GB764686A - Improvements in and relating to measuring bridges incorporating thermally variable resistors - Google Patents

Abstract

764,686. Electric indicating systems. ENGLISH ELECTRIC CO., Ltd. Oct. 22, 1954 [Oct. 30, 1953], No. 30036/53. Class 40 (1). [Also in Group XXXVI] A thermistor bridge network comprising two four-armed bridges containing each three linear resistors and a thermistor with a diagonal unbalance detector is energized from an adjustable power source, and the respective thermistors are supplied with heat energy from a first independent source and a second independent source; the heat energy from the first source being measurable in terms of that supplied from the second by adjusting the network for balance of both bridges in absence of heat from the independent sources, unbalancing the first bridge by supplying heat to its thermistor from the first source, rebalancing such bridge by adjusting the energizing electric power source, and restoring balance of the second bridge by varying the heat energy supplied to its thermistor from the second source as a function of the energy supplied from the first source. Fig. 1 shows bridges 11, 12 respectively comprising thermistors 18, 22 interconnected with resistances 15, 16, 17 and 19, 20, 21 which are energized from a source of direct E.M.F. 13 over variable resistance 14 and have balance detectors 23, 24 across the remaining diagonals. Initially, the bridges are balanced by variation of the resistance 14 in series with the energization and by varying resistance arm 17 of bridge 11, and thereafter electric power P to be measured is introduced to heat thermistor 22, after which resistance 14 is adjusted to rebalance bridge 12. Bridge 11 is then rebalanced by readjusting resistance 17 (which may be calibrated as a measure of the power input) or by introducing a known power W to P heat thermistor 18. It is shown that - is a W stated function of the bridge resistance arms and independent of the thermistor characteristics. In Fig. 2 resistance arms 15, 16, 19, 20 are replaced by equal common resistance arms 25, when it is shown that P/W = R17/R21. Fig. 3 shows a thermistor bridge comprising equal resistance arms 26, and a thermistor 29 in series with a resistance 27 and equal series blocking capacitances 28, 30 energized from an oscillator 31; the bridge unbalance signal being applied to adjust the oscillator output for bridge balance in the absence of heating power supplied to the thermistor 29. An independent thermistor bridge circuit is completed by adding arms comprising resistance 36 and variable resistance 40 in series with thermistor 38 energized from the oscillator over equal blocking condensers 37, 39, whose unbalance signal energizes amplifier 33 and phase-sensitive rectifier 34 to develop an output potential E dissipating power in the thermistor over resistances 41, 42 and measurable by voltmeter 35. The output of the rectifier is decoupled to earth over condenser 43 while the loading on the thermistor arm is compensated in the bridge by resistance 44 in series with earthed condenser 45. Resistance 40 is adjustable for zero balance, at which potential E is maintained constant. A power P to be measured is then applied to heat thermistor 29, and the oscillator excitation is varied to restore balance of the first bridge, after which the value of E changes to restore balance of the second bridge. It is shown that, where W is the power dissipated in thermistor 38, R 41 +R 42 = n R 36 , and the bridges are initially balanced, The two bridges may be energized from the oscillator over separate input transformers, and the apparatus may be incorporated with a Pirani gauge for determining low gas pressures by introducing the measuring thermistor into a tube containing the gas, whereby a balance attained under atmospheric conditions is disturbed by the additional thermistor heating by the excitation current due to the low conductivity of the gas, whose pressure the system may be calibrated to indicate.