| Temperature scale consists of temperature fixed points, interpolating instruments and formulas, and the temperature fixed points are an important part of the Temperature scale. Temperature fixed point plays an important role in establishing a temperature scale because of the advantages of accurate value, perfect reproducibility and small temperature drift over time. The International Temperature Scale of 1990(ITS-90) was established by interpolating a series of defined temperature fixed points with standard platinum resistance thermometer in a temperature range between 13.8K and 1234K. Resistance thermometer has been widely used as standard thermometer and working thermometer in contact temperature measurement area because of the the advantages of wide useful temperature range, good agreement, easy to use and easy with measuring instrument. However, the resistance of the resistance thermometer will be changed due to the impact of pollution by impurities and the stress and the resistance-temperature characteristics of the resistance thermometer will drift over time, so the resistance thermometer need to be calibrated regularly to ensure its accuracy. In contrast, the reproducibility and stability of the temperature fixed point is much better than the resistance thermometers, so the stability test and the calibration of the resistance thermometer is also uses on the fixed point. ITS-90 prescribe 17 defined fixed points and there temperatures in the temperature range between 13.8K and 1234K and the lowest temperature fixed point is the hydrogen triple point temperature,13.8033K, whose thermodynamic temperature uncertainty is 0.5mK and the best practical reproducibility is 0.1 mK. There is no defined temperature fixed point under 13.8033K. Therefore, to find and develop good temperature fixed point below 13K is an important basis of the temperature scale study, thermometer study and measurement.The atomic weight of helium is 4 and the normal boiling point is 4.224K. When pumping the liquid helium with vacuum to cool it down, the normalfluid helium(HeI) will change to the superfluid (HeII) at the superfluid transition temperature Tλ=2.176K, which is not curing at the triple point temperature like the normal liquid. The thermal conductivity of normal phase liquid helium (HeI) is limited. Once the temperature of the helium dropped below the superfluid transition temperature, the thermal conductivity of the superfluid phase liquid helium (Hell) suddenly increases of several orders of magnitude. Meanwhile, the specific heat of liquid helium show a narrow peak-like near the superfluid transition temperature, so the superfluid phase transition of liquid helium is called as theλtransition of liquid helium. When substance occurs phase transition at the triple point, melting point, freezing point, condensation point and the boiling point, the phase transition is a "first phase transition", so there will be a latent heat with the phase transition, and the reproduction can be obtained by adiabatic. The superfluid transition of liquid helium is the second phase transition, and there is no latent heat with the transition but the specific heat, the thermal conductivity, the magnetic susceptibility and other parameters. So the phase change temperature plateau can not be obtained with conventional adiabatic method, and the phase transition temperature can be obtained only by scanning temperature method with the changes of the related parameters. ITS-90 text provides that the liquid helium superfluid transition temperature is 2.1768K at the saturated vapor pressure, but there is no practical application method to reproduce the liquid helium superfluid transition temperature.Owing to the dramatic change in the thermal conductivity of 4He when its temperature crosses the transition of superfluid (HeI) and normalfluid (HeII), a sealed-cell with a capillary is used to realize the lambda transition temperature, Tλ. A small heat flow is controlled through the capillary of the sealed-cell so as to realize the coexistence of HeI and HeII and maintain the stay of Hel/Hell interface in the capillary. A stable and flat lambda transition temperature "plateau" is obtained. Because there is a depression effect of Tλcaused by the heat flow through the capillary, a series of heat flows and several temperature plateaus are made and an extrapolation is applied to determine Tλwith zero heat flow. A rhodium-iron resistance thermometer with series number A34 (RIRT A34) has been used in 24 Tλ-realization experiments to derive Tλwith the standard deviation of 0.022mK, which proves the stability and reproducibility of Tλ. A reference standard facility of rhodium-iron resistance thermometer (RIRT) was established. A new cryostat was designed to measure the five RIRTs which used in the realization.42 basis temperature points were measured using this set of reference standard facility which proved the accurate of the RIRTs. |
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