Technical perspective
A turbine oil can appear clear while polar degradation products are accumulating in solution, suspension or on cooler system surfaces. Membrane Patch Colorimetry, commonly abbreviated MPC, concentrates insoluble material from a prepared oil sample onto a membrane and measures the resulting colour difference. The test is designed to support trending of lubricant-generated insoluble colour bodies in in-service turbine oils.
MPC is often discussed as a varnish test. That shorthand is useful, but incomplete. A patch colour is evidence within a varnish investigation; it is not a photograph of every deposit in the machine and it does not identify the chemistry of every retained material.
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What ASTM D7843 measures
ASTM D7843 extracts insoluble contaminants from an in-service turbine-oil sample onto a membrane patch. A spectrophotometer analyses the colour and reports a Delta E value on the CIELAB colour scale. ASTM describes the result as a condition-monitoring trending tool within a comprehensive programme and notes that the method is not appropriate for turbine oils containing dyes.
- The result represents the colour difference of retained insoluble material under the defined procedure.
- A larger colour difference generally indicates a darker or more strongly coloured patch, not a direct mass measurement.
- The method is intended for in-service turbine oils within its stated applicability.
- Trend quality depends on consistent sampling, sample preparation and test timing.
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Why sample preparation and timing matter
Varnish precursors and degradation products respond to temperature, ageing and storage. Material that was soluble in a warm circulating system may become less soluble as the sample cools. Conversely, inconsistent settling or preparation can make two otherwise similar samples behave differently during filtration.
For useful trending, collect from the same active sampling point, document operating temperature and recent filtration activity, and follow a consistent laboratory preparation protocol. Samples collected immediately after an oil change, top-up, purification run or system upset should be clearly identified rather than compared as if nothing changed.
Sampling point
Use a repeatable location representative of circulating oil, not a stagnant drain unless the investigation specifically requires it.
Operating state
Record load, oil temperature, recent shutdowns and any abnormal servo-valve or filter behaviour.
Oil history
Provide oil brand, grade, service age, top-ups, purification and filter-change dates.
Trend discipline
Compare like with like and investigate step changes before assigning a maintenance action.
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What MPC can and cannot establish
| MPC can support | MPC alone cannot establish |
|---|---|
| Trending of coloured insoluble material retained by the method | The exact chemical identity of every deposit |
| Early investigation of deposit-forming conditions in applicable turbine oils | That all internal machine surfaces are clean or varnished |
| Comparison of repeat samples collected and prepared consistently | A universal alarm limit for every oil and turbine design |
| Review of purification or corrective-action response | The root cause without operating, contamination and oil-condition evidence |
A dark patch may contain oxidation products, external contamination, wear debris or a combination of materials. A modest result does not exclude deposits already attached to system surfaces. The colour must be interpreted with the equipment symptom and the rest of the condition-monitoring evidence.
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Build the complementary test plan around the symptom
If servo valves are sticking, record temperature dependence and examine whether deposits are present on affected components. If filters block repeatedly, retain and characterise the material rather than presuming every dark filter is varnish. If oil has been topped up or changed, investigate compatibility and additive condition separately.
- Viscosity, acid number and water for basic oil condition and contamination context.
- Particle count or microscopic examination when solid contamination is part of the symptom.
- Antioxidant or oxidation-related testing where the oil, method and decision require it.
- Filter, deposit or component examination when physical material is available.
- Compatibility review when different oils, top-ups or flushing fluids may have mixed.
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Use the trend to choose the next maintenance action
The next step may be to repeat the sample, inspect a component, improve contamination control, review oil compatibility, evaluate purification or plan an oil change. Select the action from the combined evidence and verify the response with repeatable follow-up samples. Avoid using one MPC result as an automatic drain-or-continue instruction.
Apply this guidance
