The International Temperature Scale (ITS-90) was approved by the International Committee of Weights and Measures in 1989, and is based on 17 fixed points. The fixed points consist of pure chemical substances which may be in melting, freezing states or in the three phases concurrently (steam, liquid and solid), and have defined numerical values, which are the best approximate values of thermodynamic temperature T, in compliance with the results of measurements made at the time that ITS 90 was approved. The ITS 90 fixed points are given in the following table.
The International Temperature Scale of 1990 (ITS-90) defines both scales: the International temperatures Kelvin, with symbol T90, and the international temperatures Celsius, with symbol t90. The relationship between T90 and t90 is the same as that between T and t, for example:
t90 / °C = T90/K – 273.15
The unit of the physical quantity T90 is Kelvin, the K symbol, and the unit of the physical quantity t90, is the degree Celsius, symbol °C, as it is in the case of thermodynamic temperature T and the temperature Celsius t.
The fixed points of ITS 90 and respective values
|Substance and respective state||Value defined in Kelvin||Values defined in Celsius degree|
|Ratio between pressure-temperature of Helium-3 (equation)||(0.65 to 3.2)||(−272.50 to −269.95)|
|Ratio between pressure-temperature of Helium – 4 below its lambda value (equation)||(1.25 to 2.1768)||(−271.90 to −270.9732)|
|4 below its lambda value Helium – 4 and above its lambda value (equation)||(2.1768 to 5.0)||(−270.9732 to −268.15)|
|Ratio between pressure-temperature of Helium (equation )||(3 to 5)||(−270.15 to −268.15)|
|Triple point of hydrogen||13.8033||−259.3467|
|Triple point of neon||24.5561||−248.5939|
|Triple point of oxygen||54.3584||−218.7916|
|Trriple point of argon||83.8058||− 189.3442|
|Triple point of mercury||234.3156||−38.8344|
|Triple point of water||273.16||0.01|
|Melting point of gallium||302.9146||29.7646|
|Freezing point of gallium||429.7485||156.5985|
|Freezing point of lead||505.078||231.928|
|Freezing point of zinc||692.677||419.527|
|Freezing point of aluminium||933.473||660.323|
|Freezing point of silver||1234.93||961.78|
|Freezing point of gold||1337.33||1064.18|
|Freezing point of copper||1357.77||1084.62|
The laboratory of temperature has equipment that ensures traceability in the International Temperature Scale ITS-90. This equipment is used for the dissemination of the unit of temperature through comparison calibrations in the ranges from -80 o C to 500 o C.
The traceability of thermodynamic temperature to the SI system unit is ensured through direct calibration of SPRT thermometers in ITS-90 fixed points, from the Triple Point of Argon (-189.3442 o C) up to the freezing point of Aluminium (660.323 o C).
The primary duties of the laboratory of temperature are:
– to ensure traceable measurements on the International Temperature Scale, ITS 90;
– to realise temperature in the range between the triple point of mercury (-38.3844oC) and the freezing point of zinc (419.527oC).
– to disseminate the unit of thermodynamic unit through direct calibration of platinum resistance thermometers in ITS 90 triple points and through the comparison method by using various means of calibration for different temperature ranges.
At primary level the laboratory has:
The triple point of mercury (-38.8344 oC)
The fixed point of water (0.01oC)
The melting point of gallium (29.7646oC)
The freezing point of indium (156.598oC)
The freezing point of lead (231.928°C)
The freezing point of zinc (419.527 oC)
In order to realise ITS 90 in these points, temperature stabilisation baths, high accuracy thermometry bridge < 0.1 ppm, model F18 and standard platinum resistance thermometers are used.
At secondary level, the laboratory performs calibrations of different thermometers such as liquid-in-glass thermometers, industrial resistance thermometers, digital thermometers/analogue display thermometers, calibration of baths and thermostats, incubators, etc.
In order to provide these services, the laboratory has:
– Quartz-Sheath Standard Platinum Resistance Thermometers Pt-25 (SPRT)
– Thermometry bridge of AC model, F700B
– Standard resistor 10 Ω , 25 Ω and 100 Ω, 300 Ω and thermostatic oil baths to maintain standard resistors
– Alcohol baths for low temperatures (range from–80 °C to 110 °C).
– Silicon oil bath (range from 40 °C to 300°C).
– Salt bath (range from 200 °C to 550 °C).
– Standard platinum resistance thermometers, Pt100, etc
– Multimeters / Voltmeters
This laboratory is in the process of implementation of the Quality Management System according to SSH ISO/IEC 17025:2005 in order to ensure the accreditation of the calibration services offered at secondary level and the self-declaration of calibration and measurement capabilities at primary level. To this end serves the participation in inter-laboratory comparisons, such as:
– Regional inter-comparison project for SEE countries, CARDS 2006 Project “Calibration of digital platinum resistance thermometer”, completed.
– The EURAMET Project no.1164 “Calibration of platinum resistance thermometers in the range from -56 °C to 420° C”, completed.
– The EURAMET Project no. 1251 “Calibration of platinum resistance thermometers with comparison in the range -60oC up to 300o C”, ongoing.