Patent GB-805016-A (GB)
Title: Acoustic flowmeter
IPC: G06G7/57
Filed: 1956-06-08
Abstract: 805,016. Sonic fluid-flow meters. KRITZ, J. June 8, 1956 [July 29, 1955], No. 17804/56. Class 118 (2). 'The mass flow of a fluid through a pipe 10, Fig. 1, is determined by generating (1) a train of electrical pulses 421 of constant duration when recurrence frequency is proportional to the fluid velocity X; (2) a train of rectangular pulses 29 of recurrence frequency proportional to the propagation velocity V of acoustic waves in the fluid; and (3) a D.C. voltage proportional to the acoustic impedance pV of the fluid where p is the density of the fluid. The amplitude of the pulses 29 is controlled in a clamp circuit 35 in accordance with the voltage pV and the resultant pulses 351 are integrated at 37 and detected at 34 to give a D.C. voltage proportional to p which is applied to a clamp circuit 43 to control the amplitude of the pulse train 421 so that the average current produced by the resultant pulse train 431 as indicated by a meter 45 is proportional to the rate of mass flow pX and the time integral of the pulse train 431 as indicated at 47 is proportional to the total mass flow, i.e. the total mass of fluid passing the measuring point in any period. The imputs to the indications 45 and 47 may also be used to control the fluid flow. The pulse trains 421 and 29 are produced by a method similar to that disclosed in Specification 776,526. As shown, transmitting and receiving piezo-electric transducers 12 and 13 obliquely mounted in opposite walls of the pipe 10 are interconnected by a feedback circuit comprising an amplifier 15, detector 16 and modulator 17 to which is coupled a carrier wave generator 18 such that the system operates as a closed loop oscillator and the recurrence frequency f1 of the resultant square wave modulation signal output from detector 16 is proportional to (V - X cos #) where # is the inclination of the propagation path 12, 13 to the direction of fluid flow 11. A corresponding arrangement 19, 20, 22-25 with its propagation path inclined in the opposite direction gives an output signal of recurrence frequency f2=(V+X cos #) and the two output signals are mixed at 39 and detected at 40 to give a signal of frequency (f2-f1) proportional to the velocity X which is shaped in a pip generator 41 and applied to trigger a monostable multivibrator 42 to produce the pulse train 421. The D.C. voltage output from detector 34 of amplitude proportional to p is produced as in Specification 771,637. As shown, an amplitude-stabilized constant-frequency oscillator 31 is coupled to a transducer 30 arranged parallel to transducer 12 through an inductor 32 such that the voltage across transducer 30 is proportional to p(V - X cos #) and this voltage is rectified at 33 and applied to a clamp circuit 35 to control the amplitude of a rectangular pulse train 29 produced by triggering a multivibrator 28 by the output of frequency f1 from detector 16. The resultant pulse train 351 is integrated at 34 giving a sawtooth wave of amplitude proportional to p which is detected at 34 to give a D.C. voltage of corresponding amplitude. The integrator 46 for integrating the pulse train 431 is shown in Fig. 2. The pulse train 431 is applied to the grids of a double triode 50 whose anodes are connected (1) to B+ through capacitors 51, 52; (2) through cathode followers 60, 61 to a comparison circuit 62 to which a reference voltage 631, Fig. 3, is applied from a potentiometer 63; and (3) to the contacts 71, 56 of a switch whose arm is connected through a diode 58 and resistor 59 to B+, thereby clamping the potential of the anode selected by switch 57 (i.e. the right-hand anode, as shown). Valve 50 is rendered conductive during each positive pulse of the applied pulse train 431 and the capacitor connected to the unclamped anode (capacitor 51, as shown) is progressively charged during successive pulses, causing the potential of the unclamped anode to decrease as shown by curve 501, Fig. 3, the slope of each discrete decrease being proportional to the amplitude of the applied pulse train 431 (i.e. proportional to p). Since the recurrence frequency of the pulses is proportional to the fluid velocity X, the time taken for the potential of the unclamped anode to decrease to the reference potential 631 will be equal to the period in which a predetermined mass of fluid passes the measuring point. When the potential of the unclamped anode reaches the reference potential 631, the comparison circuit 62 becomes conductive and actuates a pulse generator (output 641, Fig. 3), to trigger a monostable multivibrator 65 whose output is applied through an amplifier 66 to trigger a bistable trigger circuit 671 whose output 671 reverses the energization of relay coil 72 thereby operating switch 57 to unclamp the right-hand anode of valve 50 and clamp the left-hand anode. Winding 72 also operates a switch 73, 74, 75 coupled to an electromechanical counter 80 which indicates the total mass flow. In order to unclamp the righthand anode immediately when the multivibrator 65 is triggered, a negative pulse 581 is applied from the amplifier 66 to cut off the diode 58 until the switch arm 57 has operated.