Abstract: A method of, and apparatus for, measuring the free volume of a confined space or container is described. The method comprises applying to the gas or vapor within the confined space or container repeated compressions of predetermined waveform by means of a barrier or diaphragm; generating electrical signals representative of the resultant variation of pressure within the confined space or container; repeatedly sampling said electrical signals at regular predetermined instants, or over regular predetermined time ranges, corresponding to predetermined points or regions of said waveform; combining or integrating the sampled signals to generate thereby a time-averaged output signal; and calculating the free volume of the confined space or container from the output signal thus obtained.