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Kelvin

The kelvin (K) is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.

The triple point of water is the only thermodynamic state in which the three phases of water vapour, water and ice can co-exist in equilibrium. Provided all three phases are present, temperature and pressure remain constant, and are independent of the respective amounts of substances in each of the individual phases. The triple point of water and absolute zero point given by nature define the thermodynamic temperature scale.

The practical realisation of the temperature scale is usually done by means of a number of highly stable temperature fixed points whose thermodynamic temperature values are determined by means of primary thermometers - e. g. gas thermometers. The values of these fixed points as well as specified methods of interpolating between them have been chosen by international agreement and constitute the International Temperature Scale 1990 (ITS-90).

The thermometric basis at METAS consists of about 30 fixed-point cells allowing for redundant coverage of the temparature range from -189 °C to 961 °C. With the help of these cells, quartz glass-coated standard resistance thermometers, containing a wire spiral of high purity platinum (SPRT: Standard Platinum Resistance Thermometer), are calibrated. According to ITS-90, the SPRTs can then be used as interpolating instruments for calibrations between the fixed points.