Precision class 0.015 dc comparator bridge

Our industry produces a large number of precision wire-wound resistors, for instance, types MVSG, $5-5, $525, S5-42, etc. With a tolerance for deviations from their nominal values of 6 = i(0.03-0.1)%. The instruments most suitable with respect to precision and efficiency for testing these resistors consist of class 0.02 type R337 and class 0.005 type ShCh-30 digital ohmmeters in conjunction with resistance boxes types R32q and R326 (with corrections). However, these bridges are expensive, in short supply, and require highly-trained personnel for their testing and repai~. The measurement error of these ohmmeters is determined virtually by the precision class of the reference resistance box. By selecting appropriately the ratio-arm resistances of the dc bridges shown in Fig. 1 and employing the suggested method for choosing its supply voltage, it is possible to assemble from the components mass produced by our industry a comparison bridge for testing resistances in the range of 0.1-100 ka with an error not exceeding 0.015%. The bridge ratio-arm resistances R 1 and R z consist of resistors typeR331 with nominal values of either 1000 (for the measurement range of 0.1-10 k~) or 10000 ~ (range of 1-100 k~). The actual values of the resistors should not differ by more than ~: 0.001% with their temperature coefficients of resistance being close to each other. The resistors are selected by means of their certified records obtained in state testing. Such a selection from 25-30 available resistors with the above nominal values does not present any difficulties. The resistance R consists of the class 0.01 resistance box type R327. In measuring on this bridge the resistor Rx made with a tolerance of ~0.05% is balanced with adecade resistance box by setting it to the nominal value Rxn of the resistor. Owing to the difference between Rx and Rxn , the bridge will be slightly unbalanced. It is known [1] that the unbalanced voltage in this case (when the above-mentioned requirements with respect to the precision of R, R 1, and R 2 are met) can be determined with sufficient precision from the expression