Permeability of MDT chambers to water vapor

Tests of MDT chambers performed at the GIF facility and in the H8 test-beam area have shown relative high levels of water vapor contamination in the gas-mixture at the detector output. This effects significantly the drift properties of the MDTs. This note shows that amount of water observed is compatible with approximate estimates based on the permeability of Noryl, used in the tube end-plugs, and of EPDM, used in the O-rings of the on-chamber gas distribution.