GB805544A - Improvements in or relating to sonic gas analysers - Google Patents

Abstract

<PICT:0805544/III/1> The velocity of propagation of sound waves in a gas or vapour of molecular weight M is inversely proportional to M 1/2 and this property is utilized to determine the molecular weight M1 of a sample gas in a tube. As shown, a tube 2 contains the sample gas and a similar tube 1 contains a datum gas of known molecular weight M2. Electro-acoustic transmitters 3 and 4 at the left-hand ends of the tubes are energized from a common electrical A.C. source and the outputs from electro-acoustic receivers 5 and 6 at the opposite ends of the tubes are applied through separate amplifiers 10 and 11 to a phase comparator 7 producing a D.C. output proportional to (M1 1/2 -M2 1/2 ) which is combined with a bias voltage of amplitude M2 1/2 from a potentiometer 9 giving a resultant M1 1/2 which is applied to a non-linear moving-iron meter to give a linear indication of the quantity M1. Alternatively a single tube containing sample gas may be employed, the inputs to the phase comparator comprising (1) the input to a single transmitter and the output from a single receiver spaced therefrom; or (2) the outputs from two spaced receivers receiving sound waves from a single transmitter; in either case the calibration of the meter scale is set initially by adjustment of the bias voltage. The phase comparator may be of the type which produces a train of pulses each of duration proportional to the phase difference of the A.C. inputs, said pulses being initiated and terminated respectively by two trains of unipolar pulses derived by shaping the A.C. inputs, Fig. 1 (not shown).