Patent GB-826969-A (GB)
Title: Improvements in circuits for obtaining indications dependent on capacitance
Inventor: GREATOREX GORDON VICTOR
IPC: G01F23/26
Filed: 1956-06-19
Abstract: 826,969. Electric measuring systems. SIMMONDS AEROCESSORIES Ltd. June 14, 1957 [June 19, 1956], No. 18948/56. Class 40 (1). [Also in Group XXXVI] An electrical circuit for indicating small capacitance changes, particularly for use in a liquid contents gauge, comprises, Fig. 1 of Provisional Specification, a pair of bridges in each of which the centretapped secondary 5, 21 of a transformer 1, 15 provides a pair of inductive arms 5a, 5b and 21a, 21b, and capacitators 8 (capacity can depend on liquid level), 9, 11 and 23 (can be totally immersed in liquid), 24, 25 provide the other pair of arms, an electrical connection between the centre taps 6 and 22 (e.g. by earthing), a variable rheostat 18 connected across the primary 16 of the transformer 21 and arranged for connection to the same supply 4 as the transformer 1, a servomotor 37 arranged to operate the wiper arm 17 of the rheostat 18, an amplifier 35 connected through the transformer 33 to the points 30, 31 of the bridges opposite the centre taps 6 and 22 to supply the servomotor, and means 39 to indicate the position of the servomotor, the arrangement being such that when the bridges are unbalanced the servomotor operates the wiper arm 17 in a direction to balance the bridges. The capacitators 11 and 25 represent the dead capacitance of the liquid tank and leads. The rheostats 7 and 3 are adjusted to give correct readings on the indicating means 39 for " empty " and " full " conditions. In order to obtain the best compensation for changes of dielectric constant of the liquid the ratio of the capacitances 11 and 8 is made equal to the ratio of the capacitances 25 and 23. A float-operated device 45 mechanically coupled to the wiper arm 46 of the rheostat 19 to vary the resistance thereof in proportion to the specific gravity of the liquid being gauged, comprises a cylindrical metal float 47 carried by equal rods 48, 49 each pivoted on the float at one end and rigidly secured at the other end to the free end of bellows 50, 51, an extension 52 of the rod 49 operating the wiper arm 46. This arrangement enables the indicator 39 to be calibrated in terms of mass of liquid. Volume indications can be obtained by removing the devices 19 and 45. In the arrangement shown in Fig. 1 of the Complete Specification rheostats 61 and 62 shunt a few central turns on the secondaries 5<SP>1</SP> and 211 and are connected by the wipers 63 and 64 to earth for correct " empty " indication, and the centre points 301 and 31<SP>1</SP> are connected together to earth through the transformer primary 321. A rheostat 3<SP>1</SP> allows for adjustment of the circuit for correct " full " indication. Switches 65, 66 are connected to the primaries 16<SP>1</SP>, 2<SP>1</SP> of the bridges, each bridge being set up separately. The specific gravity adjustment means 19<SP>1</SP>, 25<SP>1</SP>, 461 are associated with the first transformer 1<SP>1</SP>, the rheostat 19<SP>1</SP> being connected in parallel with the primary 2<SP>1</SP>. In modifications the specific gravity adjustment means 19, 45, 46 or 19<SP>1</SP>, 45<SP>1</SP>, 461 can be replaced by a resistor varied with specific gravity, e.g manually set, or by the device described in Specification 786,113, [Group XV]. In order to facilitate loading and unloading of a tank the points 70, 71, 72 of the arrangement shown in Fig. 2 are connected to opposite sides of the rheostat 18 or 18<SP>1</SP> and to the wiper 17 or 17<SP>1</SP> respectively, either rheostat and wiper of the two circuits described above being designated 1811 and 17<SP>11</SP> respectively. The points 70, 71 are connected to a rheostat 73 which is similar to the rheostat 18<SP>11</SP> and has a wiper 74 connected to a relay 75 which is connected to the point 72 and closes a contact when the potential across it is zero. The wiper 74 is mechanically connected to a hand wheel 76 and a pointer 77 moving over a dial 78 marked in mass of liquid. The pointer 77 is set at a desired reading and filling or emptying of the tank begun. When the desired quantity of liquid is contained in the tank the potentials across the relay 75 become equal and the contact therein is closed, such closure being arranged to operate a lamp or stop a pump. Fig. 3 shows an arrangement for providing an indication corresponding to that of the indicator 39 or 39<SP>1</SP> at a remote position. Points 80, 81, 82 are connected to opposite sides of the rheostat 18 or 18<SP>1</SP> and to the wiper 17 or 17<SP>1</SP> respectively, which are designated 18<SP>111</SP>, 17<SP>111</SP>. The points 80, 81 are also connected to a rheostat 83 having a wiper 84 mechanically driven by a servomotor 85 controlled by the output from an amplifier 86 fed with the potential difference between the wipers 17<SP>111</SP> and 84. The motor 85 is mechanically coupled to an indicator 87. Either of the two circuits described with reference to the Figs. 1 of the Provisional and Complete Specifications can be used to provide indications due to small capacitance changes in general, e.g. by using fixed capacitators 8 and 11 and forming the unknown capacitance 23 or 23<SP>1</SP> by one fixed and one moving electrode, accurate distance measurements being obtained by suitable calibration of the indicator 39 or 39<SP>1</SP>.