Gas analysis by sonic phase measurement

The relation between the phase difference of the two received signals in a twin tube analyser and the mean molecular weight of the gas mixture in one tube is developed. The advantages of the twin tube system for temperature compensation are shown. The types of transducer used as both transmitter and receiver are discussed, and the advantages, particularly as regards the transducers, of using a continuous wave rather than a pulse or frequency modulated system to determine the sonic time delay are shown. Previous calculations take no account of the standing waves produced in the tubes by the reflection of energy from the receiving ends. The means of overcoming this effect are discussed. The practical requirements of the tube construction, and the minimizing of pressure, flow and temperature effects are considered. The phase measurement of the received signals is done electronically. The salient features of the amplification, limiting, phase comparator and output circuits are discussed. The phase relationships, and the calibration checking facilities in the complete instrument are described.