Patent GB-796874-A (GB)
Title: Improvements in or relating to electrical measuring apparatus
Inventor: GREENSHIELDS FRANCIS
IPC: G01K7/02
Filed: 1954-12-10
Abstract: 796,874. Voltage measurement. METROPOLITAN WATER BOARD. Dec. 12, 1955 [Dec. 10, 1954], No. 35854/54. Class 37. [Also in Groups XXXVIII and XL (b)] Apparatus for measuring the magnitude and polarity of D.C. signals as a measure of physical variables and derived from, e.g., thermocouples and photo-cells comprises a converter producing an A.C. corresponding in magnitude and phase sense to the D.C. amplitude and polarity, a motor driven by the A.C. signals to produce a rotary motion dependent thereon in velocity and sense, which drives an indicator disc for stroboscopic observation and a tachometer generator whose output deflects a chopper-bar centre zero galvanometer arranged to close contacts at a predetermined deflection or deflections to operate an alarm circuit. A number of pairs of thermocouples disposed, e.g. at different levels of a water supply reservoir are respectively connected in series opposition to corresponding pairs of contacts 5, 6, &c. of banks 7a, 7b of a rotary selector switch stepped by a magnet energized periodically by circuitbreaker 36 and switch 35 opened and closed by motor-driven cam 34, the arms of which are connected through a converter 11 comprising, e.g., a vibratory switch energizing alternate primary halves of a transformer to produce an A.C., representing in amplitude and phase angle the magnitude and sense of the thermocouple output and thus the temperature difference between the couples. After variable amplification the signal energizes winding 14 of bi-phase motor 15 whose other winding 16 is energized from a quadrature source to drive through gearing 18 an indicator disc 17 stroboscopically observable for speed determination, and a D.C. generator 19 having a linear output/speed characteristic. The latter energizes through variable resistance 20 a centre zero galvanometer 21 having a clock-controlled chopper-bar arrangement 24 effective to periodically close one or more pairs of critically positioned contacts 23 when the needle 21b lies thereupon, and complete an alarm circuit through relay 26 controlling mercury switch 27 to energize a lamp 28, a bell 29, and a control mechanism 30 at a selected value or values of the temperature difference of the thermocouples. The alarm and control circuit operates when the temperature difference is such that a rapid inversion of water levels is likely, and initiates, e.g., chlorination of the outflow to purify stagnant water rising from the lower depths. Bank 7c of the rotary switch selectively energizes lamps 37 to 40 to indicate the appropriate pair of thermocouples in circuit. The D.C. generator output may energize a recording galvanometer or a computer, and the thermocouples may be applied to other process apparatus (e.g. distillation columns and cereal bulk storage) to operate indicator, alarm, and/or control circuits at critical temperature differences. The thermocouples may be replaced by opposedly connected photo-cells illuminated by a prism divided light beam, so as to operate the indicator alarm and/or control circuits on reduction of the intensity of either beam by the interposition of solid, liquid, or gaseous matter.