Many types of elastomers are available and used for demountable seals in vacuum service. One important consideration in the selection of the elastomer material is the permeation rate. The permeation rate for gasses like helium can differ by orders of magnitudes for different seal materials. In this study, the gas load through a single O-ring due to permeation was compared for Viton® E and for Chemraz®653. The procedure was to use a MSLD (mass spectrometer leak detector) with helium and log the leak signal at intervals often a few seconds apart as a controlled flow of helium was applied to the seal. This was done both at room temperature and at elevated temperatures up to 140o C. The test O-ring was located in a specially constructed fixture that allowed an O-ring to be compressed to five different values ranging from 15% to 27% compression. Leak signals due to permeation through the 2-227 sized O-ring (nominal 0.139 inch cross section) were found to vary by orders of magnitudes. For instance at 22% compression and room temperature, the peak leak signal from the Viton® seal was approximately 1x10-10 mbar-l/sec while for the Chemraz®653 seal it was approximately 2x10-8 mbar-l/sec. When the seals were at 140o C with the same compression, the leak signals became approximately 1x10-8 mbar-l/sec for the Viton® and approximately 8x10-7 mbar-l/sec for the Chemraz®653 seal.

 

Leak signals due to permeation of these magnitudes are very important when troubleshooting or qualifying equipment since leak rate specifications on equipment are often lower. The same set up was also used to help find ways to distinguish a response on the leak detector due to a leak from a signal due to permeation. Using the rapid data recording and graphical display of the leak signal, it was found that permeation responses had a characteristic shape. With a response due to permeation, there is a short time before the response starts, a ramp up time, and then a slow decay. By contrast, signals from known leaks were shown to have a very rapid response time, and a quick decay or ‘clean up’ when the probe gas was removed. Using a graphing display, and comparing the response to known examples, operators can distinguish the two situations.