The precision or accuracy of the measuring instruments is a static feature that identifies the degree of agreement between the indication of the instrument and the characteristics of the measurand in measurement or monitoring.
By measurement precision we mean:
- according to ISO-IMV (International Metrology Vocabulary): “…closeness of agreement between indications or measured quantity values obtained by replicate measurements on the same or similar objects under specified conditions”;
- according to IEC-IEV (International Electrotechnical Vocabulary): “..quality which characterizes the ability of a measuring instrument to provide an indicated value close to a true value of the measurand” (Note: call in this case, however, not Precision but Accuracy);
- or we could deduce the following practical definition from the previous ones:“…by testing a measuring instrument under conditions and with specified procedures, the maximum positive and negative deviations from a specified characteristic curve (usually a straight line)”.
Therefore, the concept of linearity is also inherent in the measurement precision term (which is currently very limited in the digital instrumentation), while the concept of hysteresis is not included (although this is considered, as it is included within the maximum positive and negative deviations found).
Furthermore, the concept of repeatability of the measurement is not included (which is instead considered in the case of verification of precision over several measuring cycles.
Therefore, in the practical verification of the precision of the measuring instruments with a single up and down measurement cycle (generally conducted for instruments with hysteresis, such as, pressure gauges, pressure transducers, load cells, etc.) a calibration curve is obtained of the type found in Figure 1, where we can deduce the concept of tested accuracy (accuracy measured) that must be included within the so-called nominal accuracy (accuracy rated) or the limits within which the imprecision of an instrument is guaranteed by its specification.
The metrological confirmation is the verification that the measuring instrument keeps the accuracy and uncertainty characteristics required by the measurement process over time.
Sometimes this concept of imprecision for some common types of instruments (such as gauges, resistance thermometers, thermocouples, etc.) is also called precision or accuracy class, which according to the International Reference Vocabularies ISO-IMV and IEC-IEV : “class of measuring instruments or measuring systems that meet stated metrological requirements that are intended to keep measurement errors or instrumental measurement uncertainties within specified limits under specified operating conditions” (ie, the accuracy measured must be less than accuracy rated: See also Figure 1).
Figure 1 – Exemplification of measurement accuracy concepts