Many physical properties, such as the density of samples, are dependent on temperature. For this reason, a very good agreement of temperature measurements is necessary for a comparability of precision measurements from different laboratories. As the temperature measuring equipment of the laboratories often differs greatly from the standard thermometers used to define the temperature scale, it is best to undertake the temperature comparison directly at the measuring temperature. For the usual measurement temperatures, there are, however, no temperature fixed points with which the thermometers can be calibrated.
