10061179Calibration of Thermometers Avoiding the Use of MercuryClosedCollaborative R&DInnovate UK**Calibration of Thermometers Avoiding the Use of Mercury** High-accuracy laboratory thermometers need to be calibrated by using 'fixed point cells'; cells are used to calibrate thermometers to the highest accuracy, and cells rely on the phase change of a pure substance. At higher temperatures, pure metal ingots are encased in an assembly into which a thermometer can be placed. As the metal ingot melts or freezes the temperature remains constant. These fixed points are specified in the International Temperature Scale of 1990 (ITS-90), the internationally agreed approximation to the SI unit of temperature, the kelvin. The most used low-temperature fixed point cell uses pure mercury, which provides a reference of -38.8344°C. Using mercury is a very convenient and time-honoured (and only) way of calibrating the standard platinum resistance thermometers (SPRTs) used for realising and disseminating the ITS-90\. However, mercury is a hazardous substance, and its use is now prohibited in electronic equipment. The scientific community has great concern that mercury could soon be banned for calibration purposes; it is already very difficult and problematic to ship mercury as cells need to be both supplied for new installation, and moved for re-calibration. Scientists are researching alternatives, including carbon dioxide (CO2, -56.6 °C) and sulfur hexafluoride (SF6, -49.6 °C). Isotech has developed a carbon dioxide (CO2) fixed point cell, it operates at approximately -56.6 °C. We are unable to realise the triple point or to measure the temperature with a sufficiently low uncertainty, not for lack of equipment or commercial service, but for the need of novel realisation and measurement techniques for this new calibration point, both for the thermal performance and for determining the purity of the CO2\. We believe that by leveraging the experience and expertise of the NMI community (i.e. NPL) we can commercialise the CO2 cell, which would solve or reduce the need to use hazardous mercury, reassuring the measurement community and eliminating the environmental concerns over mercury, and giving Isotech a significant commercial advantage in the international marketplace.