# WP2 (CNAM, LNE, INRIM, PTB, TIPC-CAS)

## Measurement of *T – T*_{90} over the temperature
range ~5 K to ~200 K

The aim of this work package is to determine of *T* – *T*_{90} in the range ~5 K to ~200 K using a variety of techniques and with a target standard uncertainty in the order of 0.5 mK.

The laboratories involved in this work package have developed suitable techniques and apparatuses to be used as primary standards for the implementation of the redefined unit of temperature. Acoustic Gas Thermometry (AGT), Dielectric-Constant Gas Thermometry (DCGT) and, to a lesser extent, Refractive-Index Gas Thermometry (RIGT) were developed to determine the Boltzmann constant and to significantly improve of *T* – *T*_{90} values around the triple point of water down to ~150 K in the InK-1 project.

In this work package AGT, DCGT and RIGT will be further developed and* combined* (AGT+RIGT) to be extended below 200 K, down to temperatures close to 1 K, in order to establish a complete low-uncertainty data set of *T* – *T*_{90}. The major scientific and technological challenges are the accurate determination of the compressibility of the materials used to build DCGTs and RIGTs, the assessment of gas purity below the part-per-million level and the measurement and control of gas pressure at the highest degree of accuracy. A close scientific cooperation is needed to solve these issues and to implement the improvements to attain the required low uncertainty.

**T****-****T**_{90}** by Dielectric-Constant Gas Thermometry between 40 K and 200 K**

The aim of this task is to measure *T*-*T*_{90} between 40 K and 200 K, using Dielectric-Constant Gas Thermometry as a thermodynamic temperature primary standard, with target uncertainties in the order of 0.5 mK at the fixed points, or with uncertainties of the order of the ITS-90 uncertainty in the intermediate ranges.

**T****-****T**_{90}** by Refractive-Index Gas Thermometry between 5 K and 200 K**

The aim of this task is to measure *T*-*T*_{90} between 5 K and 200 K by refractive-index gas thermometry, with target uncertainties in the order of 0.5 mK at the fixed points, or in the order of the ITS-90 uncertainty in the intermediate ranges.

**T****-****T**_{90}** by Acoustic Gas Thermometry between 5 K and 200 K**

The aim of this task is to measure *T*-*T*_{90} between 5 K and 200 K by acoustic gas thermometry, with target uncertainties in the order of 0.5 mK at the fixed points, or in the order of the ITS-90 uncertainty in the intermediate ranges.

*Cylindrical resonator in a pressure vessel.*