RULER Test Overview
by David Doyle, CLS, OMA I, OMA II
General Manager, Tribology

From time to time end-users and lubricant suppliers have inquired about the application of the RULER analysis and the test results provided. In-service Turbine Oil and Industrial Fluids analysis by RULER instrumentation indicates the percent remaining active antioxidant in the form of aromatic amines and hindered phenols. For an accurate determination of the remaining oxidation life of an in-service fluid a new oil reference is required in order to gauge the active antioxidant in the used oil against the original level in the new oil. Generally ALS Tribology will use 25% remaining active antioxidant concentration as a condemning limit in a used fluid. This test is useful if used in conjunction with other test parameters that monitor fluid degradation and fluid life, such as acid number, oxidation number by FTIR, and RPVOT for oxidation life and serviceability. Anti-oxidants work in two ways; they either deactivate oxidation compounds and free radicals from reacting with the surface of the oil molecule, or they act as sacrificial substitutes that oxidation agents are more readily are attracted to. Once the RULER reading drops the ability for an in-service fluid to inhibit oxidation compounds and free radicals is reduced in terms of service life.

RULER stands for 'Remaining Useful Life Evaluation Routine'. The ASTM methods generally referenced for Turbine Oil Application are ASTM D6971, 'Measurement of Hindered Phenolic and Aromatic Amine Antioxidant Content in Non-zinc Turbine Oils by Linear Sweep Voltammetry', or ASTM D6810, 'Measurement of Hindered Phenolic Antioxidant Content in Non-Zinc Turbine Oils by Linear Sweep Voltammetry'. Though the ASTM methods reference the application to non-zinc containing turbine oils, the RULER instrumentation also provides application to zinc containing hydraulic and industrial fluids and well as crankcase engine oils.

This test method is a key ingredient in the test matrix ALS Tribology provides for determination of Varnish Potential in Turbine and Industrial Fluids. The test method can also be provided as a standalone test parameter. But as previously indicated the test is most useful when used in conjunction with other tests for monitoring fluid service life or to compliment a test matrix for determination of varnish potential.