GB1384390A - Electronic surveillance apparatus - Google Patents

Abstract

1384390 Measuring liquid levels under variable pressure SENSORS & SYSTEMS Ltd 14 Feb 1973 [16 Nov 1971] 53037/71 Heading G1N [Also in Division G3] The level of a liquid in a high-pressure vessel 1, Fig.1, is found from the pressure differential between the liquid at pipe 2 and the vapour at pipe 3. The liquid level is related to this pressure differential by a function which depends on the pressure of the vapour because the liquid density varies with pressure. The pressure differential signal formed at transducer 4 is therefore modified by the vapour pressure signal from transducer 7 at compensator 10 so that the level measured is correct over the whole range of pressures which may occur in the vessel. The vessel may be a steam boiler, and the level signal may control water inlet valves or the speed of an inlet pump to maintain the level constant. The vapour in Fig.1 is steam, and it condenses into a constant head device 5 whose pipe 6 is connected both to the differential pressure transducer 4 and the pressure transducer 7. The basic equation relating the water level to the various pressures, heights and densities indicated in Fig.1 reduces to the equation h-b = F 2 (p) - F 1 (p) #p, where h is the level, b is a constant, F 1 (p) and F 2 (p) are functions of pressure generated in compensator 10, and #p is the pressure differential. Fig.7 shows in block form the integers of the compensator 10. The blocks marked Gain Function and Offset Function generate F 1 (p) and F 2 (p) by using diodes and resistors at the inputs of operational amplifiers, Figs.8 and 9 (not shown); the outputs of the operational amplifiers are a series of linear portions matched to the desired functions, Figs.3 and 4 (not shown). The outputs of the function generators are multiplied and produce the required corrected level signal, Fig.10 (not shown). A power supply is given, Fig.1 (not shown), as are all component values.