In thermodynamic systems, heat flows from one system to another whenever there is a temperature difference between them. Heat flow takes place by heat conduction, heat convection, and thermal radiation.
The calibration of platinum resistance thermometers, thermocouples, and the measurement of heat flow require the precise determination of temperature.
In 1989, the International Committee on Weights and Measures adopted the International Temperature Scale of 1990 (ITS-90). The unit of the fundamental physical quantity known as thermodynamic temperature, symbol T90, is the kelvin, symbol K. The relationship between the International Celsius Temperature, symbol t90, and T90 is t90=T90/K-273.15. ITS-90 also defines specific fixed-point temperatures corresponding to various states of substances, like the triple point (TP) temperature of water (0.01°C, or 273.16 kelvins). A complete list of these fixed points is given in Ref. 76. These fixed points are used in the calibration of platinum resistance thermometers and thermocouples by evaluating the coefficients of the polynomials that represent their outputs.
According to ITS-90, a platinum resistance thermometer, in the range from 0°C to the freezing point of aluminum, would be calibrated at the TP of water (0.01°C) and the freezing points of tin (231.928°C), zinc (419.527°C), and aluminum (660.323°C or 933.473 kelvins).
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