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This article describes the construction and thermal characterization of a cryogen-free cryostat designed for single-pressure refractive index gas thermometry (SPRIGT) of helium-4 in the range 5–25 K. The system is cooled using a G-M type two-stage pulse tube cryocooler. The temperature can be controlled with a standard uncertainty of 20 µK over the whole working range. The use of a thermal switch to attenuate the modulated heat flux arising from the periodic temperature oscillation of the cryocooler is described. From the temperature evolution over the whole system the thermal characteristics are estimated using a simple model.

A cryogen-free cryostat cooled using a 4 K commercial GM or pulse tube cryocooler (PTC) displays temperature oscillations caused by the intrinsic working principle of the regenerative cryocooler. To dampen such oscillations usually requires either a large heat capacity or a large thermal resistance. To understand this phenomenon better and suppress it more effectively, both the step response characteristic and the intrinsic oscillation characteristic of cryostat have been used to obtain the complete transfer functions of a simulation model. The latter is used to test and optimize traditional PID feedback control. The results showed this approach has almost no effect on the temperature oscillation amplitude. Based on this simulation model, a novel active method was proposed and tested numerically. Simulation results predict the method should suppress the amplitude of the original temperature oscillation by a factor of two., Comment: 16 pages, 10 figures

In this work, a Helmholtz free energy equation of state for R290 + R134a based on the multi-fluid approximations model was developed to describe the thermophysical properties of the binary mixture. The present equation shows more competitive than the default equation in REFPROP 9.1 (Lemmon et al., 2013). It covers a temperature range of (252–400) K and pressures up to 6.1 MPa. The estimated standard uncertainties of the present equation are 0.70% and 0.90% for the bubble point and dew point pressures, 0.26 K and 0.29 K for the bubble point and dew point temperatures, 0.13% and 0.60% for the saturated liquid and vapor densities, respectively. The main purpose is to provide a useful model for R290 + R134a, and thus to promote its better applications for the design and optimization of the refrigeration systems.

The SPRIGT team in TIPC has achieved the most stable long-term pressure in the world in the range from 30 to 90 kPa. The pressure instability is less than 0.1 ppm (0.0032 Pa at 30 kPa, 0.009 Pa at 90 kPa), which can build the foundation for the success of SPRIGT.

We describe measurements of thermodynamic temperature in the range 5 K to 24.5561 K (the triple point of neon) using single-pressure refractive-index gas thermometry (SPRIGT) with 4He. In the wake of the May 2019 re-definition of the kelvin and its associated mise en pratique, the main purpose of the work is to provide values of T–T 90, the discrepancy between thermodynamic temperature and that of the International Temperature Scale of 1990 (ITS-90). The link to ITS-90 is made via calibrated rhodium-iron resistance thermometers. Innovations required to reach the level of accuracy required for meaningful measurements (uncertainty in T–T 90 less than the expected deviation) include the suppression of temperature oscillations in a cryogen-free cryostat, a pressure stabilization scheme based on a non-rotating piston balance, modelling of the hydrostatic head correction and refinements of the measurement of microwave resonances in a quasi-spherical copper resonator. The accuracy of measurements varies from 0.05 mK to 0.17 mK and is competitive with that of all previous ones in this temperature range using other techniques. The improvement stems partly from the new techniques used for the new definition of the kelvin as well as ab initio calculations of the thermophysical properties ofgaseous 4He. In addition to confirming the validity of SPRIGT as an accurate, easier-to-implement alternative to other low-temperature primary thermometry techniques (e.g. acoustic gas thermometry) yet with scope for improvement, the results should provide important input data for any future revision of ITS-90.

A cryogen-free cryostat cooled by a closed cycle cryocooler is compact, can provide uninterrupted long-term operation (up to ten thousand hours) and is suited to temperatures from 3 K to 300 K. Its intrinsic temperature oscillation, however, limits its application in experiments requiring high thermal stability at low temperature (below 77 K). Passive suppression methods are effective but all suffer from drawbacks. We describe a novel, active suppression scheme more efficient than traditional proportional-integral (PI) control. The experimental results show that it can reduce the standard deviation of the temperature oscillation by a further 30% compared with PI feedback. To the best of our knowledge, this is the first time such active suppression of temperature oscillations has been implemented with the cryogen-free cryostat. The results also show, however, that an unwanted lower frequency thermal noise will be generated, which appears to be the limit of the method. Nevertheless, the approach could be used to improve the temperature stability in all cryogen-free cryostats.

A novel single pressure refractive index gas thermometry(SPRIGT) allows the determination of thermodynamic temperature of helium gas filled in quasi-spherical microwave cavity resonator(QMCR) by combination of microwave measurements, temperature and pressure control. A long-term temperature stability better than O.2mK is demanded for the QMCR from 5K to 24.5561 K. The uncertainty below 4ppm for pressure needs to precisely control the oscillation of temperature along the gas line. Microwave resonance frequency measurement in QMCR should be better than 2ppb. To meet these requirements, SPRIGT has been built in China and the first run experimental data was analysis in this paper.

Quasi-spherical microwave cavity resonators are widely used in primary thermometry. To realize high-accuracy microwave resonant frequency measurement, it is essential to close the two quasi-hemispheres correctly. We show how to do this using microwave resonance signals to monitor shape and sealing as torques to equatorial bolts are increased. The present resonator was closed with torques up to 105 cNm. The method should be useful to monitor the closing of other quasi-spherical cavities.

Single-pressure refractive-index gas thermometry (SPRIGT) is a new type primary thermometry jointly developed by TIPC of CAS in China and LNE-Cnam in France. To realize a competitive uncertainty of 0.25 mK for the thermodynamic temperature measurement, a cryogen-free cryostat with high-stability better than 0.2 mK should be designed. This paper presented the first experimental results of temperature control for this cryostat. To realize this objective, multi-layer radiation shields combined with a thermal-resistance method were used to isolate the thermal-noise from surroundings. Besides, a new temperature control method based on a gas-type heat switch and proportional-integral-derivative control method was proposed, which was applicable to different temperature ranges by changing the working modes of the heat switch. After optimizing, the ultra-high precision temperature control in the range of 5-25 K has been fully realized, which was the temperature instability (with standard deviation) of 0.021 mK at 5.0 K, 0.05 mK at 5.7 K, 0.042 mK at 7.4 K, 0.029 mK at 14.3 K, and 0.022 mK at 25 K with the sampling time of 0.8 s. This was almost the best reporting result in the world and showed its great potential in SPRIGT.