能量天平及千克单位重新定义研究进展

Progress of joule balance and redifination of Kilogram

质量单位千克是国际单位制7个基本单位中惟一一个仍以实物定义和复现的基本单位.作为一种实物,其量值必然会因为环境因素及使用时的磨损而发生变化.但由于缺少更高一级的参考标准对其进行考察,国际千克原器的真实变化情况无从得知.国际计量委员会建议采用普朗克常数对千克重新定义,号召各个国家开展普朗克常数的精密测量研究工作,并要求至少有三种独立方案提供有效测量数据.自20世纪70年代起,英、美等国采用功率天平方案进行研究,国际阿伏伽德罗常数合作组织则采用了X射线单晶硅密度的方案.为了应对国际单位制的重大变革,2006年中国计量科学研究院提出了用能量天平法测量普朗克常数的新方案,其特点是可避免国外方案中困难的动态测量.2013年原型实验装置研制成功,证实了能量天平方案原理可行.此后,中国计量科学研究院开始了新一代能量天平装置的研制.2017年5月,中国计量科学研究院提交了普朗克常数的测量结果,不确定度为2.4×10^(-7)(k=1),该数据被国际科学数据委员会收入参考数据库.但由于数据的不确定度尚未进入10^(-8)量级,未被用于普朗克常数的定值.目前中国计量科学研究院正对几项主要的不确定度来源进行研究,预计在未来的两年内达可到10-8量级的不确定度.

Kilogram, the unit of mass, is the last one of seven base units in International System of Units（SI） which is still defined and kept by a material artifact. 1 kg is defined as the mass of the International Prototype of the Kilogram（IPK） kept at the Bureau International des Poids et Mesures（BIPM） in Paris. One of the major disadvantages of this definition is the fact that the amount of material constituting the IPK changes with time. Because a more stable mass reference does not exist, the variation of IPK is completely unknown so far. The International Committee for Weights and Measures（CIPM） recommended redefining the kilogram by fixing the numerical value of the Planck constant h and called on every national metrology institute to study the measurement of the h. To avoid possible system errors from one method, more experiments especially based on different principles are expected and encouraged for the final determination of the Planck constant. The CCM required that at least three consistent results should be obtained before the redefinition. Since 1970 s, the Kibble balance（also known as the Kibble balance） experiment has been used by a number of national metrology institutes such as NPL, NIST, METAS, LNE and BIPM. The IAC including the PTB,NMIJ and NMIA used the XRCD method to measure the Avogadro constant. To make contribution to the redefinition of kilogram, the National Institute of Metrology of China（NIM） proposed a joule balance method in 2006, which is also an electrical way but different from the watt balance method in that the dynamic phase is replaced with a static phase to avoid the trouble in the dynamic measurement. The progress of these approaches and the current situation of the redefinition of the kilogram are presented in this paper. In 2013, a model apparatus was built to verify the principle of the joule balance. Then NIM started to build its new joule balance aiming to obtain an uncertainty of 10^-8 level since 2013. In Dec. 2016, the new apparatus was built and could be used to measure the Planck constant h in vacuum. In May 2017, the measurement result was submitted to the Metrologia and accepted by the CODATA TGFC as the input data. However, the measurement result has an uncertainty bigger than 10-8 and was not used for the final determination of the h value. At present, the joule balance group of NIM, together with the Harbin Institute of Technology, Tsinghua University and China Jiliang University is still making great efforts to improve the joule balance apparatus. The uncertainty of 10^-8 level is expected to be achieved in the next two years.