NIST Authors in **Bold**

Author(s): | Michael R. Moldover; Mark O. McLinden; |
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Title: | Using Ab Initio "Data" to Accurately Determine the Fourth Density Virial Coefficient of Helium |

Published: | June 25, 2010 |

Abstract: | We combine accurate ab initio calculations of the second and third density virial coefficients, B(T) and C(T), of 4He with measurements of its pressure-density-temperature behavior to determine the fourth density viral coefficient D(T). The measurements were made with a two-sinker, magnetic-suspension densimeter at pressures up to 38 MPa. The measurements on isotherms from 223 K to 323 K were previously published; new measurements from 323 K to 500 K are presented here. On each isotherm, a regression of the virial expansion was constrained to the ab initio values of B(T) and C(T); the regression determined D(T) as well as two apparatus-dependent parameters that compensated for systematic errors in the measurements. The relative uncertainty of D(T) ranged from 2.9 % at 223 K to 10.7 % at 400 K to 26.4 % at 500 K with a coverage factor of k = 2. This uncertainty is 1/6th of the uncertainty obtained without the ab initio values of B(T) and C(T). The apparatus-dependent parameters can be used to calibrate the densimeter, and this will reduce the uncertainty of future measu |

Citation: | Journal of Chemical Thermodynamics |

Volume: | 42 |

Issue: | October 2010 |

Pages: | pp. 1193 - 1203 |

Keywords: | helium; virial coefficients; two-sinker densimeter; calibration; equation of state; reference data; gas metrology; fourth virial coefficient |

Research Areas: | Physical Properties, Chemical Engineering & Processing, Metrology, Thermophysics |