Special Problems When Realising the Triple Point of Hydrogen as a Defining Fixed Point of the ITS-90

摘要

The two special problems, which appear when realising the triple point of hydrogen as a temperature fixed point due to the influences of the deuterium content and the catalyst, have been investigated by filling a large number of sealed triple-point cells. The cells differ with respect to the deuterium content (29 mmol D/mol H to 155 mmol D/mol H), type of catalyst (hydrous ferric oxide and gadolinium oxide), catalyst-to-liquid volume ratio (a few per cent to more than 100 per cent) and cell design. At higher catalyst-to-liquid volume ratios, the shape of the melting curve depends on the type of the catalyst, but only the melting temperature of that sample part being in direct contact with the catalyst and thus distorted is changed.