According to the "International Vocabulary of Metrology" (VIM), measurement uncertainty is a "non-negative parameter that characterizes a range of values attributed to a measurand". We have not to confuse the error with measurement uncertainty; the error is defined as the difference between a single test result and the known value of the measurand, the measurement uncertainty is, on the contrary, an estimate formed by several sources, each of which is called the "component of uncertainty" and it has the form of an interval. We usually use the expanded uncertainty U that is U = K*u, where K is the coverage factor 2 which gives a level of confidence of approximately 95%, as prescribed by EA (European Accreditation). The coverage factor K must always be indicated in the laboratory test report, so that it can be traced to the composite type uncertainty of the measured quantity.
The factors that affect measurement uncertainty can be summarized as follows:
· Measuring instrument
· Environmental conditions
· The user
· Measurement method
· The measurand
Metrological characteristics of measuring instruments:
To know metrological characteristics of instruments is essential for a company, for a correct choice of the equipment required for product measurement requirements. Metrological characteristics have an immediate influence on product measurement; they are essential for checking the instrument errors and for a correct estimation of measurement uncertainty balance.
The main metrological characteristics are:
• Unit of format
• Resolution, repeatability, reproducibility, precision
The errors attributable to these characteristics contribute to the estimation of measurement uncertainty.