Abstract: A method of measuring the volume (V) of a gas in a container (20) comprises varying the container volume according to a sine wave between a minimum and a maximum value, the variation being made at a constant frequency but at varying amplitude, to obtain a predetermined pressure difference (.DELTA.p), and the volume difference (.DELTA.V) corresponding to said predetermined pressure difference (.DELTA.p) being determined by detecting the points of time when at least two points on the sine wave are attained which correspond to two predetermined values of the container volume. By measuring the time between the two points on the sine wave, the sine wave peak values for the volume difference (.DELTA.V) corresponding to said predetermined pressure difference (.DELTA.p) may be calculated and, thus, the said volume difference (.DELTA.V). By means of the formula V=.DELTA.Vxp.sub.0 x.eta./.DELTA.p, wherein p.sub.0 =the static pressure and .eta.