Ear thermometers are widely used in a clinical setting for the measurement of patient temperature, whereas forehead thermometers have been increasingly used during the pandemic as a method of fever screening. For reliable body temperature measurement, the performance of the devices needs to be validated by calibration using a suitable blackbody source. For a source to be suitable it must have a sufficiently high emissivity, sufficiently large opening to completely fill the field-of-view of the thermometer, good temperature uniformity and a means to measure the temperature which is traceable to the International Temperature Scale of 1990 (the ITS-90). The need for better understanding of the performance and calibration of both forehead and ear thermometers has been recognised internationally by the BIPM Consultative Committee for Thermometry (CCT) - [https://www.bipm.org/en/committees/cc/cct/guides-to-thermometry][0] The ISO 80601-2-56:2017 standard provides the specifications for suitable blackbody sources, but these are based on stirred liquid baths which are relatively cumbersome to use. Designing and developing a blackbody which has the required specifications, but which does not rely on a water bath, would provide users with a more practical solution for the calibration of ear and forehead thermometers and help ensure they are fit for the intended use. We have a range of existing products used to calibrate non-contact thermometers but these are primarily designed for the more generic infrared thermometers, not medical thermometers. The requirements for the laboratory performance of medical thermometers, and hence the blackbody sources used to calibrate them, tend to be more stringent, and it is not clear, without further study, whether or not Isotech's sources are suitable for the calibration of medical thermometers in their current design. Working with NPL, Isotech will be able to determine the suitability of an existing calibrator for these medical thermometers, from analysis and modeling we can learn of the optimal geometry and then have evidence of the performance. Then we will be able to sell our product into a new market both in the UK and overseas via our global distribution network. Having a portable product, of known performance, will enable routine calibration of the devices by users. [0]: https://www.bipm.org/en/committees/cc/cct/guides-to-thermometry

**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.