HELIUM GAS PURITY MONITOR BASED ON LOW-FREQUENCY ACOUSTIC-RESONANCE

Monitoring gas purity is an important aspect of gas recovery stations where air is usually one of the major impurities. Purity monitors of K atherometric type ate commercially available for this purpose. Alterna tively, we discuss here a helium gas purity monitor based on acoustic resonance of a cavity at audio frequencies. It measures the purity by monitoring the resonant frequency of a cylindrical cavity filled with the gas under test and excited by conventional telephone transducers f ixed at the ends. The use of the latter simplifies the design consider ably. The paper discusses the details of the resonant cavity and the e lectronic circuit along with temperature compensation. The unit has be en calibrated with helium gas of known purities. The unit has a respon se time of the order of 10 minutes and measures the gas purity to an a ccuracy of 0.02%. The unit has been installed in our helium recovery s ystem and is found to perform satisfactorily.