Long-term stability of sulfur dioxide permeation tube standard reference materials

Summary A series of NIST (National Institute of Standards and Technology) sulfur dioxide permeation tubes from several Standard Reference Materials (SRMs 1625 and 1627) were studied for their long-term permeation characteristics at temperatures of 20, 25 and 30°C. It was found that contrary to previous reports the steady-state permeation rate was not constant, but showed a very slight downward trend with time. During the day-to-day use of such devices the downward trend in permeation rate is small enough to go unnoticed. In addition, the permeability at 30°C and permeability activation energy of SO₂ in FEP-Teflon are also evaluated for tubes that were part of the National Institute of Standards and Technology permeation tube SRM inventory from 1982 to 1989.