Viscosity, acid number and base number: is the oil still fit for service?
Viscosity decides the oil film. Acid number and base number show how far the oil has aged or been used up. Know what a shift means before you change the oil.
Viscosity: the first number to check
Viscosity decides whether the oil can keep the surfaces apart. It is measured as kinematic viscosity at 40 C for most industrial oils. A change in either direction is a message. Thinner oil can mean fuel dilution or a wrong, lower grade. Thicker oil can mean oxidation, soot, water emulsion, or a higher grade added by mistake.
| Shift | Likely causes | Check next |
|---|---|---|
| Viscosity falls | Fuel or solvent dilution, a thinner oil added, shear of a polymer-thickened oil | Fuel dilution test, elemental additive check |
| Viscosity rises | Oxidation, soot, water emulsion, a thicker oil added | Infrared oxidation, soot, water, elemental |
| Large step, such as 249 against an expected 150 mm2/s at 40 C | A different viscosity grade added | Compare with the new oil and the top-up record |
Setting a viscosity limit
A common rule is to alarm when the viscosity differs from the grade by a set percentage, and to tighten the band for critical equipment. A viscosity check alone is not enough, because two different oils of the same grade have the same viscosity. Use it together with elemental or infrared checks.
Acid number and base number
As oil ages, oxidation forms organic acids. The total acid number (TAN) measures the acid in the oil. The total base number (TBN) measures the reserve alkalinity left from the additives. Both are reported in mg KOH per gram of oil, the amount of potassium hydroxide equivalent needed to neutralise one gram of sample.
| Number | Used for | How to read it |
|---|---|---|
| TAN | Gear oils, hydraulic oils, turbine and compressor oils | Rises with oxidation. A rise above the new-oil value by a set amount is the usual alarm. |
| TBN | Diesel and gas engine oils | Falls as the additive reserve is used up neutralising combustion acids. A drop to a set share of the new-oil value is the usual alarm. |
| Marine diesel TBN | Slow and medium speed engines on high-sulphur fuel | Starting values can be as high as 70 mg KOH/g. TBN is checked often, sometimes weekly, and fresh oil is added to hold it. |
Measuring them
- Titration is the reference method (ASTM D664 for TAN, D4739 for TBN) but needs a trained operator, reagents and waste handling.
- Infrared methods give TAN and TBN quickly if they are calibrated for the right oil family. They are best for trends and screening, with titration for the cases that matter.
- Always compare with the same oil type. TAN and TBN depend on the additive package, and different oils have different starting values.