General speaking, the national references are the materialization of the units or sometimes the measuring bench that allows to access the quantity consider. Regarding the thermal quantities, the references are shown under four sections :
Scheme of traceability between the different quantities
The unit of thermodynamic temperature (the kelvin) is defined on the basis of the value assigned to the triple point of water, 273,16 K (or 0,01 °C).
The temperature standards should be, in principle, the materialization of the kelvin or of certain values of the thermodynamic temperature or the apparatuses for absolute measurement of the thermodynamic temperature.
IThere are direct measuring instruments of the thermodynamic temperature and it is possible to consider the temperature standards as these instruments. This is also done for the temperatures close to absolute zero. However, these instruments are complex to implement and above all, the experimental results obtained with these instruments show great dispersion.
That’s why, since 1927, the metrologists have tried to overcome this problem by developing one temperature scale. All of the different scales proposed over time are based on a certain number of states of thermodynamic equilibrium which value is set using these absolute measuring instruments. These states of equilibrium (phase transitions of pure substances) are called fixed points and constitute the basis of one temperature scale. The scale is fully defined by associating to these fixed points one specific measuring instrument and one interpolation formula.
Two scales are currently used: the ITS-90 (International Temperature Scale of 1990) and the PLTS-2000 (Provisional Low Temperature Scale of 2000):
Second-sound thermometer and helium-3 melting-pressure thermometer assembled on the LNE-INM dilution refrigerator.
|Fixed point temperature (in K)||Substance||Type of point|
|from 3 to 5||Helium||Vapour pressure|
|about 17||Hydrogen (or Helium)||Vapour pressure (or gas thermometer)|
|about 20,3||Hydrogen (or Helium)||Vapour pressure (or gas thermometer)|
More particularly, for the most common temperatures, the ITS-90 defines:
Standard platinum resistance thermometer
A book "Supplementary Information for the International Temperature Scale of 1990" published by the BIPM) describes the techniques and means required for implementing the ITS-90 in a laboratory. The French version is the monography n° 17 published by the BNM "Compléments d’information à l’Echelle internationale de température de 1990". It also includes the full text of the ITS-90.
The temperature standards (or references) are therefore the practical realizations of these fixed points associated with the instruments specified in the ITS-90. The specified means necessary for the generation of the phase transitions of the substances have been implemented.
A thermometer is placed in thermal equilibrium with the pure substance. By observing the evolution of the pure substance temperature over time, the phase change is located and the temperature corresponding to this point is assigned to the thermometer to be calibrated.
Owing to the range of the ITS-90 (from cryogenic temperatures to the highest temperatures), the materialization of the fixed points requires very different techniques. The wide-ranging research works that have been carried out over these last years have led to the development of original and high performance methods and instrumentations.
The French national laboratories (LNE-INM and LNE) hold all the materializations of the fixed points of the ITS-90 (most of which were devised by the LNE-INM), and the means for the development of these fixed points as well as the measuring and calibrating instruments matching these references.
Thermometric cells at the LNE-INM
Regarding the field of contact thermometry, for each fixed point, the references comprise a batch of thermometric cells that are regularly compared between themselves and maintained at the best metrology level. The standard platinum resistance thermometers are regularly calibrated against these references.
Concerning the area of measurement by radiation thermometry, the LNE-INM has devised and realized three blackbodies associated to the three fixed points of the ITS-90 (silver, gold and copper) together with a radiance comparator. The latter allows measuring the temperature of the blackbodies (lamps or cavities) in relation to these fixed-points blackbodies.
Humidity equals the quantity of water contained in a quantity of dry gas. According to the field of application, different quantities are used in hygrometry. Three of them are particularly used in metrology: the mixture ratio, the dew point and relative humidity.
View of the CETIAT humid air generator
during calibration of the condensation hygrometer
Since 1994, thanks to the new means (high performance saturators) developed in France for the calibration of hygrometers, humid air temperature in the saturators was taken as a reference. From the metrology point of view, humidity is therefore related to temperature, and thus to the ITS-90. The standard instrument is a platinum resistance thermometer associated to a humid air generator.
The range of dew point measurement, at the highest metrology level, extends chiefly from -60 °C to +80 °C. Additionally, it is possible to cover the calibration needs for the very low dew points ranging from -100 °C to -60 °C.
The conduction thermal exchanges are primarily characterised by three thermophysical properties of materials related between themselves:
For these three quantities, the LNE holds measurement benches at different metrological levels though. For the time being, only two benches have been sufficiently characterized (calculation of uncertainties) to be considered as reference benches :
Emissivity is a property characterising the capacity of a material to exchange heat through radiation. Concerning the emissivity of materials, the LNE holds three reference measurement benches. Although the measurements of thermoradiative properties call for radiometry techniques, they are classified under the “thermal quantities” field since they are used to calculate the radiance temperature of the blackbodies applying Planck’s radiance law.
Measurement bench for HT° emissivity
for good thermal conductors