Setup of a Highly-Accurate Cryogenic Densimeter, April 2025:
The new Magnetic Suspension Balance (MSB) is tested in our laboratories. The revised cryostat design requires a long thin metal wire for the connection between sinker and MSB to reduce surface tension effects. The control of the MSB cannot handle long thin wires. Due to oscillations the balance readings fluctuate, do not become stable. A solution was found that allows for stable balance readings with minimal surface-tension effects.
Thermometers and the corresponding resistance bridges have been delivered. A calibration plan has been developed with PTB; the thermometers are at PTB by now. The secondary Cernox thermometers, which will be used for the measurement of temperature gradients in the cell, will be calibrated against the PRTs by ourselves.
The thermal design of the measuring cell and of the pre-cooling pipe has been finalized. Finite-element heat transfer calculations prove that it is possible to position the catalyst required for ortho/para conversion in a supply pipe of the measuring cell rather than in the cell.
The production of the equipment for helium handling is completed. The equipment is currently tested at the labs of the manufacturer. At RUB, the equipment for helium recycling has been installed and tested.
Investigations on the dynamics of the ortho/para conversion show that it would be very valuable to measure the ortho/para distribution of the off-gas from the measuring cell. In cooperation with colleagues at the University of Western Australia (UWA) in Perth, a concept is beeing developed to measure the ortho/para distribution. The corresponding Raman measurements have high priority by now, since they are also required for measurements with our existing densimeter down to 100 K. We plan to send a PhD candidate to UWA for three months from August on to gather experience with this measurement technology.
Experimental Campaigns with Established Equipment, April 2025:
Further measurements with our existing low-temperature densimeter confirmed the results obtained for the system helium/neon. First measurements were carried out for another relevant pre-cooling mixture, for neon/argon. These measurements show that the current reference equation of state for mixtures of helium, neon and argon does not accurately describe properties in the homogeneous region of the system neon/argon as well. Measurements are ongoing. Mixtures are currently prepared to investigate the third system, the system helium/argon.
Viscosity measurements for hydrogen and for mixtures of hydrogen with carbon dioxide, methane and ethane (impurities that can be observed in blue-hydrogen production) are completed, the results are published.
Modelling of Hydrogen Properties, April 2025:
Comparisons between heat capacities calculated based on an analysis of experimentally determined energy levels (by spectroscopy), on energy levels calculated from fits to spectroscopic data and on ab initio calculated energy levels resulted in an improved understanding of the uncertainty of the ideal gas heat capacities and of the temperature dependence of the equilibrium composition. The ideal-gas part of the current reference equation of state for hydrogen is based on an unsuitable functional form. A more suitable and numerically less demanding functional form has been identified and will be used as basis for the new reference equations of state.
Measurements with our low-temperature densimeter down to 100 K have improved the data base for mixtures of helium and neon and for mixtures of neon and argon. Based on the new data and on literature data for phase equilibria, mixture models are being developed.
