A sensor method and apparatus are described for measuring the pressure of a gas within a sealed commercial pressure vessel mounted into an assembly. A sonic transducer is used to apply an oscillating force to the surface of a vessel. The frequency of the sonic wave is swept through a range which caused resonant vibrations of the gas in the vessel. A receiving transducer measures the amplitude of the resultant vibration at the vessel surface and reveals the resonant frequency of the gas at peaks in the amplitude of the sweep. The resonant frequency obtained depends upon the composition of the gas, its pressure and temperature and the shape of the confining vessels. These relationships can be predetermined empirically so that the pressure inside the vessel can be calculated when the composition of the gas, its temperature and shape of the confining vessel are known. The output of the receiver is fed into a computer which is programmed to calculate the pressure based upon these predetermined relationships which are stored in the computer.