Patent GB-1535423-A (GB)
Title: Apparatus which measure the rate of flowing fluid
IPC: G01F1/80
Filed: 1976-03-12
Abstract: 1535423 Measuring fluid mass flow-rate ELDEC CORP 12 March 1976 [9 July 1975] 10039/76 Heading G1N A flowmeter provides first and second electrical signals representative respectively of mass flow-rate and volumetric flow-rate at inputs 144, 146 respectively. These signals are converted to respective D.C. voltages at points A, B respectively by pulse shaper 148, averaging filter 152 with a ten second time constant, D.C. source 156 and frequency to voltage converter 162. The voltage at B is multiplied in a variable gain amplifier 170 by a signal at input 172 representative of the density of the fluid metered. Thus at point C an output voltage representative of the fluid mass flow-rate is provided which has the response time of the volumetric flowmeter and which is said to follow transient changes in it practically instantaneously. This output voltage at C is applied to a flow indicator (not shown) and also to another averaging filter 176 with time constant equal to that of the first averaging filter 152. The signal at the filter output D is applied to the inverting input 178 of summing amplifier 180 of which the non-inverting input 182 receives the output signal from the first averaging filter. The output (error) signal from the summing amplifier at E is integrated in an error amplifier 184 and provides the signal at F which is fed to the input 172 of the variable gain amplifier as the densityrepresenting signal. Figure 4 This illustrates the operation of the circuit of Figure 2 for a step increase in density at time T 1 , coincident with a step decrease in volumetric flowrate whereby the mass flow-rate remains constant so that the voltages at points A, B are as shown at 186, 188. The amplifier 170 has a time constant of about ten milliseconds providing a more slowly changing signal 189 at point C of which the changed level would be as shown at 189a, in the absence of a correction to the density-representing input signal 196. As a result the output at C would be erroneous since the mass flow-rate has been assumed to be constant. The signal 189 is applied to the filter 176 which provides a gradually decreasing output signal 192 at point D due to the time constant (ten seconds) of the filter. Since the non-inverting signal input to the summing amplifier is constant in this case the decrease at D produces an increasing error signal 194 from the summing amplifier at point E. Consequently the integrated error signal at F is increased from its level before time Ti reflecting the increase in density. The output 170 therefore returns to a level 189b which is equal to its level before T 1 . Steady state conditions now prevail. Figure 1 The volumetric flow-rate metering assembly 22 provides from pick-up coil 60 and magnets 56 an A.C. signal of frequency proportional to flow-rate, which is supplied to the input 146 of Figure 2. The mass flow-rate metering assembly 65 has a motor assembly 70, 72 driving shaft 66 to which cylinder 100 is secured bearing magnets 108. The shaft also drives an impeller 120 through a spring 130 whereby the angular displacement of the impeller relative to the cylinder is proportional to the mass flow-rate of the fluid, the spring 130 having a linear torque/ deflection characteristic. Thus the phase difference between the A.C. signals induced in respective coils 110, 138 by the rotating magnets 108, 136 is representative of the mass flow-rate and provides the input signals at 144 of Figure 2.