Abstract: A gas sample flowing through the lower chamber of a diffusion cell diffuses through a membrane into the upper chamber at a rate above a determined minimum. As the sample diffuses through the membrane, substantially all of the water and any particulates are removed from the gas. A clean carrier gas flows through the upper chamber at a constant rate to dilute the diffused gas and convey it to gas analyzers. Upon acquisition, a gas sample is maintained at a temperature above its dew point until it passes from the lower chamber and the membrane is maintained at a substantially constant temperature.