GB798323A - Improvements in or relating to methods of and means for detecting changes in the velocity of sound or of ultrasonic vibrations in gases - Google Patents

Abstract

<PICT:0798323/III/1> <PICT:0798323/III/2> When the velocity of sound waves in two different gases differs appreciably, the velocity in a mixture of said gases depends upon the relative proportion of the gases in the mixture and according to the present invention this principle is utilized to determine the composition of such a mixture by spacing an electroacoustic receiver 7 a distance d from an electroacoustic transmitter 6 in the gas mixture and determining the phase difference between electrical waves applied to said transmitter and the corresponding electrical output from said receiver, the distance d being such that said phase difference is less than 2p . Alternatively effects due to temperature variations can be eliminated by having a single transmitter radiating into two tubes each provided with a receiver, one tube containing the gas mixture and the other a reference gas and means being provided for measuring the relative phase of the outputs from the two receivers. As shown, the sinusoidal output from an oscillator 20 is applied through an amplifier 21 to a transmitter 6 and to a squarer 22 and differentiating circuit 23 (output Fig. 5, not shown) and the delayed sinusoidal electrical output from receiver 7 is applied through amplifier 24 to squarer 25 and differentiator 26 (output Fig. 8, not shown), the positive pulse outputs from circuits 23 and 26 being selected and inverted by a mixer 27 and applied to the Y plates of a cathode-ray tube 28 whose X plates are coupled to a timebase generator 29 synchronized by the negative pulse output from circuit 23 so that the displayed pulse 40, Fig. 9, derived directly from oscillator 20 appears in the centre of the trace and the phase of the displayed pulse 41 derived from receiver 7 is preset by adjusting the bias on squarer 25 and/or a phase shifter (not shown) so that the displayed pulse 41 appears at the extreme right of the trace when only the gas having the smaller sound propagation velocity is present. Thus the displayed pulse 41 will move toward the left of the trace as the relative proportion of the gas of higher sound velocity increases and hence the position of pulse 41 as read off a calibrated scale indicates the composition of the mixture. Alternatively the output from mixer 27 may be applied to deflect or intensity modulate a circular timebase, or the input to transmitter 6 and the output from receiver 7 are applied to the X and Y plates to produce Lissajous figures. An aural and/or visual warning is given when the displayed pulse 41 appears on the right-hand half of the trace by applying the outputs from circuits 26 and 22 to the control and screen grids respectively of a gate valve 45 which triggers a thyratron 46 by applying a negative pulse to the cathode thereof to actuate a relay which controls an indicator 47 and which may also switch on the E.H.T. power supply for the cathode-ray tube and/or automatically control the composition of the gas mixture. The range of gas mixture indicated by the warning device 47 may be varied by adjusting the bias on squarer 22 and the device 47 may be reset by a push-button switch which breaks the anode circuit of the thyratron. The operation of the device 47 may be tested by reversing the phase of the input to transmitter 6. The transmitter 6 and receiver 7 may be mounted on sound insulating material at the opposite ends of a telescopic tube, Fig. 1 (not shown), which is lined with insulating material to prevent the formation of standing waves. For operation at a frequency of 3.5 kc/s. the transmitter 6 may comprise a telephone earpiece and receiver 7 a crystal microphone; for operation at ultrasonic frequencies crystal transmitters and receivers may be used. Temperature effects may be compensated by choosing a microphone such that phase changes due to variations of its resonant frequency with temperature neutralize the phase changes due to variation of the velocity of sound. An indication may also be produced by (1) causing a valve to be switched on by the negative transmitter pulse and switched off by the receiver positive pulse, and measuring the anode current of the valve; (2) applying the transmitter and receiver voltages to a phase discriminator either directly or through a phase shifter which is adjusted to produce a predetermined phase difference; or (3) applying the receiver voltage to illuminate a stroboscope driven by the transmitter voltage.