Patent GB-776526-A (GB)
Title: Acoustic flowmeter
IPC: G01F1/66
Filed: 1955-12-05
Abstract: 776,526. Determining velocity of sound and/or flow of a fluid. KRITZ, J. Dec. 5, 1955 [Dec. 10, 1954], No. 34777/55. Class 118 (2). Apparatus for determining the velocity of flow of a fluid and/or velocity of sound waves in a fluid comprises transmitting and receiving means for propagating and receiving an acoustic carrier wave in the fluid, the transmitter and receiver being interconnected by a feedback circuit including an amplifier and wave packet generator so that each received wave packet is amplified without demodulation and triggers the generator at a predetermined instantaneous voltage to cause wave packets to be repropagated in a sustained manner. Any change in the flow velocity of the fluid produces a corresponding change in the repetition frequency of the wave packets and a frequency meter is used to determine the unknown velocity. In Fig. 1 a wave packet generator 14 and amplifier 13 are associated with transducers 11 and 12 for sending waves through the fluid in a pipe 10 in a downstream direction while generator 14<SP>1</SP> and amplifier 13<SP>1</SP> are connected to transducers 11<SP>1</SP> and 12<SP>1</SP> for upstream transmission. Pulse signals corresponding to the repetition frequencies f1 and f2 of generators 14 and 14<SP>1</SP> respectively, are fed to a mixer 15 and detector 16 which provides sum and difference outputs for measurement by a frequency meter 17. The difference frequency (f1-f2) is directly proportional to the flow velocity and independent of the propagation velocity of acoustic waves in the fluid, while the sum frequency (f1+f2) is directly proportional to the popagation velocity of the waves and independent of the flow of the fluid. Calibration may be effected by taking a reading at a known flow velocity in the one case and for a known value of propagation velocity in the other case. Alternatively, calculation may be made from simple formulµ based on the dimensions of the system 11, 11<SP>1</SP>, 12, 12<SP>1</SP>. Due to the resonant character of the transducers and other circuit elements the generated wave packet (Fig. 2, not shown) is received with a build-up as in Fig. 3. Triggering of the generator 14 by the carrier wave without demodulation avoids the time delay introduced by a detection process, and also improves the stability of the firing time since any variation of the point P in the “-cycle wherein the voltage attains triggering value Ex will be a smaller time variation at this higher frequency. Additional stability is obtained by incorporating automatic gain control into the amplifiers 13, 13<SP>1</SP>; a voltage comparator circuit whose output is fed to the gain control elements of the amplifiers maintains their outputs at a level determined by a fixed reference potential (Fig. 4, not shown). This overcomes fluctuations in signal strength due to impurities in the fluid or other factors. The circuit of the wave packet generator is shown in Fig. 5. A triode 30 is arranged as a squegging oscillator with its anode circuit tuned by inductance 37 to the resonant frequency of transmitting crystal 11. It is normally quiescent due to diode 44 which is biased to be non-conductive by about 8 volts drop across resistor 47 in series with diode 41, 42. A signal received by the crystal transducer 12 is amplified and fed to the load circuit of a cathode follower 50. This comprises an inductance 53 and diode 54, such that only positive voltage swings are passed on via condenser 55 to the grid 32 of valve 30. When the positive voltage swing attains the value Ex (Fig. 3) the amplified voltage across the winding 39 overcomes the bias due to resistor 47 and regeneration takes place in valve 30. During each positive peak of these oscillations the condenser 55 receives an additional charge and produces an increasing negative potential on its side towards grid 32 until this is sufficient to cut off the valve 30 and stop the oscillation. This negative bias then leaks away till the normal quiescent bias of about 8 volts across resistor 47 is reached when diode 41, 42 will conduct to maintain this potential until another signal is received (Fig. 6, not shown). The period during which the negative bias is higher forms a protection against spurious triggering. In order to start the generator when first switched on, or after any interruption in acoustic transmission, a condenser 56 and resistor 57 are connected via diode 48, 49 to the grid circuit of valve 30, and to positive supply, as shown in Fig. 5. The time constant of this circuit is made much longer than that of condenser 55 and resistor 47 or of any possible transit time around the complete system loop but in the absence of a received signal a positive voltage is built up to trigger the valve 30 and start the system operating as described above. When valve 30 oscillates the positive potential on diode 44 causes diode 48, 49 to be non-conductive and effectively disconnects condenser 56 from the remainder of the circuit each time a wave packet is generated. Condenser 56 thus forms a convenient place to obtain a signal whose repetition rate is that of the wave packets and this is fed to the mixer 15 as in Fig. 1. The transducers may be modified to provide wave packets of improved waveform by having a metal interface between the crystal and the fluid (Fig. 7, not shown) or by the addition of a further material between such an interface and the fluid (Fig. 9, not shown) and the effect of various thicknesses in terms of the wavelength is stated. Further modifications consist in providing transmitting and receiving crystals in a common housing (Figs. 10 and 11, not shown) to facilitate the maintenance of equal path lengths in each direction of transmission (Fig. 12, not shown) and eliminate errors due to different conditions of the fluid in the two paths. Greater accuracy is obtained by giving the transducers concave faces (Fig. 14) to cause the paths to cross (as in Fig. 15). The crystals may be concentrically disposed in their housings (i.e. one being annular) as in Figs. 17 and 18 (not shown) and the housings arranged as in Figs. 19 and 20 (not shown) such that the annular crystal of each transducer is focused on to the central disc crystal of the other.