Uncertainty and Calibration of Instruments

Abstract

Quite often measuring instruments are received for calibration. Scale of the measuring instrument is calibrated at few points only. The correction or the value of standard input is assigned at those selected points of its scale and uncertainty of measurement is also stated at those points only. In most cases, the values of the standard input versus scale readings are given at the selected points. When an instrument is used in the field, the scale readings are recorded, which in general, may not be the same points at which the instrument was calibrated. The correct value is obtained from the corrections at the two nearest calibrated points just by linear manipulation. In this method only small interval containing the observed reading is considered which may not be always justified. However, it is advisable to consider all the points at which calibration is carried out. It is, therefore, necessary that a mathematical relation between the scale reading and standard input is given. So that the user can substitute the value of observed scale reading in the relation and get the value of the input to the instrument. For example an ammeter with range of 100A and with 100 divisions on the scale is calibrated normally at four points, say at 25A, 50A, 75A, and 100A graduation marks, but in practice the instrument may read 60A; then naturally the user would like to know as to what will be the real value of the current passing through it, when the instrument is reading 60A. This chapter is mainly based on my research paper [1] published in MAPAN – Journal of Metrology Society of India, in 1999.