The density of high viscosity working liquids (BM-1C, LEYBONOL LVO 500, and Alkaren-D24) for diffusion vacuum pumps have been measured over the temperature range from 273 to 473 K at atmospheric pressure. The measurements were made using a newly designed densimeter based on hydrostatic weighing method. The combined expanded uncertainty of the density, atmospheric pressure, and temperature measurements at 0.95 confidence level with a coverage factor of k = 2 is estimated to be U(rho) = 1.0 kg-m(-3), U-r(P) = 0.01, and U(T) = 0.02 K, respectively. To confirm the reliability, accuracy, and correct operation of the densimeter, the density of pure n-heptane, ethane-1,2-diol (MEG), and propane-1,2-diol (MPG) with well-established PVT properties (REFPROP/NIST/TDE) over the temperature range from 273 to 473 K at atmospheric pressure were measured using the new developed hydrostatic weighing densimeter (HWD). Also, in order to additional validate the reliability of the measured density data all diffusion vacuum pump liquid samples were measured using the pycnometric method. The present study showed that the densities measured using the new HWD agree with the values obtained with pycnometric method within 0.03-0.08 %.