Physikalisch-Technische Bundesanstalt

The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. 

Only at a single temperature – 273.16 K (or 0.01 °C) – do the three states of aggregation of water (solid, liquid and gaseous) coexist. This temperature can be realized with the aid of a triple point cell. The triple point cell is a quartz glass cell, similar to a sealed test tube. Highly pure water filled into this vessel is cooled until it reaches the triple point. This point of equilibrium is more suitable for the realization of the temperature than the ice point used formerly (the point at which water freezes), because the latter depends on air pressure and the amount of oxygen dissolved – factors which are regarded as being not precisely reproducible.

The triple point is used for the calibration of thermometers which serve as standard instruments for the dissemination of the unit of temperature. As a single thermometer cannot cover the entire temperature range, further fixed points are required, each of which defines the properties, or rather a point of equilibrium, of certain molecules or atoms: for example the point at which liquid helium passes into the gaseous state (between 3 and 5 K), or the point at which copper begins to melt (with 1357.77 K at the top of the list of defining fixed points).

Basically, the kelvin scale is nothing else but the Celsius temperature scale whose zero point has been shifted.
–273.15 °C correspond to 0 K. The advantage of the kelvin scale thus is that there are no negative temperatures. The intervals within the scales are, however, the same: one step in kelvin corresponds to one step in Celsius. 

Through the determination of the temperature of the water triple point to 273.16&degr;K, the kelvin is at present associated with a rather "random" material property. As in the case of the other units, it would be advantageous to link the temperature unit up with a fundamental natural constant and to determine the value of that constant, because in that case no temperature value (and no measurement method either) would be assigned. In the case of the kelvin, the Boltzmann constante k would have to be determined, because in fundamental physical laws, the temperature always occurs as "thermal energy" kT. It would thus be imaginable to define the kelvin as the temperature change which – in the case of an ideal gas composed of 10³⁰ point particles without inner degrees of freedom – leads, according to today's state of knowledge, to a change in the internal energy by 20 709/nbsp;755 J.

Basically, the Boltzmann constant can be deternined with any primary thermometer, by measuring – when the temperature is known, and in the ideal case, at the triple point of water &8211; the product kT and by calculating k from it. The actual value of k was determined at the NIST with the aid of acoustic gas thermometry, i.e. by measurement of the sound velocity in a gas. A study recently completed at PTB showed that another variant of gas thermometry – dielectricity constant gas thermometry with helium – offers good chances to achieve the required further reduction in the uncertainty, above all as the required polarizability of the helium atom can meanwhile be very exactly calculated by means of quantum mechanics. In the case of this method of measurement, the temperature- and pressure-dependent dielectricity constant of helium is determined from the capacity change which occurs when the gas is pumped off from a capacitor filled with helium.

It is, therefore, PTB's aim to improve dielectricity constant gas thermometry, which it has successfully used for many years in the low-temperature range, in such a way that the relative standard uncertainty of 15 · 10^–6, which can today be achieved for the determination of the Boltzmann constant, will in a first step be reduced to 2 · 10^–6 and in a second step by another order of magnitude.
This project, which will in total surely require one decade, has now been tackled with first investigations on a newly established gas thermometer system. Spectral radiation measurements on the basis of Planck's law will be performed concurrently with the project and help to ensure the results.

Temperature and Synchrotron Radiation Division
"In focus": "Temperature Quiz"