A thermometer without traceable calibration to National Standards is like a Clock without the Pips. Does a thermometer being used at John O’Groats agree with one being used at Land’s End? We all buy thermometers without calibration and assume that the manufacturer has been conscientious enough to supply an accurate instrument.
The only way of being confident in a thermometer is to have it calibrated against known Standards.
Temperature is one of the most commonly measured physical quantities. Try to think of any business or process that does not use temperature as a part of their organisation, from chemical reaction to the comfort of their offices. But how we define temperature is not widely understood. Unlike other quantities, such as mass and time, temperature is defined on a theoretical set of conditions whereas other units are based on real, physically realisable, defined conditions. For instance the perfect kilogram is in Paris (soon to be changed!) and time is based on atomic transitions in a caesium atom.
When a thermometer is calibrated it should be to ITS-90. This is the current, International Temperature Scale, which was updated in 1990. ITS-90 (and previously IPTS-68 & IPTS-48) are scales based on international agreement to assign temperatures to naturally occurring physical phenomena. A Triple Point of Water is a piece of equipment that enables water to co-exist in its three states – Liquid, Vapour and Solid. This produces an exact temperature of +0.01°C and can provide an accuracy of ±0.0001°C.
Other fixed points on the ITS-90 Scale include the Triple Point of Argon at -189.3442°C and the Freezing Point of Gold at +1064.18°C. As technology improves, better measurements can be made, and about every 20 years the practical scale is updated. An example of this is the freezing point of Zinc, which in 1948 was determined as 419.505°C, in 1968 was 419.58° C, but in 1990 became 419.527° C. Zinc still freezes at the same temperature but our measurement ability has improved.
National Standards of the ITS-90 are maintained at the National Physical Laboratory and for confidence in calibration any measurement should be traceable back to these Standards. Any Calibration Laboratory that has obtained UKAS Accreditation will have had their calibration procedures independently approved and shown that temperatures measured during calibration are traceable back to National Standards.