Modernization of the International Temperature Scale ITS-90: Triple Point of Carbon Dioxide Instead of Triple Point of Mercury

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.