ASTM D3238 Calculation of Carbon Distribution and Structural Group Analysis of Petroleum Oils by the n-DM Method
Carbon distribution and ring content data are used in characterization of oils in the refining-manufacturing process and can also correlate to critical performance properties. Refractive Index, Density and Molecular Weight (n-DM) are used to calculate the following values:
| % CA | Percentage of Aromatic Carbon |
| % CN | Percentage of Naphthenic Carbon |
| % Cp | Percentage of paraffinic Carbon |
| RA | Average Number of aromatic rings per Molecule |
| RN | Average Number of naphthene rings per Molecule |
| RT | Average Number of rings per Molecule |
The mass % Sulfur must be determined in order to calculate values for RT and RN.
The n-DM and Sulfur content are determined by the following:
| Refractive Index (n) | ASTM D1218 |
| Density at 20°C (d) | ASTM D1481 |
| Average Molecular Weight (M) | ASTM D2502 |
| Sulfur Content (mass %) | ASTM D2622 |
Applicability of the method is limited to the following:
In terms of carbon distribution – up to 75% carbon atoms in ring form; % aromatic less than or equal to 1.5 times the % naphthenic rings.
In terms of ring content – up to four rings per molecule with not more than half of them aromatic.