Triple point of carbon dioxide to replace fixed point of mercury

Reports of the 24th General Conference on Weights and Measures contain information on ways to modernize ITS-90, one of them being developing a fixed point based on a new substance to replace fixed point of mercury. The need is dictated by dangers of using mercury and signing the Minamata Convention on Mercury. To solve this, the D.I. Mendeleyev Institute for Metrology developed a cell for realization of triple point of CO₂. 

The advantage of this design is ability to carry out measurements of both capsular and long-stem thermometers. The cell is filled with 99,9995 % pure substance. Its advanced construction does not require expensive materials. To realize it, the cell is placed in a bath, and the well is cooled with liquid nitrogen (Fig. 1). The best plateau length is 40 hours, with plateau degradation not exceeding 0.3 mK. Reproducibility of triple point of CO₂ is 0.25 mK. Measured temperature of the triple point according to ITS-90 is ─56.5583 °C with standard uncertainty (k = 1) of 0.29 mK. Analysis of sensitivity coefficients shows that using triple point of CO₂ as the main ITS-90 fixed point can reduce calibration uncertainty by 1.5 times (Fig. 2). Results demonstrate feasibility of triple point of CO₂ at level of national secondary temperature standards.

Contact: Dr. Anatoliy I. Pokhodun

Head of Thermodynamics Department

E-mail: A.I.Pokhodun@vniim.ru