Total acid number, reported in mg KOH/g and measured by titration under ASTM D664 or D974. New industrial oils start low, often 0.1 to 0.5 depending on additives, and a rising TAN signals oxidation or acidic contamination. A rise of a few tenths over the new-oil value is a warning in turbine oils, while gear and hydraulic oils tolerate more. Some AW additives read slightly acidic, so every program needs a new-oil baseline.
TAN jumped from 0.4 to 1.6 in one quarter, that compressor oil is cooking.
One TAN reading means little; the slope across several samples is what tells you oxidation has taken off.
Also heard as
- acid number
- AN
- neutralization number
Do not mix it up with
- TBN: TAN measures acids that have built up; TBN measures the alkaline reserve left to fight them, mainly in engine oils.
Related terms
TBN
Total base number: the alkaline reserve left in an engine oil to neutralize combustion acids.
Oil analysis
Lab testing of in-service oil samples to judge oil condition, contamination and machine wear.
Antioxidant
Additive that slows the chemical aging of oil by intercepting the radicals and peroxides that drive oxidation.
Varnish
Thin, sticky, amber-to-brown deposit from degraded oil that coats valves, bearings and coolers.