Errors and uncertainties of imbalanced bridge-circuits as primary converters for RTD sensors

Abstract

The original way of evaluating accuracy of the resistance-to-voltage converter (imbalanced bridge-circuit) is presented. It is important because this converter is used as the input stage of industrial and laboratory temperature measurement systems. From circuit theory point of view it is the two-port circuit of type X with four variable resistances and it is supplied by a current (or voltage) source. Formulas of transfer coefficients are presented in general forms and in relative units. Moreover, two particular cases of this converter used in temperature measurements, i.e. with one or two RTD elements, are considered in detail. Error propagation functions are determined in two-component form, i.e. separately for both initial value and relative increment of a transfer coefficient (dependent on temperature). Values of limited errors and expanded uncertainties are calculated for a few variants of this converter with the Class A and B industrial Pt100 sensors. Formulas and example results allow to determine accuracy measures for the selected configuration of bridge-circuit and to compare with other configurations. Null adjustment, inside and outside the bridge, is also considered.