Temperature calibration of thermometers is performed using the International Temperature Scale of 1990 (ITS-90) specified by the International Committee of Weights and Measures (CIPM). The ITS-90 ranges from -272,50 °C to 1084,62 °C and is defined through 14 calibration points in the range from -259,35 °C to 1084,62 °C and based on various thermodynamic properties of 13 pure chemical elements as well as water. The substances are commonly referred to as fixed points.

These defining fixed points utilizes the thermodynamical property that a substance undergoing a phase-transition will remain stable at a fixed temperature. A well-known example of such a transition is boiling water which remains at exactly 100 °C under normal conditions such as standard atmospheric pressure in a laboratory. Fixed points are selected to be as stable and reproducible as possible.

DFM is the leading laboratory for non-contact accredited temperature calibration in Denmark with more than 50 years of experience. In order to enhance temperature calibrations at temperatures above 660 °C, DFM has acquired a new fixed point made of ultra-pure silver that defines 961,78 °C.

The metal is encapsulated in a graphite cup allowing heating to temperatures above the melting point of the metals. The fixed point is placed in a custom-built furnace which is specifically optimized to ensure homogeneous heating. Using fine control of the furnace the fixed point is heated above the melting temperature until the metal is completely melted. The temperature of the oven is then lowered and the metal starts freezing at the freezing temperature, and maintain this temperature for a long time. The freezing temperature can then be measured and used for calibration of equipment. Measurements can be done using non-contact thermometry with for example a thermal camera or a so-called pyrometer.

To ensure utmost performance of the new DFM fixed point in Denmark, it is necessary to verify performance by comparing with standards of other countries. To obtain this goal the DFM equipment was moved to Sweden for a measurement campaign at the thermometry laboratory of the metrological institute of Sweden, RISE. With the gracious help of the experts at RISE, the DFM silver fixed point was verified to be within 25 mK of the Swedish silver fixed point with only a few mK uncertainty. This impressive performance of the Danish silver fixed point confirms that we are able to define 961,78 °C with the lowest possible uncertainty of the transfer to measurement equipment.

DFM is continuously improving our working standards and measurements methods to ensure highest performance and the establishment of the new high temperature fixed point is the latest step in improving our temperature laboratory calibration facilities.

Read more about our accredited temperature calibration services here: https://dfm.dk/en/services/calibration/thermometry/