GB689747A - Improvements relating to indicating, recording and control apparatus - Google Patents

Abstract

689,747. Electric indicating systems. FIELDEN (ELECTRONICS), Ltd. Feb. 6, 1951 [Feb. 27, 1950; Sept. 1. 1950], Nos. 4940/50 and 21583/50. Class 40 (i). A liquid level varied in accordance with a variable such as fluid-fllow, pressure, specific gravity, temperature &c. is measured by moving one of the electrodes of a capacitor in accordance with the level to permit or block oscillations in an electric circuit whereby means are operated to restore the capacity to a predetermined value and at the same time actuate an indicator or recorder. Fig. 1 shows a manometer for measuring differential pressure in which variations of the liquid level vary the capacity of a ring condenser 2, the capacity variations controlling a two phase motor 4 which rotates a lead screw 3 in the corrective sense to return the ring electrode 2 to its neutral position relative to the liquid surface and at the same time actuate an indicator or a recorder 7, 8. In another embodiment, Fig. 2, the movable electrode 12 is carried by a float 13 in one arm of the manometer tube and the motor 4 raises or lowers a plunger 16 in the other tube arm to bring the float 13 and hence the electrode 12 back to its neutral position. In this case a relay in the motor control circuit may give an alarm in response to rapid changes of the variable in question. In another embodiment the movable electrode is placed in a glass or other non-conducting tube immersed in the liquid, and actuated as above described. Temperature may be measured by using a mercury thermometer, and pressure air-speed, flow, specific gravity &c. may be measured by using a U-tube containing mercury. Motor control circuit, Fig. 3. The grid cathode circuit of an oscillator 18 includes a series connected capacitor 19 and inductor 20, in parallel with the series connected measuring capacitor 2 (or 11, 12, Fig. 2) and inductor 21. This impedance bridge arrangement is such that the valve will oscillate only when the capacitor 19 is greater than the measuring capacitor. The oscillator output is rectified by the grid-cathode circuit of a thyratron 22, so that during the positive half-cycles of the supply 25 when oscillation occurs the condenser 24 is charged negatively to prevent the thyratron firing during the alternate half-cycles. When oscillator 18 is blocked and the thyratron conducting the variable phase of the two phase motor 4 is fed in series with the thyratron anode current and the motor runs in one direction to drive the ring electrode 2, Fig. 1, away from the liquid surface. After a predetermined separation of the electrode 2 from the liquid surface, the valve 18 oscillates and blocks the thyratron. The phasg of the current in the variable phase of the motor is reversed, being now supplied in series with the oscillator anode current. The motor therefore drives the electrode 2 in the reverse direction. The motor thus oscillates the moving electrode about the neutral position.