Precision, Classes of

an overall characteristic of measuring devices that is used as an indicator of the limits established by state standards for systematic and random errors, as well as other parameters affecting precision. For example, for end-to-end measurements of length, classes of precision characterize the limits of allowable deviations from the nominal dimension and the effect of temperature changes, as well as the permissible lack of parallel orientation of working surfaces and their deviations from an ideal plane surface. The introduction of classes of precision facilitates the standardization of measuring devices and their selection for measurements with the required degree of precision.

Because of the variety of measuring devices and quantities being measured, introduction of a uniform method for defining the limits of allowable errors and of a uniform set of designations for classes of precision is impossible. If the limits of error are expressed as reduced error (as a percentage of the upper measurement limit, the range of measurement, or the length of the instrument’s scale), as well as relative error (as a percentage of the true value of the quantity), then the classes of precision are designated by numbers corresponding to the magnitude of the error. For example, class 0.1 corresponds to an error of 0.1 percent. Many indicating instruments (ammeters, voltmeters, manometers, and so on) are standardized according to reduced errors, expressed as a percentage of the upper limit of measurement. The following series of classes of precision is used in these cases: 0.1., 0.2, 0.5, 1.0, 1.5, 2.5, and 4.0. The classes of precision are circled if they are standardized according to relative error.

For weights, measures of length, and instruments for which the error limits are defined in units of the quantity being measured, the class of precision is usually designated by numbers (1st, 2nd, and son on) in order of decreasing precision. The word “precision” is usually omitted in designating a specific class. Series of classes of precision, their designations, and the corresponding requirements with respect to the measuring devices are part of GOST (the All-Union State Standard) for the various types.