Fluke lesmsc92 free download

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Reprinted from the Proceedings of the Measurement Science Conference Symposium and Workshop, January 1992 A Wheatstone Bridge for the Computer Age Les Huntley Metrology Manager John Fluke Mfg. Co., Inc. Everett, Washington 98206 ABSTRACT Guildline's 9975 Current Comparator Resistance Bridge [3, 4] provides excellent accuracy for values smaller than 10 kilohm, but is a manual instrument which requires a skilled operator. (Another system for making automated measurements of resistance in this range is presented at this conference [5].) Furthermore, while it excels in the measurement of low-valued resistors, its performance degrades rapidly at values above about 100 kilohm. Instruments for measuring high valued resistors automatically and with the desired accuracy were not available at the time this effort was undertaken. This paper describes a modification of the familiar Wheatstone bridge circuit which has the desired qualities. INTRODUCTION Application of Process Metrology [1, 2] to the calibration control of the 5700A Multifunction Calibrator forced Fluke's Standards Laboratory to find means for rapidly and automatically verifying all ranges, levels and functions of the instrument. Since the calibrator contains internal resistance standards ranging in value from 1 ohm to 100 Megohm, and the internal metrology of the 5700A makes it possible to measure these resistors with very good accuracy, it was essential that the Standards Laboratory find means for making these same measurements with even greater accuracy. The Wheatstone Bridge, the venerable workhorse of the resistance measuring laboratory, has seemed to be a candidate for inevitable replacement by modern, computer controlled measuring systems. A recent revision of the traditional circuit, in which two of the resistors are replaced by precision direct voltage calibrators, admirably adapts the Wheatstone bridge to the computer age. Addition of a bus-controlled switch for connecting standards and unknowns makes this a completely automated resistance measuring system, and provides means for comparing 4-terminal resistors. This paper describes the implementation in the Fluke Primary Standards Laboratory and presents results for measurement of resistors in the range 100 ohms to 290 Megohms. THEORY The Wheatstone bridge circuit (Figure 1) has been widely used for many purposes. Its equations are simple: il = V/ (R1 + R2) i2 = V/ (R3 + Rx) At null, iIR2 = i2Rx from which, substituting the currents, Rx/R2 = R3/Rl In a common c