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The saturated-vapour pressure/temperature relation for liquid helium provides such a good and reproducible scale that its use as such long pre-dates any internationally agreed scale in the helium range; in fact, it even pre-dates ITS-27. However, it proved difficult to reach international agreement on a helium vapour pressure scale.
1958 |
The CIPM adopted a Table of 4He vapour pressure against temperature data, proposed to it by the CCT. The Table was based upon gas thermometry data smoothed by magnetic thermometry and, below 2.2 K, by thermodynamic calculations. It covered the range from 0.5 K to 5.23 K and became known as the 1958 4He Scale, temperatures measured on it being denoted by T58.
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1962 |
Shortly after the adoption of the 1958 4He Scale, a further proposal was made in respect of a vapour pressure scale for 3He. This was based upon comparison of the vapour pressures of 3He with the 1958 4He Scale above 0.9 K, and with thermodynamic calculations below 0.9 K. The Scale was accepted by the CIPM and become known as the 1962 3He Scale, temperatures measured on it being denoted by T62.
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Complete revisions have since been made of both the 3He and 4He scales, making them consistent with the results of gas, noise, acoustic and magnetic thermometry carried out since the original versions of the scales were adopted.
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