Computational analysis of the accuracy in the evaluation of the mean molecular weight of petroleum oils from the ASTM Standard D 2502-92

It is well known that the kinematic viscosity of a petroleum oil is very sensible to small changes in temperature. In this work the author takes into account the experimental errors involved in the measurements of the kinematic viscosity of petroleum oils and applies the error theory to a set of equations recently published that successfully fit the data provided by the ASTM standard D 2502–92. This standard proposes a method for the evaluation of the mean molecular weight (relative molecular mass) of petroleum oils from kinematic viscosity measurements and uses a viscosity-molecular weight chart, which unfortunately involves unavoidable interpolation errors. Due to the relative complexity of the new set of equations, a PC program is designed and used in order to obtain some instructive figures that show the behaviour of the relative error linked to the mean molecular weight of petroleum oils versus different parameters. All the figures are plotted by supposing a relative error in the kinematic viscosity measurements of 1%. Our results show that, under these conditions, the relative errors associated to the evaluation of the mean molecular weight of petroleum oils are always below 2.6%, which permit us to recommend this computational method as a useful and quick alternative to the classic techniques for direct molecular weight determination.