Paper VT-TuA12
Investigation of the use of Viton as Certified Reference Material for Outgassing

Tuesday, October 20, 2015, 6:00 pm, Room 230B

Goal of this study was to make samples with well known outgassing rate which can be used as a reference for calibration and validation of different outgassing measurement facilities.

Main criteria for selection of suitable material were (i) sufficient solubility of different gases in the material (ii) diffusion constant for different gasses in the range from 10^-8 cm²/s to 10^-6 cm²/s, and (iii) material shall be "clean" and compatible with high vacuum, which means it shall not release volatile organic compounds. Only materials which have sufficient repeatability and reversibility of gas absorption could be used as reference samples. Materials with desired properties can be found among polymers and we selected Viton (FPM-fluoropolymer) as a good candidate for outgassing reference samples. Use of Viton in high vacuum systems down to 10^-7 Pa is well established.

We have prepared outgassing reference samples in the following way: pieces of Viton were placed in a gas loading cell and were evacuated on an ultrahigh vacuum system for sufficiently long period to get fully degassed state. Then the loading cell was filled with a pure gas or gas mixture at a pressure up to several 100 kPa and the samples were kept in the gas for several days to penetrate into material to a fully saturated state. Samples were removed out of the loading cell just before they were placed into the outgassing rate measurement apparatus.

A reference sample can be first loaded with a certain gas or gas mixture and measured by a "primary" outgassing measurement system (like the system in PTB-Germany which was developed in the frame of EMRP-IND12 project), yielding a certified reference value of the outgasing rate as a function of time. Then the same sample can be re-loaded with the same gas and under the same conditions and sent in the loading cell to a user in industry or another laboratory for the measurement in their system.

Certified reference samples can be used in round robin tests for the proficiency testing of systems in different laboratories. Distinctive feature of newly developed reference samples based on Viton material is that they can be reloaded many times and with many gases, and also with mixtures of gases at arbitrary concentration ratios. Gases which do not react chemically with Viton are expected to produce reproducible outgassing rates. This was confirmed in our tests with gases H₂, He, Ar, Kr, CH₄, N₂, O₂ and CO.

Support through the EMRP IND12 project is gratefully acknowledged. The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union.