Calibration of the measurement channels of measurement systems following graduation of the channels
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CALIBRATION OF THE MEASUREMENT CHANNELS OF MEASUREMENT SYSTEMS FOLLOWING GRADUATION OF THE CHANNELS
A. A. Danilov and Yu. V. Kucherenko
UDC 621.317
Questions related to determining the parameters of the transfer function of the measurement channels of measuring systems, insertion of corrections, and subsequent estimation of the metrological characteristics of the channels are considered. Keywords: measuring systems and channels, metrological characteristics, transfer function.
Graduation of the transfer function of the measuring system followed by insertion of correction factors is performed in the course of periodic testing of the state of metrological assurance of operating measuring instruments for the purpose of increasing their precision. In those cases in which graduation of the measuring system (Fig. 1) is one of the stages in calibration of the system (or of testing, which is essentially the same thing as calibration except for deciding whether the metrological characteristics correspond to previously established standards), it is necessary to deal with certain features of the metrological assurance of the measuring system. The chain of successively executed procedures is identified by boldfacing in Fig. 1 and these will be considered below. It is known [1] that graduation and calibration of a measuring system should be conducted with the use of different (at least two) copies of the working standards. One example of the relatively few measuring systems for which such a procedure has been implemented is that of electronic balances, the delivered complement of which includes graduated balance scales. The metrological characteristics of electronic balances are determined by means of a balance scale from a different set. Note that besides the use of different copies of a standard, several alternative ways of employing one and the same copy of a working standard for both graduation and calibration of measuring systems may be recommended [1]. Unfortunately, in actual practice such a method of cross-checking is usually not used, which reduces the reliability of calibration and testing of measuring systems. In fact, one and the same copy of a working standard serving both for graduation and for calibration may yield an overly optimistic result for the metrological characteristic of a calibrated measuring system if point rather than interval error estimation is employed. It is precisely for this reason that it should not be forgotten that the following estimates must be included among the metrological characteristics of measuring systems for which graduation is carried out: • nonexcluded systematic error; • standard deviation of random error; and • variations. Moreover, an estimator of the nonexcluded systematic error of a measuring system should obviously include the same type of error of the measuring system (this is sometimes forgotten). If graduation and calibration of the measurement channels of measuring systems are to be performed on complete units, they will most likely be performed under the effective
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