Patent GB-616272-A (GB)
Title: Improvements in or relating to electrically operated indicating devices
IPC: G01F23/26
Filed: 1945-10-29
Abstract: 616,272. Indicating liquid-levels. LIQUIDOMETER CORPORATION. Oct. 29, 1945, No. 28525. Convention date, Feb. 1. [Class 37] A liquid-level gauge or other displacement indicator is controlled by two or more circuits each responsive to capacitative changes. At least one of the capacitors in each circuit is variable in accordance with the displacement to be measured and means are provided for compensating relative changes of the capacitance of the two or more capacitors caused by a change of the dielectric constant of an identical component of each-capacitor so that the indication is independent of variations in this dielectric constant. The invention is particularly applicable to liquid level gauges where the liquid forms part of the dielectric of a condenser and compensates for changes in the dielectric constant of the liquid. As shown, the liquid level in one tank or the amount of liquid in one or more tanks 15, Fig. 1, varies the capacitance 5<1> correspondingly. The capacitance 5<1> and a similar capacitance 5, i.e. one formed by plates immersed to a constant depth in the same liquid are connected in respective bridge circuits fed from a source 1, the bridge outputs being fed through rectifiers 8, 8<1> and smoothing condensers 10, 10<1> to the respective coils 9, 9<1> of a resultant field indicator 12 having a permanent magnet rotor 11. The bridges are both balanced when capacitors 5, 5<1> have air as dielectric. Liquid is then admitted to fill condenser 5, whereupon the coil 9 is energized to move the pointer 12 to the empty position. Admission of liquid to the measuring condensers 5<1> then energizes coil 91 to move the indicator correspondingly. Any change in dielectric constant of the liquid will affect both bridges and therefore both coils 9, 9<1> so that the indicator reading is independent of such changes. In a modification, Fig. 2, the condensers 5, 5<1> are fed in parallel from source 1 through respective rectifiers 8, 8<1>. Initially, with air forming the dielectric of both condensers, the currents therethrough are balanced against the currents in inductors 41, 41<1> in parallel therewith, so that both meter coils 9, 9<1> are de-energized. Liquid is then admitted as before, first to the condenser 5 to establish the " empty " position and then to 5<1>. In another modification, Fig. 3, three condensers 51<1>, 51<11>, 51<111> are connected in the oscillatory circuits of respective oscillators 1<1>, 1<11>, 1<111>. The three condensers are combined in a single assembly, Fig. 4 (not shown), their capacitances varying differently as the liquid level rises. As the oscillators are locked to the same frequency these capacitance variations cause corresponding phase-shifts at the points 6<1>, 6<11>, 6<111>, whereby the coils 91, 9<11>, 9<111> of the resultant field indicator are energized and produce a reading corresponding to the liquid level. The condenser plates are shaped to produce the desired deflection law of the indicator and one of them may be extended below the others to produce a zeroizing current in the meter coils. If, however, the coil currents are all zero at "empty," a zeroizing permanent magnet 102 may be provided. The pointer may be arranged to deflect several revolutions. Condenser constructions are described. The condenser 5, Fig. 1, may be fitted in the same tank and so arranged that as the liquid therein rises the air above it is forced out through a valve, whereby the condenser 5 may be maintained full even although the liquid level of the tank may fall. The invention is applicable to electrically-conducting liquids in which case some of the condenser electrodes must be insulated, and to powdered materials or semi-liquids. The invention is also applicable to capacitance arrangements which indicate the approach of a body into an electric field.