THE TEMPERATURE–TIME DEPENDENCE OF THE TRIPLE POINT OF WATER

Abstract

The important, but elusive, temperature–time dependency of the triple point of water has been thoroughly investigated in 10 triple point cells from two sources. During the first 2 days after preparation of the cells, the temperature was found to increase by amounts ranging from 0 to 5 × 10−4 °C with the average rise being 2 × 10−4 °C. After the second day the temperature continued to rise at a rate of about 0.1 × 10−4 °C per day for about a week and finally stabilized. In practice, if an ice mantle in a cell is allowed to age for about three days before the cell is used the temperature should be reproducible to about 10−1 °C.A series of experiments are described which suggest that this initial temperature rise may well be due to the growth of crystals in the ice and/or strains in the freshly prepared ice. The slow rise after the second day could be accounted for by crystal growth. These two possibilities are discussed in detail and a formula relating the temperature to crystal size is compared with the observed results.Tests in pyrex cells up to 5 years old showed that they contain no significant amount of impurities and, therefore, that the segregation of impurities during the freezing process is not likely to be the cause of the initial temperature variations.On the assumption that the above explanations are true, a number of methods of eliminating this troublesome initial temperature rise were tested. Since none of these tests was completely successful, methods of extending the usefulness of old mantles were examined.Different methods of preparing and using the cells were critically examined; the earlier method of supercooled freezing was found to be quite inadequate. The effect of different thermal bonds in the thermometer well and of different cell environments was investigated. As a result of this work a new importance is attached to the standard practice of melting the inner layer of ice next to the thermometer well.The effect of the temperature–time dependency on previous measurements of the difference in temperature between the ice and triple points of water is discussed.