TBN and TAN—What does it Mean?


22 AUG 2019 David Doyle

You may have seen or heard the terms TBN or TAN being used. TBN stands for Total Base Number, while TAN is short for Total Acid Number. Both of the tests monitor the effective life of in-service lubricants. The terms for “Total Base Number” and “Total Acid Number” are generally outdated terms and the test are currently designated as Base Number, or BN, and Acid Number, or AN. So, what is the significance of these tests and when should they be used?

Base Number

Base numbers generally apply to diesel engine crankcase oils. Base number represents the level of alkalinity reserve available for neutralizing acids formed during the combustion process and may be introduced through recirculated exhaust gases. As the lubricant ages and the additive package depletes, the base number will decrease from its initial fresh oil value. A severe depletion in the alkalinity reserve additive compounding, or base number, will correlate so an increase in sulphation and oxidation, wear metals, corrosion and viscosity.

Since new API CJ4 engine oils start out with a lower base number and the level of sulfur available to create sulfuric acid during combustion has been reduced, then condemning limits for in-service CJ4 oils have also been reduced appropriately.

Condemning limits for in-service API CJ4 oils are much lower than previous API formulations. Another factor that has always been relevant is; it isn’t always a matter of how high the base number starts out for a new engine oil, but the rate at which the alkalinity reserve depletes, which is represented by the rate of decrease in the base number. This is generally a matter of “hard” Base Number vs. “soft” Base Number.

Acid Number

Acid number is a trending tool generally applied to industrial lubricants and natural gas engine oils. Acid number in a new lubricant represents a certain level of additive compounding. This can come from anti-wear additives, rust inhibitors, or other additives. The acid number can drop a bit after a lubricant has been in service for a certain period, which indicates some initial additive depletion. After a time the acid number will start to increase, which indicates the creation of acidic degradation products related to oxidation. The acid number is a means of monitoring fluid service life. A significant increase in acid number should correlate with increases in oxidation and nitration numbers and viscosity.