Measuring Device Error

the deviation of a metrological property or parameter of a measuring device from the nominal property or parameter affecting the accuracy of the measurement results obtained with the device. Those components of the errors that are dependent on the measuring-device error are called instrumental errors. Measuring-device errors are expressed as absolute, relative, or reduced errors. Absolute errors are expressed in the units of the measured quantity. Relative errors are fractions or percentages of the quantity. Reduced errors are percentages of the upper measurement limit, of the measurement range, or of the length of the scale.

Measuring-device errors that occur under normal usage conditions of the devices are referred to as fundamental. When quantities influencing the measurements, such as temperature or electric-current frequency, deviate from their normal values, the resulting errors are called additional errors. Limits of permissible error are established for each type of measuring device. These limits determine theclass of precision of the device. When constant quantities are measured, steady-state readings are used, and the measurement results are affected only by static measuring-device errors. When variable quantities are measured, dynamic errors are added to the static errors, and the total error is increased.

Measuring-device errors can be systematic or random. Systematic errors remain constant or change in a regular manner, whereas random errors vary in a random way. For example, systematic errors can be caused by an incorrectly calibrated scale or an inaccurately adjusted set of measures, such as balance weights. Friction between moving parts in an instrument can result in random errors. Systematic measuring-device errors can be eliminated by introducing corrections or by multiplying the readings by correction factors.