ASTM D2502 has a chart that is used to estimate the molecular weight of a petroleum oil based on two viscosities of the oil. The chart requires several estimation and interpolations, which can introduce mistakes. A calculation from mid 1985 using the 2 viscosities is presented (LINK->Calculation of a Petroleum Oil Molecular Weight). The calculated molecular weights compared to the ones determined from the chart have a difference of 2.3 standard deviations. The calculation is valid for the entire chart. The Microsoft document is available by contacting the author for a Dropbox link.
An Excel workbook (MWPetOilFrom2Vis_VisCalcs.xlsm) and the exported VBA module (MWPetOilFrom2Vis_VisCalcs.bas) are available. The function MW40_100 calculates the molecular weight from cSt viscosities at 40C and 100C. It converts the viscosities and checks if they are within the bounds of the chart.
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