Technical perspective
Total Acid Number (TAN) and Total Base Number (TBN) are both titration-based measures used in lubricant analysis, but they answer different questions. TAN reflects the amount of base required under the method to neutralise acidic constituents, while TBN reflects reserve alkalinity measured under its applicable test method.
Neither number is a direct measurement of lubricant life in hours. Their usefulness comes from comparing the right property with the right lubricant type, fresh-oil baseline, operating environment and historical trend.
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Where TAN is most useful
TAN is widely trended in industrial oils where oxidation and acidic degradation products are important. An increase can support evidence of oxidation, contamination or chemical change, but the fresh oil may already have a non-zero TAN because some additive chemistries themselves contribute to the measurement.
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Where TBN is most useful
TBN is commonly used for engine oils formulated with alkaline reserve to neutralise acids produced during combustion and service. A downward trend can indicate depletion of this reserve. Interpretation depends on engine design, fuel, lubricant formulation, drain interval and the exact analytical method.
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Do not use TAN and TBN as universal mirror images
- The two tests use different chemistry and may use different methods; their numeric values are not interchangeable.
- A crossover point between TAN and TBN is not a universal oil-change rule.
- Fresh-oil reference data is valuable because additive formulations create different starting values.
- Viscosity, oxidation, soot, water, wear and contamination should be reviewed alongside acid/base trends.
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Trend the property that matches the failure mode
For a turbine or hydraulic oil, acid-number growth may be more relevant than TBN. For a heavy-duty engine oil, reserve alkalinity may be a central drain-interval indicator but still should not be used alone. The laboratory test plan should be built from the lubricant formulation and machine risks rather than from a generic checklist.
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Using TAN and TBN in Indian maintenance decisions
TAN and TBN should not be treated as universal health scores. Their usefulness depends on whether the lubricant is an engine oil, industrial oil or another formulation and on the limits recommended for that application.
For Indian DG sets, gas engines and industrial equipment, trend the relevant parameter against oil hours, make-up oil and other degradation evidence. An apparent improvement after a large top-up, for example, may represent dilution rather than recovery of the used lubricant.
Apply this guidance
