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Electronic "fixed point" for 20 °C

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.

The temperature of the gold-plated copper insert is measured with two thermistors and regulated by means of a heater with wire resistors.

An electronic device was therefore developed at PTB which very precisely and reproducibly realizes the reference temperature of 20 °C. Within this device, a microprocessor-controlled temperature regulator sets the temperature in two copper blocks in two consecutive stages. An additional dry block calibrator ensures a stable ambient temperature and sufficient cooling of the copper blocks. In three Working Groups of the PTB and two laboratories of the Italian metrology institute INRIM, the temperature of the electronic reference point was determined with the respective temperature measuring devices. In this way it could be proved that the temperature measurements at 20 °C in Italy and Germany agree with a precision of approx. 1 mK.

This is of importance in particular for the Avogadro Project, in which the number of atoms in a 1 kg silicon sphere is determined from the distance of the atoms in the single crystal silicon and the volume of the sphere. Because of the thermal expansion of the crystal, a very good agreement of the temperature measurements is a necessary prerequisite for this method.

Contact at PTB:

Phone: +49-531-592-0