The Immersion Profiles With Water Triple Point Cells And Investigations On The Influences Of Bushing

The triple point of water (TPW) is not only the sole reference point for Kelvin Thermodynamic Temperature Scale(KTTS), but also a fundamental defining fixed point of International Temperature Scale of1990(ITS-90). It is great significance in the realization of the thermodynamic temperature measurement, the international temperature scale, and the more and actual temperature measurement. In the thermometry field more concerns have been paid on the accurate realization of the triple point of water. The improvements of technology and level of realization of TPW provide guarantees of the development of temperature measurement and reliability of temperature traceability. Therefore, it is urgent to carry out the investigation on TPW.The immersion profile of TPW cells is to investigate the influence of hydrostatic pressure on measuring the TPW. By measuring the different height measurement results of the temperature sensing element of platinum resistance thermometer (SPRT) within well of TPW cells, and the hydrostatic pressure coefficient is obtained with a least square fitted experimental results, the hydrostatic pressure coefficient is defined:dT/dh=7.3μK/cm. The immersion profile of TPW cells is an important index to evaluate the influence of heat leakage. Therefore, in international comparison of TPW cells, the various national laboratory comparison reports must provide measurement results which the immersion profile of TPW cells. On CCT-K7key comparison of water triple point cells,20national metrology institutes have significant difference between the hydrostatic pressure correction coefficient and the theoretical value based on the immersion profiles. P.P.M Steur, the INRiM experts, he thinks that the diameter difference of thermometer well of TPW cells is the main cause of deviation. In order to investigate the effect of diameter of thermometer well to immersion profile of TPW cells, the thermometer well of two cells had diameters of16mm and18mm were developed. In order to accurately control the SPRT in different heights of well, with labview programming and01sc403stepper motor controller, established an automated system for measurements of immersion profiles. The device through the stepper motor controller to control the height of SPRT in thermometer well, and use the accuracy is0.02ppm AC bridge to measure the resistance which in different heights of SPRT in well. Through linear fitting, get the hydrostatic pressure coefficient. Based on this, the immersion profiles of TPW cells are made by experiments. The experimental results show that the hydrostatic pressure correction coefficient from experiments and the theoretical value are in good agreement within1μK/cm. There are many factors that can affect TPW, P.P.M Steur, the INRiM experts, he found that a bushing in well can affect the realization of value:50μK or higher have been observed in TPW cells with a thermometer well of12mm to13.6mm diameter during measurement. In order to investigate the effect of bushing to TPW, the copper bushing had diameters of15.5mm and thermometer well of two cells had diameters of16mm and18mm were developed. Experimental results show that the influence of copper bushing for values of TPW form experiments is less than40μK, and under the condition of the same measure, if the diameter of the copper bushing is more similar to the TPW cells, the influence of copper bushing for values of TPW is smaller.In addition, the frozen process of the traditional TPW cells is complicated, it not only needs to master certain frozen skill, but also is inconvenient, therefore, Hart has developed a mini TPW cell. This mini TPW cell has a matched frozen system device. Its operating process is quick and convenient. The mini TPW cell in passing a short type precision grade platinum resistance thermometer is used widely. In order to compare the realization of traditional TPW cells and mini TPW cells, in this paper, we developed each of these two types of cells. The experimental results show that the realization of two types of cells are in good agreement within±0.1mK, mini TPW cells’stability is better, and it can be used in the calibration thermometer.