By means of the invention, the pressure in a cavity can be measured, without ever reducing the gas flow to zero. This is achieved by bleeding an intermediate vessel slightly with its inlet valve closed and its outlet valve open and then being able to extrapolate from the pressure decrease whether the desired pressure has been reached. If the result of the extrapolation indicates that the desired pressure has not been reached, then pumping is repeated, measuring is repeated and so on and so forth until the extrapolation indicates that the desired pressure has been reached. Thus extrapolations are made instead of allowing the intermediate vessel to empty completely.