Temperature Sensor Calibration

Calibration is a broad topic and includes the ultimate reference sources, such as the national metrology laboratories, who are the custodians of the International Temperature Scale, and those services that are directly traceable to the the national standards.

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Yes, there is an “official” temperature scale. It’s based on the true, thermodynamic temperature scale, but the real, working temperature scale is something practical which can be repeatably produced anywhere in the world given the correct practices and equipment.

This is a lot like the freezing and boiling points of water but developed and refined to a higher degree (pardon the pun).

It is revised periodically as our technical abilities improve and as those who specialize in the field of metrology (measurement science) are able to come to some consensus.

The link above provide detailed information about the newest revision of the temperature scale used in calibration and measurement.

This is the scale that the National Measurement Institutes (NMIs) or “labs”, or those affiliated with them, refer to in the calibration certificates of reference devices.

This could be for sensors used in corporate or university or other measurement laboratories which provide a more local service such as to working instruments in a process plant or experimental apparatus.

The International Temperature Scale of 1990 (ITS-90
(A technical paper describing the scale, in fact a site dedicated to the ITS-90 and related topics)

The International Bureau of Weights and Measures (Bureau International des Poids et Measure or BIPM)
The task of the BIPM is to ensure world-wide uniformity of measurements and their traceability to the International System of Units (SI).

It does this with the authority of the Convention of the Metre, a diplomatic treaty between fifty-one nations, and it operates through a series of Consultative Committees, whose members are the national metrology laboratories of the Member States of the Convention, and through its own laboratory work.

Temperature and Humidity are but two of the principle measurement areas of interest.

It is interesting that most of the fixed reference points on the ITS-90 are melting and triple point temperatures of pure materials. One example used to be the Ice Point, 0 Degrees Celsius.

But it was replaced in 1967 by an even more precise temperature called the Triple Point of water. The name of the Centigrade temperature scale was officially changed to Celsius at the same time.

In the early days of thermometry, the Boiling Point of water was also used as a key reference temperature for the Celsius Scale. It is nominally 100 Degrees Celsius (hence 100 degrees or the Centigrade temperature scale). However, it was found that the boiling reference point was not as stable as either the freezing or triple points; it is quite sensitive to the pressure on it from its surroundings.

An excellent video on the subject was created by the folks in responsible for the Periodic table of Videos at the School of Chemistry at The University of Nottingham in the UK:: http://www.nottingham.ac.uk/chemistry

Have a look: “Boiling Water – Periodic Table of Videos”

(At YouTube: https://www.youtube.com/watch?v=sCh2T9axLyY)


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