An apparatus for testing the gas impermeability of watch cases or other small articles comprises two enclosures. A bellows forms part of a hermetic separation between the enclosures, the bellows serving to move the movable plate of a variable condenser. The condenser is connected in an oscillator circuit to vary the oscillator frequency in accordance with the differential pressures between the two enclosures. The oscillator output frequency is compared with the output frequency of a stable frequency generator to provide a measurement indication. A watch case is placed within one enclosure and both enclosures brought to equal pressures after which the measurement of gas impermeability may begin.