Using Ab Initio "Data" to Accurately Determine the Fourth Density Virial Coefficient of Helium

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