Published the standard EN ISO 1:2022 (Standard reference temperature)


The EN ISO 1 was published in June 2022 under the project leadership of Alessandro Balsamo, INRiM's Research Director. It was a good example of cooperation between primary metrology (CCL and CCT) and standardization body (ISO/TC 213). Beside the standard, it resulted in a permanent official liaison of the BIPM (the CCL in particular) and the ISO/TC 213, of which the INRiM is liaison officer.

The definition of the metre (based on the second and the speed of light in vacuo) is independent of the temperature. But when moving from primary to dimensional metrology (about forms and dimensions of physical objects), the temperature does count because all expands and shrinks with it and deforms with thermal gradients and inhomogeneous materials. Furthermore, the dimensional and geometric tolerances that designers put on drawings do not make sense without agreement on the temperature they hold at.

The need of fixing a conventional temperature is well known. The ISO was founded in 1947 and developed its first standardization project on this subject, resulting in ISO/R 1:1951 (standards were called Recommendations at that time) which fixed the temperature for “industrial length measurements” at 20 °C, following up a resolution of the CIPM in 1937. 

The standard was revised several time since.

  • In 1975 it was converted to an International Standard (ISO 1 ed. 1); 
  • In 2002 it was included in the ISO GPS (Geometrical Product Specification) system of standards and its title and scope was amended accordingly (from industrial measurements to ISO GPS, ed. 2); 
  • In 2016 the definitions of reference temperature (which can be of any value as long as agreed upon) and standard reference temperature (fixed at 20 °C) were separated to support applications forced to temperatures far off 20 °C (e.g. a bridge in the artic or at the equatorial). In the 90’s whether to move from 20 °C to 23 °C was questioned, similarly to the electric standard and to save conditioning energy worldwide. Eventually no change was decided because of the unacceptable consequence of keeping two versions of same spare precision components on the market, e.g a 200 mm component 1975 and a 200 mm component 199X.

The edition 4 (published on 2022‑06‑14) was born form a point raised by the CCL in 2018 and supported by official documents of the CCT: the thermodynamic temperature and the international temperature according to the ITS‑90 differ of 2.8 mK at 20 °C. Not much in dimensional metrology (3 × 10‑8 for steel), but within the resolution of top level dimensional calibrations. Which of the two temperatures the 20 °C value stated in the standard was assigned to?

The INRiM started a consultation between the CCL and the CCT, brought the issue to the ISO/TC 213 in charge of the ISO 1, and was eventually appointed as project leader. The new edition 4 clarifies that 20 °C is the value according to the ITS‑90, toward which a very large majority of thermometers are calibrated worldwide.

Because of the ISO/CEN cooperation agreement (Vienna agreement) the standard was published jointly as EN ISO 1:2022 (2022‑06‑29); as a European standard, the UNI has to adopt it shortly as UNI EN ISO 1:2022.