Uncertainties in calibration of long stem platinum resistance thermometer according to the ITS-90.

Thermodynamic temperature is a macroscopic measure of the degree of particle agitation in a system. The higher the temperature, the more intense the microscopic agitation of the particles. It is defined on the basis of the first and second principles of thermodynamics. It has a universal definition and coincides with the absolute temperature measured with perfect gas thermometers (see "The International Temperature Scale: ITS-90" [R 2 510] ).

The unit of thermodynamic temperature, a fundamental physical quantity, is the kelvin, symbol K. The kelvin is one of the seven base units of the International System of Units (SI).

Manufacturers generally seek to reproduce a temperature, rather than actually knowing the thermodynamic temperature involved in their manufacturing procedures. These considerations led to the introduction in 1927 of a practical temperature scale based on repeatable and easily identifiable physical phenomena. This scale was revised in 1948, 1968, 1976 and 1990, so that the measurements carried out in this scale are increasingly in close agreement with thermodynamic temperature values.

Rather than measuring thermodynamic temperature, we'll refer to this as "locating the temperature on the scale". The scale currently in use is the 1990 International Temperature Scale (ITS-90). It is based on :

• a series of defining points based on pure-body phase transitions;

• specified instruments with parameterized interpolation or extrapolation formulas.

To meet the needs of industry, calibration laboratories generally use simplified techniques that enable them to approach the international temperature scale. However, these laboratories are becoming interested in the ITS-90 for a variety of reasons. Laboratories wishing to offer their customers low calibration uncertainties are looking to limit the impact of their standard's connection uncertainty. If the metrological characteristics of the standard allow (quality of the elements used in its construction, mounting technology, purity of the platinum wire making up the sensitive element, etc.), the most appropriate calibration method is based on the use of the fixed points and interpolation functions defined in the text of the ITS-90.

Many laboratories also use a water triple point (0.01°C) or a gallium fixed point (29.764 6°C) to monitor the drift of their standard between two connections.

Finally, some laboratories wish to perform calibrations to ITS-90 over limited temperature ranges.

The aim of this article is to provide...