| A multiparametric and fundamental equation of state is presented for the fluid thermodynamic properties of helium. The equation is valid for temperatures from the lambda-line (~2.17 K) to 1500 K and for pressures up to 2000 MPa. The formulation can calculate all thermodynamic properties, including density, heat capacity, speed of sound, energies, entropy and saturation properties. A new equation of state is necessary to overcome difficulties associated with the current standard in the asymptotic region between the lambda-line and 3 K and also difficulties related to lack of data, extrapolation performance, and accuracy at higher temperatures.;Below 50 K, the uncertainties in density are 0.20% at pressures up to 20 MPa. From 50 K to 200 K the uncertainties decrease to 0.05 % at pressures up to 80 MPa. At higher temperatures the uncertainties in density are 0.02 % up to pressures of 80 MPa. At all temperatures and at pressures higher than listed here, the uncertainties may increase to 0.3% in density. The uncertainties in the speed of sound are 0.02%. The uncertainties in vapor pressure are less than 0.02% and for the heat capacities are about 2%. Uncertainties in the critical region are higher for all properties except vapor pressure. |
