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More reliable temperature fixed points

lly based – via many intermediate steps –, on the International Temperature Scale. In the temperature range between 20 °C and 1000 °C, which is important to industry, the fixed points of this scale are defined by the melting and freezing points of high-purity metals. Due to the contamination of the metals, uncertainties between 0.2 thousandths Kelvin and 2 thousandths Kelvin have so far been assumed. At PTB, the influence factors which have so far been theoretically assumed have now been replaced by concretely measured values so that the uncertainty can be reduced by factor 3.

Influence of different contaminations on the fixed point temperature of some high-purity metals. Whereas contaminations from magnesium or chromium, for example, do not affect indium fixed point cells, this does not apply to contaminations from silver or copper.

The calculative correction developed by PTB is based on a chemical analysis. As no reliable data about the behaviour of solved substances are known in the range of extremely small contamination concentrations, these data must all be obtained by experiment. The applied method determines the amount of contaminations with uncertainties of a few ppb (this corresponds to 10–9 mol/mol) and determines its influence on the phase transition temperature of the metal.

To avoid use of the large fixed point cells the employed so far, special minimized cells have – on the basis of investigations of the heat transport in a fixed point cell – been developed which are selectively contaminated to observe the change of the melting temperature. The data obtained in this way on the fixed points of indium, tin and zinc are – as far as the uncertainty and amount at low costs are concerned – unequalled in the international comparison. The result of the investigations was surprising: For the first time it could be demonstrated that not all contaminations affect the fixed point as expected. A detailed analysis as well as calculations showed that elementary contaminations sometimes form insoluble compounds which do not have to be taken into account for the correction.

Contact at PTB:

Phone: +49-531-592-0