The most commonly and most frequently measurable variable in industry is temperature. Temperature greatly influences many physical features of matter, and its influence on e.g. quality, energy consumption and environmental emission is significant. Temperature, being a state of equilibrium, makes it different from other quantities. A temperature measurement consists of several time constants and it is crucial to wait until thermal equilibrium is reached before measuring. Metrology contains mathematic formulas for calculating uncertainty. The polynoms are specified in ITS 90 table (International Temperature Scale of 1990). For each measurement, a model that includes all influencing factors must be created. Every temperature measurement is different, which makes the temperature calibration process slow and expensive.
While standards determine accuracy to which manufacturers must comply, they nevertheless do not determine the permanency of accuracy. Therefore, the user must be sure to verify the permanency of accuracy. If temperature is a significant measurable variable from the point of view of the process, it is necessary to calibrate the instrument and the temperature sensor. It is important to keep in mind an old saying: all meters, including sensors, show incorrectly, calibration will prove by how much.