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INTERCOMPARISON ON FIXED POINTS

Objective


Four sealed cells for the realization of the freezing points of tin, zinc, aluminium and silver were manufactured and characterized by the Istituto di Metrologia 'G. Colonnetti' (IMGC, Italy).

In the first phase of the project, the tin and zinc cells were circulated among twelve other national standards laboratories. It was shown that, at the tin freezing point, the temperatures of the local cells agree with that of the circulating cell within +/- 1.07 mK, and that the temperature differences among the local cells lie in a range of 1.70 mK. When the results for the tin cell of one laboratory are excluded (because of a likely wrong pressure inside the cell) and the new IMGC tin cell is considered instead of the old one, the differences among the local tin cells lie in a range of 1.18 mK. At the zinc freezing point, the temperature of the local cells agree with that of the local cell within +/- 1.03 mK, and the temperature differences among the local cells lie in a range of 1.33 mK. These agreements are considered to be very satisfactory, for both tin and zinc cells. There are indications that most of the temperature differences obtained are principally due to the different metal sources and impurity of the 6N purity metals used in the freezing point cells of the above standard laboratories.

In the second phase of the project, the aluminium and silver cells were circulated among two other national standards laboratories. It was shown that, at the aluminium freezing point, the temperatures of the local cells agree with that of the circulating cell within +/- 1.55 mK, and that the temperature differences among the local cells lie in a range of 2.65 mK. At the silver freezing point, the temperatures of the local cells agree with that of the circulating cell within +/- 9.5 mK, but the temperature differences among the local cells lie in a range of 4.68 mK. Therefore, the silver transfer cell, manufactured with a 6N purity metal from Leico Industries (USA), was 5 to 10 mK lower than the local cells, all manufactured with 6N purity metals from Cominco Electronic Materials (Canada). The different metal sources and impurities could have greatly influenced the freezing point of the silver transfer cell, although it was realized with a 6N purity sample.
The practical temperature scale is realised in the range up to 1600 C by means of the fixed melting points of pure metals.

Different design and construction of the cells, sources of supply and handling of the metals, procedure and equipment used to reach equilibrium, can cause discrepancies in these values.

The project involves the intercomparison of the Zn and Sn points in a first phase followed by the intercomparison of Al and Ag points in a second phase.

STATUS

Most of the laboratories have completed their measurements on the Zn and Sn cells, and the circulation of the Al and Ag cells has begun.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Imgc

Participants (8)

CENTRO ESPAÑOL DE METROLOGÍA
Spain
Dansk Teknologisk
Denmark
Istituto Portuguese da Qualidade
Portugal
National Physical Laboratory (NPL)
United Kingdom
Address
Queen's Road
TW11 0LW Teddington
PHYSIKALISCH-TECHNISCHE BUNDESANSTALT
Germany
Address
Bundesallee 100
Braunschweig
Statens Provningsanstalt
Sweden
Swiss Federal Office of Metrology
Switzerland
VSL
Netherlands