维里系数及维里方程是对真实气体非理想性的客观描述。氦-4气体作为氖三相点（24.5561 K）温度以下，温度及压力计量研究中唯一实用的工作气体，其准确、高精度的密度维里系数对前沿计量科学研究和量子力学发展具有重要意义。维里系数的获取有实验测量和理论计算两种，实验测量时很难实现低温区温度和维里系数的高精度、原位测量，所得结果受外界非理想因素影响大；理论计算基于量子力学“从头算”对计算机资源要求较高，多采用近似计算模型，依赖于计算机资源和模型精度，计算精度受限。本项目基于既有的定压气体折射率基准测温装置，研究拓展其维里系数测量功能，通过提高频率测量精度、获取准确等温压缩系数及非理想修正方案，借助维里系数和温度联合计算方程得出密度维里系数。将实验测量数据和理论计算数据进行建模分析和比对校验，建立二者的融洽性评估体系，为温度、压力计量及低温流体热物性量子力学“从头算”提供准确、可靠的基础物性数据。

Virial coefficient and virial equation are objective descriptions of the non-ideality of real gases. Helium-4 is the only practical working gas below the neon triple point (24.5561 K) in temperature and pressure measurement research. The accurate and high-precision density virial coefficient acquisition of He-4 is of great importance to the research of cutting-edge metrology and development of ab initio potentials. There are two kinds of methods to obtain the virial coefficient: experimental measurement and theoretical calculation. It is difficult to achieve high-precision in-situ measurement of temperature and virial coefficient in low temperature region during the experimental measurement. The results are greatly affected by external non-ideal factors. Theoretical calculations are based on the ab initio potentials and requires relatively high-level computer resources. Generally, approximate calculation models are used which caused the theorical results depend on computer resources and model accuracy, so the calculation accuracy is limited. This project is based on the existing Single Pressure Refractive Index Gas Thermometry device and research on expanding its measurement function of virial coefficient. The density virial coefficient can be obtained by the scheme of improving the frequency measurement accuracy, obtaining accurate isothermal compression coefficients and non-ideal correction and together with the the combined calculation equation of virial coefficient and temperature. The experimental measurement and theoretical calculation data are modeled, analyzed, compared and verified to establish a harmonious evaluation system of these two, which provides accurate and reliable basic physical properties data for temperature, pressure measurement and ab initio calculation of thermal properties of cryogenic fluids.