Patent GB-907801-A (GB)
Title: Improvements in or relating to electrical measuring systems for measuring liquid quantities
IPC: G01F23/26
Filed: 1958-11-14
Abstract: 907,801. Electric tests for determining liquid level. MINNEAPOLIS-HONEYWELL REGULATOR CO. Nov. 14, 1958 [Nov. 14, 1957], No. 36716/58. Class 37. In a system for measuring the quantities of liquid in a number of containers each of which has an electrical liquid level sensor, all of the sensors are fed with an electrical signal having one electrical characteristic and selected ones only of the containers are fed with an electrical signal having a second electrical characteristic. The outputs of all the sensors are fed to a common channel which drives two signal generators respectively responsive only to signals of the first and second electrical characteristic and which balance out respectively the signals of the first and second characteristics which are present in the common output channel. In this way not only is an indication given of the total quantity of liquid in all of the containers, but also the quantity of liquid in selected ones only of the containers. Fig. 1 shows a system for indicating the liquid level in containers 11, 12 and also the total liquid level in all of the containers 10, 11, 12. From a common A.C. supply, two A.C. signals, phase A, phase B, having a phase difference between them of 90 degrees are fed to transformers 30, 100 respectively. The phase A signal is fed to capacitive sensors 14 and 15 so that the input to amplifier 20 is proportional to the sum of liquids in containers 11, 12. The output from the amplifier controls motor 84 which positions the slider of potentiometer 52 to balance out the signal due to sensors 14, 15. The empty tank signal is balanced out by signal from capacitor 67 so that the sum of the signals from capacitors 60, 67 is of equal amplitude and opposite phase to the whole signal from capacitive sensors 14, 15. Thus the setting of wiper 54 as indicated on scale 55 is indicative of the quantity of fuel in tanks 11, 12. The phase B signal is fed to all three tanks in parallel so that the signals from all the capacitive sensors 13, 14, 15 are fed to amplifier 20 the output of which drives motor 174 which is sensitive only to phase B signals and positions wiper 150 to balance out the total phase B signal in the output channel. The signal across the lower portion of transformer winding 120 is 180 degrees out of phase with reference to phase B signals, hence the re-balance signal in potentiometer 150 is 180 degrees out of phase with that in potentiometer 122 and this is fed via potentiometer 52 and capacitor 60 to amplifier 20 so as to cancel the effect of phase B signals from sensors 13, 14, 15 due to the presence of fuel in the tanks. If the fuel level changes in any tank the input to amplifier 20 changes to control motor 174 to position potentiometer wiper 150 to restore the balance, so that the indication on the associated scale 151 is indicative of the total quantity fuel in all of the tanks. Instead of using A.C. signals of differing phase it would be possible to feed the transformers 30, 100 with A.C. signals of differing frequencies. In the modification shown in Fig. 2 both the total quantity of fuel in all the tanks and the amount in any one of the three tanks may be measured. Whatever the position of the switch 570 all three capacitive sensors have the phase B signal applied to them simultaneously, but whereas two have phase B signals fed to them directly through conductor 668 the third receives it only via circuit 35, 40, 42 and thus is combined with a phase A signal. As shown in the Fig. 2 sensor 13 receives the combined phase A and phase B signal, whereas sensors 14, 15 have phase B signal supplied to them alone so that as described in connection with Fig. 1, the setting of potentiometer 140 and thus the reading on the scale 151, is indicative of the total quantity of fuel in all the tanks, whereas the setting of potentiometer 52 and thus the reading on scale 55 is indicative of the quantity of fuel in one only of the tanks as selected by switch 570. By the use of " padder '' capacitors, only one empty tank compensating signal capacitor 67 is required.