An Apparatus for the Approximation of The Triple Point of Argon

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An Apparatus for the Approximation of The Triple Point of Argon (188.87 KB)

SPRT calibration over the ITS-90 sub-range of 83.8058 K to 273.16 K requires measurement at three defining fixed points—the triple points of water, mercury, and argon. Cells and apparatus are commercially available for the realization of these defining fixed points. Several factors have resulted in the widespread implementation of only two of these defining fixed points—the triple points of water and mercury. Comparison calibration in a simple liquid nitrogen comparison apparatus is typically substituted for the triple point of argon. For this substitution to be useful, the measurement uncertainty achievable in the comparison system must approach that achievable in the argon system. Several challenges arise. At one standard atmosphere, the normal boiling point of liquid nitrogen is approximately 77.36 K, about 6.4 K below the lower limit of the sub-range, resulting in extrapolation of the reference thermometer. Additionally, the design of the comparison apparatus affects the uncertainties attainable. Finally, boiling points are highly influenced by environmental conditions, further complicating the measurements. Metrologists at Hart Scientific have developed and built an apparatus intended to attain sub-millikelvin uncertainties using comparison techniques in liquid nitrogen. The apparatus was designed to reduce effects from conduction and is capable of self-pressurization to increase the boiling point to a value closer to the argon triple point. This paper will discuss the apparatus, techniques, and results of this effort.