Determination of Binary Gas Mixtures by Measuring the Resonance Frequency in a Piezoelectric Tube

The composition of gas mixtures may be determined via changes of the speed of sound. As this affects the resonance frequency of the gas inside a tube, indirect measurements through a frequency analysis are also possible. It is demonstrated that this may be carried out with unprecedented simplicity by the novel employment of a piezoelectric tube which serves at the same time as a resonance tube and as transducer into the electronic domain. Experiments were run using a simple diecast aluminum box as the measuring cell, inside which the piezoelectric tube made from lead zirconium titanate with 30-mm length and 5.35-mm inner diameter was suspended. A small loudspeaker placed into the cell served for excitation of the resonance. Peak frequencies between 3910 and 14,590 Hz (for pure CO₂ and He, respectively) were obtained. Two component mixtures of O₂/N₂, CO₂/N₂, and He/N₂ at various composition were tested. A linear frequency change from 4790 to 5100 Hz was observed when going from pure O₂ to pure N₂.