Patent GB-810607-A (GB)
Title: Improvements in or relating to float-operated capacitance device
IPC: G01N9/18
Filed: 1956-01-05
Abstract: 810,607. Electric tests. LIQUIDOMETER CORPORATION. Jan. 5, 1956 [Jan. 19, 1955], No. 411/56. Class 37. [Also in Group XXXVIII] The weight of a liquid in a tank is determined by means of a capacitor the dielectric of which is constituted by the level of liquid in the tank and by further density-sensitive capacitors each varied by a float responsive to a particular density of the liquid and each of the densitysensitive capacitors consisting of a pair of component capacitors which are simultaneously varied in opposite senses by the movement of the float one of the component capacitors having the liquid between its plates. The capacity of the arrangement is proportional to (K - 1), where K is the dielectric constant and their use will be explained subsequently. The construction of one of the density-sensitive capacitors is shown in Fig. 3. It consists of two housings 91, 92 mounted on a base 30 beneath the liquid level. Each housing contains two outer fixed condenser plates 42, 48 and 86, 88, a central earthed shield 38, 28 and two coupled moving plates 32, 34 and 72, 74. Connections are made to the outer plates in each housing to avoid connection to the moving plates and each housing thus contains two series condensers. The plates are Maltese cross-shaped and the arrangement is such that when one capacitor is in its position of maximum capacitance the other is in its minimum. Housing 9 is sealed to exclude the liquid which enters housing 92 freely through holes 98. The moving plates in housing 9 which is of non-magnetic material are rotated by magnets 58, 62 carried by a rotatable member 64 to which the float is attached by arm 12, the magnets acting on magnetic material 54 which joins the movable plates. This yoke drives the plates in housing 92 directly by couplings 84 moving in arcuate slots 85. The two component capacitors are connected in parallel so that the change in capacitance is (C1 - C2) (K - 1), where Cl is the minimum and C2 the maximum capacity of each in air. A system for indicating directly the weight of liquid in a tank is shown in Fig. 5 in which 112 is the conventional measuring capacitor of which the liquid constitutes the dielectric, 122 is a capacitor of value equal to the measuring capacitor 112 when empty, 120 has the same value as 122, 130 is a capacitor introducing a compensation for the value of the dielectron as described in British Patent 693,600 and Ca, C S is a series of densitysensitive capacitors of the type described above and each arranged to charge from minimum to maximum value of capacity at a particular value of density as detected by the float 10. The centre-tapped secondary of mains transformer 100 feeds anti-phase currents through the measuring capacitor 112 and compensating capacitor 122 to line 116 and the upper half of the secondary feeds a potentiometer 106 from the slider of which is connected to line 116 through capacitor 120 and, through a phasereversing transformer 124 to the density and dielectric compensating capacitors Ca, Cs and 130. If the sum of the currents reaching line 116 is not zero a signal is fed to amplifier 136 which drives servomotor 132 to operate a counter 152 to indicate the total weight and rebalance the circuit by adjusting potentiometer 106. The condition of balance of the network without the float-operated capacitance is Co + CoX(K- 1) + mCo - Co - mKCo = 0, where Co is equal to the capacitance of measuring capacitor 112 when the tank is empty, X is the proportion of the condenser that is filled, K is the dielectric constant and m is the proportion of the potentiometer between the slider and ground. Thus the arrangement is balanced when m = X but if the reading of counter 152 is to be proportional to weight, m must equal Dx, where D is the density of the liquid. This occurs if mKCo in the above expression becomes This change is achieved by the addition of the float-operated capacitors Ca and Cs and the Specification shows that if the last condition is to be satisfied where there are N float-operated capacitors and n are in their maximum position where Cf is the change in capacitance of between maximum and minimum of each float-operated capacitor.