Patent GB-700466-A (GB)
Title: Improvements in or relating to apparatus for measuring the amount of liquid in a container
IPC: G01F23/26
Filed: 1951-01-29
Abstract: 700,466. Measuring liquid depth by electric tests. LIQUIDOMETER CORPORATION. Jan. 29, 1951 [Jan. 31, 1950], No. 2180/51. Class 37. Apparatus for measuring the amount of liquid in a container comprises a measuring condenser, whose reactance depends on the depth of liquid and its dielectric constant, associated with an inductance whose reactance is equal to that of the measuring condenser, when free of liquid, and a comparison condenser, wholly immersed in the liquid so that its reactance depends on the dielectric constant, associated with a further inductance whose reactance is equal to that of the comparison condenser when free of liquid'; the circuit of these components is fed with A.C. and gives an output proportional to the quantity of liquid and independent of its dielectric constant. The circuit may also have means for compensating for variation in the density of the liquid where the indication is in terms of weight, as well as means for compensating for stray capacitance of the leads &c. In Fig. 1, the measuring condenser 2 comprises two concentric cylinders immersed in the liquid 10 so that the space between the cylinders is filled to the height of the liquid 10. The comparison condenser 12 is wholly within the liquid. A circuit, Fig. 2, comprises the condenser 2 in parallel with an inductance 76, and the condenser 12 in parallel with inductance 77, the inductances having reactances equal to the reactances of the associated condensers when free of liquid. Each condenser and reactance form one arm of a bridge whose other arms are formed by resistances 54, 68 across the secondary of a transformer 36 fed with A.C. from a generator 16. The bridge output is taken from 62 and earth, amplified at 81, applied to a phase-sensing circuit 89, 94, to produce D.C. voltage between 104, 112 whose sense is dependent on the direction of unbalance of the bridge. This voltage is applied to the valves 108, 116 whose cathodes are fed with A.C. from transformer 126, so that the output in transformer 138 is A.C. of phase dependent on the direction of unbalance of the bridge this is applied to winding of motor 72 which adjusts tapping 66 on resistance 68 to balance the bridge and actuate an indicator 156. It is shown in the Specification that the output is independent of the dielectric constant of the liquid.  In Fig. 3, the inductances 232, 238 are in series with the associated capacitances 202, 204, and each series combination is shunted by a capacitance 236, 242, equal to the capacitance of the associated capacitance 202, 204 when free of liquid plus the stray capacitance due to leads &c. The output indicated at 298 is independent of the dielectric constant of the liquid and is compensated for the stray capacitance. In Fig. 4, the series combinations of inductance and capacitance and shunting compensating capacitance is fed with A.C. from 302 and a valve voltmeter 318 indicates variation of the potential at 316 which is a function only of the quantity of the liquid. A similar circuit may employ the parallel combinations of Fig. 1, Fig. 5 (not shown). If the total amount of liquid in several tanks is to be measured, additional measuring condensers may be provided, the value of the inductance being appropriately adjusted. If the dielectric constants of the liquid in the several tanks is not the same, several comparison condensers may be employed. If the crosssection of the tank is not uniform, linearity of indication can be obtained by tapering the resistance 68 Fig. 1 or by shaping the plates of the measuring condenser. Compensating for density variation due, e.g. to change of temperature. In Fig. 2, floats 160, 162, 164, 166 are designed so that as the density increases, each in turn rises to close contacts short-circuiting part of resistance 52. As a consequence the voltage supply across the arm 54 of the bridge is varied and the balance point adjusted to compensate for change in density of the liquid. Specification 684,348 is referred to.