# The Provisional Low-temperature Scale of 2000, PLTS-2000

Internationally agreed temperature scales ensure uniformity, comparability and reproducibility of temperature measurement worldwide. Temperature scales are usually based on a set of fixed points and an appropriate interpolating instrument. As fixed points serve phase transitions of high-purity materials as melting points, freezing points or triple points.

The PLTS-2000 [1] was adopted by the Comité International des Poids et Mesures (CIPM) in 2000. PTB has made substantial contribution to its establishment [2]. The PLTS-2000 covers the temperature range from 0.000 902 K to 1.0 K and is defined by a polynomial of 13th order relating the melting pressure of 3He to temperature, which is shown in figure 1. The definition of the PLTS-2000 ensures high reproducibility as well as high resolution of low-temperature measurements.

In figure 1, the diamonds mark the fixed points of the 3He melting curve with temperatures TMin, TA, TA-B und TNéel corresponding to the minimum of the melting curve, the transition from normal-fluid to supra-fluid A phase, the transition from supra-fluid A phase to supra-fluid B and the Néel transition in the solid 3He. The corresponding pressure and temperature values are defined by the PLTS-2000 and given in table 1. A temperature measurement based on 3He melting pressure basically consists of the determination of the pressure inside a closed cell filled with 3He of appropriate pressure under equilibrium conditions. Equilibrium ensures that the melting pressure moves along the red curve when the temperature changes. A great advantage of the 3He melting pressure thermometry is the fact that the pressure sensor can be calibrated in situ at the fixed points of the 3He melting pressure curve at low temperature using the data given in table 1. Once the melting pressure is measured the temperature is simply calculated applying the defining polynomial of the PLTS-2000. Therefore, in case of the PLTS-2000 the 3He melting pressure itself represents the interpolating instrument.

## Table 1. Pressure and temperature values of the fixed points of the 3He melting pressure according to the PLTS-2000

 Fixed point p3He  / MPa T2000 / K Minimum 2.93113 0.315 240 A transition 3.43407 0.002 444 A-B transition 3.43609 0.001 896 Néel transition 3.43934 0.000 902

For reaching the low temperatures, cryostats [3] are used. The equipment and procedures for the practical realization of the PLTS-2000 are documented in detail in [4]. An overview on the actual low-temperature thermometry at PTB is given in [5]. The dissemination of the PLTS-2000 to customers is provided by our calibration services.

## References

1. Comité International des Poids et Mesures (CIPM), 2000, The provisional low temperature scale from 0.9mK to 1K, PLTS-2000, Appendix to recommendation C1. http://www.bipm.org/en/committees/cc/cct/publications-cc.html
2. Rusby R L, Durieux M, Reesink A L, Hudson R P, Schuster G, Kühne M, Fogle WE, Soulen R J, Adams ED, 2002, The provisional low temperature scale from 0.9 mK to 1K, PLTS-2000, J. Low Temp. Phys. 126, 633–642. (doi:10.1023/A:1013791823354)
3. https://en.wikipedia.org/wiki/Cryostat, reference made on 21.04.2016
4. Guides to Thermometry are also listet under the following Link: http://www.bipm.org/en/committees/cc/cct/publications-cc.html
5. Engert J, Heyer D, Fischer J, 2013, Low-Temperature Thermometry Below 1 K at PTB, AIP Conf. Proc. 1552, 136. (doi: 10.1063/1.4819528)