kPa down to 100 Pa within 20 ms. Slower expansions can also be realized by means of orifices or nozzles.

In order to determine the time response during the measurement, the experimental values are compared with calculations of the time-dependent evolution of the pressure inside the small volume during the expansion. These simulations are very demanding since all of the three types of fluxes occur at any given time – viscous, molecular and transition fluxes – and their spatial transitions change over time. In addition, the changing geometry of the fast opening valve and the strong influences of the rapid pressure changes on the temperature have to be taken into account. In front of an orifice, a temperature drop down to approximately 165 K was calculated and measured. The simulations were carried out using the ANSYS CFX software and a program developed in-house based on OpenFOAM^®.

A first series of vacuum gauges has aloready been tested at the new standard. In the case of fast capacitive membrane vacuum gauges, which had been specially developed for this project by the INFICON company, an upper limit of 1.3 ms was determined for the 1/e response time in the event of a pressure change