apparatus for measuring gas comprising a gas sensing unit having a gas inlet membrane and being evacuated by a sputter ion pump via a restrictive conductance . said conductance has a very low value relative to the pumping speed of the pump , whereby a change in the pumping speed of the pump causes substantially no change in the pumping speed with which the gas sensing unit is evacuated , so that the output signal of said gas sensing unit is a true reflection of the gas entering said inlet , without distortion which would otherwise be caused by changes in pumping speed of said pump .