As oil degrades, especially in internal combustion engines, oxidation bi-products are formed. Also, nitration of the oil can occur which is indicative of oil degradation. The level of oxidation and nitration of the lube oil can be measured by infrared analysis. Different atom-to-atom bonds absorb light in different points in the infrared region. This fact can be used to detect oxidation (carbon oxygen double bonds) and nitration (carbon nitrous-oxide bonds).
The downward peaks represent light absorption for those atom bonds listed. The amount of light absorbed is measured to give a number usually ranging from 1 to 4. This number represents the % transmittance decrease divided by 10. Typically, new engine oil may have a base line oxidation/nitration of 1/0 or 1/1. These are caused by additives. A few drops of lube oil are placed in a cell with a path length of 0.005 centimeters. An infrared light beam is then passed through the cell window at wavelengths from 400 to 4500 cms. Oxidation and nitration product atom bonds will then become excited (vibrate, stretch, or bend) and absorb light at their own wavelength. The percentage transmittance decrease is measured and reported as % transmittance decrease/10.