One of the areas for improvement in ITS-90 is studied: the search for a fixed temperature point as an alternative to the triple point of mercury, e.g., the triple point of carbon dioxide. The need for this area of improvement arises from the signing in 2013 of the Minamata Convention on Mercury under the auspices of the UN and the avoidance of new measurement instrumentation based on mercury (liquid mercury thermometers, etc.). A cell for implementation of the triple point of carbon dioxide is designed. Methods for producing the triple point of carbon dioxide are introduced and the results of studies of its reproducibility are presented. These studies demonstrate that the use of the triple point of carbon dioxide offers promise as the fixed point for ITS-90 in place of the triple point of mercury, as well as the possibility of using it for calibration of both capsule and long-stem standard platinum resistance thermometers.
The results of a measurement of the indium, tin, and zinc fixed points in the KT-500 and KT-650 temperature calibrators are presented. It is shown that the error in reproducing the temperature does not exceed 0.003°C for indium and tin and 0.014°C for zinc. It is concluded that it is possible to use these points to check second-class and third-class standard resistance thermometers.
A thermal flowmeter of gaseous helium with a linear scale and an upper flow rate limit of 450 mu mol/s is described. The error of measurement is 5% of the upper limit.
We describe standard resistance thermometers made of a rhodium-iron alloy designed for operation in the temperature range 0.02-2 K. The sensitive element of the thermometer is filled with liquid helium-4 or helium3, which improves the heat exchange by several orders of magnitude. This makes it possible to significantly increase the measuring current and to use conventional technology for precise measurement of resistance.
The paper describes a cut-off valve whose volume changes by less than 0.2 mm(3) during operation, it was designed to handle pure gases.
As a part of preparatory work for the establishment of the new ITS-90 temperature scale, measurements of thermodynamic temperature by means of a gas thermometer without "dead space" were performed. The deviation of the NPL-75 scale from thermodynamic temperature was found to lie between zero and 0.7 mK. The value for the thermodynamic temperature of the hydrogen normal boiling point was found to be only 0.6 mK higher than that of Berry. The deviation of our scale, referenced to 273 K, from that of Steur and Durieux does not exceed 0.4 mK from 4.2 K to 27 K. Independent values of the second virial coefficient in the range 2.5 K to 27 K were found. In agreement with the results of other workers the deviations of the IPTS-68 from thermodynamic temperature were found to be - 8 mK near 20.3 K and +11 mK near 83 K. The maximal deviation of the IPTS-68 from thermodynamic temperature, viz 15 mK, was found near 150 K. This result is 4 mK higher than that of Kemp et al and lower by the same amount than was measured by the total radiation thermometer of Martin et al. The new values found for the thermodynamic temperatures of the fixed points are in good agreement with the results of other workers. The reference function W(T) for a standard platinum resistance thermometer is calculated.
