GB1422791A - Apparatus for indicating fluid flow velocity - Google Patents

Abstract

1422791 Ultrasonic fluid flowmeters; automatic frequency control UNITED KINGDOM ATOMIC ENERGY AUTHORITY 29 March 1973 [4 April 1972] 15554/72 Headings H4D and H3A In an ultrasonic fluid flowmeter of the type in which pulses are transmitted upstream and downstream between two spaced transducers, variable frequency oscillator means (VFO) are controlled automatically to give output frequencies N 1 /t 1 and N 2 /t 2 where N 1 and N 2 are integers and t 1 and t 2 are respectively the upstream and downstream pulse transit times, and the difference between the two frequencies is determined as a measure of the flow velocity. Preferably N 1 = N 2 = N. The difference frequency is N times that obtained from prior art flowmeters measuring the difference between upstream and downstream "sing-around" frequencies, hence the velocity readout response time is 1/Nth that of the prior art flowmeters. Respective oscillators may be used for generating the output frequencies simultaneously; or a signgle oscillator for generating them sequentially. In one embodiment using simultaneous generation two oscillators have respective retroactive frequency-control logic circuits each of which derives an error signal by comparing the occurrence time of the Nth oscillator pulse following the acoustic pulse transmission from one transducer with the arrival time t 1 or t 2 of that acoustic pulse at the other transducer. The transducers feed respective receiver amplifier channels (Fig. 1, not shown) or a switched single channel (Fig. 5, not shown). The logic circuits are inhibited except when reception of the acoustic pulse is expected. A fault indication is given if fewer than a given fraction of the transmitted acoustic pulses are received. In a second embodiment, operating in a sing-around mode, velocity resolution is improved by a factor M at the expense of response time by using frequency-control logic which ensures output frequencies N/t 1 and N/t 2 by comparing the times Mt 1 and Mt 2 for M sing-arounds with the times for MN oscillator pulses to derive the error signals. frequency control of oscillator VFO in Fig. 3 is required such that N oscillator pulses occur in the interval t 1 starting with the first VFO pulse following a reset pulse and ending when pulse RX is received (see Fig. 4). Bi-stable 15 or 16 is set depending on whether the RX or Nth VFO pulse occurs first. The bi-stable in turn triggers a monostable 19 or 23 to apply a discrete current pulse (error signal) of negative or positive polarity respectively to an integrator 22. The integrator output controls the VFO frequency in steps so as to increase or decrease it depending on whether RX or the Nth VFO pulse respectively occurred first. The size of a step may be temporarily increased by a manual control.