Constant sensitivity bridge for thermistor thermometers

Abstract

Despite the variation of the incremental temperature resistance sensitivity of thermistors, temperatures can be measured in a Wheatstone bridge circuit having constant temperature deflection sensitivity if the bridge is fed from a variable voltage supply. In the circuit described the compensation can be automatically set when the bridge is balanced, both operations being performed simultaneously on one and the same variable resistor, which is in series with a thermistor in one arm of a Wheatstone bridge and is part of a fixed resistor, the remainder of which is in series with the bridge voltage supply. As a result, the voltage across the bridge changes with the balancing resistor, so that the rate of change with temperature of the off-balance current is independent of the temperature at which the bridge is balanced. The resistance values of the other three bridge arms must be suitably chosen to achieve this automatic compensation, and a method is described for their evaluation. In a typical example, full-scale deflection was obtained for a change of 0.315 degC, i.e. a sensitivity of 2.45 μA deg-1, which was within 1% of the calculated value and constant to 1% over the working range of 15 degC.