The total acid number of the quanity of the base ,expressed in terms of the equivalent number of milligrams of potassium hydroxide that is required to titrate the strong base constituents present in 1 gram of the sample " ASTM method D 664 or D 974 "
TAN measures the amount of acid present in the lubricant, which is independent of the remaining TBN or Total Base Number. As acids form, they get neutralized by the over based detergents in motor oil and get sequestered into detergent micelles. But they are still there and contribute to the level of oxidation and eventual viscosity increase. TAN is usually used for gear lube or hydraulic fluid analysis, where the lubricant typically does not contain a robust over based detergent package. TBN is a more useful metric for used motor oil since it indicates the capacity of the lubricant to continue neutralizing acids.
Remember that for ester based synthetic oils, you don't need as robust a detergent package as for other base stocks. It is shear resistant and doesn't oxidize as long as a suitable additive package is present (the actual anti-oxidant capacity doesn't show up in oil analysis). I think that the TAN number of an unused oil can be as misleading as most of the other info that can be derived from VOA. The only thing that matters in how well on oil holds up over time in the real world.
quote:Since the detergents themselves are weak acids, I would imagine that a higher TAN in a VOA would reflect a stronger detergent package.
Detergents are generally made of base metals or chemicals that have high base numbers.
A higher starting TAN indicates that the original acid content is higher than one with a lower acid number.
The important number for motor oil is TBN and how that TBN behaves over the drain interval. The TBN will reduce over the life of the oil since the chemicals that produce the TBN will be neutralizing acids and converting them to salts.