Abstract: A device is disclosed for rapidly testing items for leaks. The device incorporates a mass concentration gauge. One such gauge has a sensor which has an .alpha.-emitting radioactive source to ionize gases in a space between two electrodes. Associated electronic circuitry is disclosed which automatically indicates when a leak is detected. Other gauges to measure mass concentration are disclosed which use sensors directed to dielectric variation, magnetic permeability variation, acoustic velocity variation and optical index of refraction variation. All sensors are sensing properties which are completely or relatively insensitive to temperature variations.