Viscosity Index and Oil Analysis
Having more information increases the accuracy of an oil analysis interpretation. The more accurate your oil analysis, the better value you get on your investment. The Viscosity Index (VI) can be calculated from viscosity measurement at 40°C and 100°C using ASTM D2270. The VI can be trended and compared with new oil specifications to detect changes in the lubricant condition. With new, high speed used oil viscometers available, measuring the oil sample at both temperatures is very cost effective.
Two-temperature viscosity measurement should be used in oil analysis will help diagnose problems.
In Engines:
| Fuel Dilution | Fuel dilution and VI shear-down will cause a viscosity drop
at 100°C. An FTIR is not entirely reliable regarding the presence of
fuel. A viscosity drop at 40°C would indicate fuel dilution rather
than VI Shear.
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| VI Shear | Viscosity Improver additives can shear under load, resulting
in reduced viscosity at elevated temperatures. The viscosity at 100°C
will drop. FTIR can indicate fuel even though none is present. A
flash test on a multigrade oil or gasoline engine oil will make it difficult
to know if it was actually fuel flashing. Look for the 40°C viscosity
to remain relatively unchanged to confirm VI Shear. VI Shear is often
incorrectly attributed to fuel dilution.
|
| Incorrect Lube Usage | Often an engine is filled or is topped up with an incorrect lube. A VI of 105 rather than the expected 168 may indicate a lower cost straight grade is being substituted for the indicated multigrade. Viscosity measurement at 100°C alone will not detect this. A multigrade oil will reduce cold start wear. |
Hydraulic Systems
| VI Shear | Hydraulic oils normally have viscosity measured at 40°C.
Viscosity improver shear-down may go undetected unless viscosity is also
measured at 100°C. A viscosity drop at 100°C, while viscosity at 40°C
remains normal, indicates this situation. High pressure hydraulic
systems are susceptible to this condition. Excessive VI shear can lead
to pump failure.
|
| Cavitation | High viscosity at the pump inlet can cause cavitation wear. A lower viscosity fluid with a high viscosity index may be required. Once again, VI shear must be carefully monitored. |
| Foaming | Excessive foaming may be caused by Viscosity Improver additives. A lower VI fluid may be required. Once again, VI measurement can confirm whether the correct fluid is in the system. |
Compressors
| Incorrect Viscosity | To reduce heat and power requirements, lower viscosity compressor oils are used. This oil must retain a high viscosity at the sealing surfaces, which are hotter. A high VI fluid is required. Monitor viscosity at both temperatures to ensure adequate lubrication while obtaining greater efficiency. A drop in VI can cause severe wear or failure. |