METROLOGICAL FEATURES OF MEASURING INSTRUMENTS
Unit of format:
The unit of format for analog instruments is the smallest division on the scale, while for digital instruments is the least increasing of significant digit. The unit of format is crucial to the introduction of another metrological feature, that is the resolution.
The resolution of an instrument is the smallest valid variation, to which a certain value can be attributed. It follows from the definition that, in the case of digital instruments, the resolution coincides with unit of format, while in analog instruments the resolution can be better than unit of format. It depends on the user’s reading capacity and the error that he can commit, so-called error of “parallax”.
Repeatability, reproducibility, precision:
The repeatability is the attitude of an instrument to provide reading values a little different between them, in consecutive readings on the same measurand, by unified procedure by the same operator, under the same conditions as influence quantities. We talk about reproducibility if these measurements are made on the same sample, but changing one or more conditions such as different locations, operators, measurement methods, measurement times and different measuring instruments. Therefore, reproducibility is more related to measurement method than the repeatability, which takes into account the single instrument and the user performing the measurement.
VIM combines the concept of repeatability and reproducibility with “measurement accuracy” defined as: “degree of concordance between indications or measured values obtained from a number of repeated measurements of the same object or similar objects, performed under specified conditions.” If specified conditions are those of repeatability, then precision becomes repeatability, but if conditions are those of reproducibility, then precision becomes reproducibility.
Measurement error (indicated in Laboratory Test report):
The measurement error is very important for a laboratory test report, the VIM defines the measurement error as “measured value of a quantity minus a reference value of the quantity”, therefore is the difference between the measured value and the measured value of standard sample (indicated on primary certificate). This difference is detected by instrument calibration and reported on laboratory test report.
The measurement error is not a contribution in evaluating the measurement uncertainty.