I have always recommended MPC test patch photo to be included in the used turbine oil analysis reports.
Turbine Engineer asked : Why MPC test patch photo should always be included in the report and not just MPC value ?
Khash Replied:
In turbine oil analysis, the MPC test is one of the most important tools for evaluating varnish potential. However, reporting only the MPC value is not enough. A professional report should include:
1. MPC value
2. MPC patch photo
3. Patch weight / membrane deposit weight, when available
4. Technical interpretation of the patch appearance
This is because varnish is not just a number. It is a deposit-forming behavior of oil degradation products, and the membrane patch gives important visual and physical evidence.
1. MPC value tells us “how much color,” not the full story
The MPC value is usually based on the color intensity of the membrane after oil degradation products are captured on the patch. It is a very useful number for trending varnish potential.
But the MPC value alone cannot explain everything.
For example, two turbine oils may both have:
MPC = 35
But one patch may be smooth, uniform, amber-brown.
The other patch may be black, spotty, sticky-looking, or uneven.
Technically, these two oils are not telling the same story.
One may indicate typical oxidation varnish precursors.
The other may indicate thermal degradation, contamination, insoluble sludge, carbonaceous deposits, or abnormal oil chemistry.
So the MPC number gives the severity, but the patch photo gives the fingerprint.
2. Patch photo is visual evidence
The MPC patch photo should be included in the report because it allows the user to see the actual membrane condition.
The photo can show:
Uniform staining
Usually linked with dissolved oxidation by-products and varnish precursors.
Dark center staining
May suggest heavier deposit concentration, poor solubility reserve, or more advanced degradation.
Ring staining
Can be related to filtration/test behavior, deposit migration during drying, or uneven distribution of degradation products.
Black specks
May suggest contamination, carbonaceous material, dirt, degraded elastomer material, thermal degradation, or particles not purely related to soluble varnish.
Patch edge staining
Can show how the oil/deposit flowed across the membrane.
Uneven or blotchy pattern
May suggest mixed contamination, unstable degradation products, poor sample homogeneity, or test handling effects.
This is why a report without the patch photo is incomplete.
3. Patch weight gives another layer of evidence
Patch weight, or membrane deposit weight, can be extremely useful when the lab method allows it.
The MPC value is mainly related to color intensity.
Patch weight is related to mass of material captured on the membrane.
These are not always the same.
A patch can be very dark but have low deposit mass.
Another patch can have high mass loading but moderate color.
This difference is important.
Example 1: High MPC, low patch weight
This may mean the oil has strongly colored oxidation products, but not a large amount of heavy insoluble material.
Possible interpretation:
The varnish potential is high, but the captured deposit may be mainly colored soluble/semi-soluble degradation products.
Example 2: Moderate MPC, high patch weight
This may mean the patch captured significant material, but the material is not extremely dark.
Possible interpretation:
The oil may contain sludge, soft contaminants, additive residue, external contamination, or insoluble material that does not produce very high color intensity.
Example 3: High MPC and high patch weight
This is a stronger warning condition.
Possible interpretation:
The oil is carrying both highly colored degradation products and significant deposit-forming mass. This may indicate advanced oil degradation, poor varnish solubility reserve, or an active deposit formation problem.
Example 4: Low MPC but visible particles / high patch weight
This can happen when the captured material is not strongly colored varnish but is still physically present.
Possible interpretation:
The issue may be cleanliness, contamination, insoluble particles, or sludge rather than classic oxidation varnish potential.
4. MPC value, patch photo, and patch weight should be interpreted together
A strong turbine oil report should not say only:
MPC = 32
A stronger report should say something like:
MPC = 32. The membrane patch shows uniform amber-brown staining with slight darker center concentration. No heavy black particulate contamination is visible. Patch weight is moderately elevated, supporting the presence of deposit-forming degradation products. The result indicates increasing varnish potential and should be trended with RULER, acid number, particle count, water, oxidation, and system operating temperature.
This is much more useful.
It gives the maintenance team a diagnostic conclusion, not just a lab number.
5. Why patch weight matters in filtration decisions
Patch weight can help decide whether the problem is mainly:
Soluble varnish precursor chemistry
or
Insoluble deposit loading / contamination
This matters because the corrective action may not be the same.
If the patch is highly colored but low in mass, the solution should focus on removing soluble degradation products, acids, and varnish precursors.
If the patch has high mass and visible particles, the solution may require additional attention to mechanical filtration, contamination control, flushing, reservoir cleaning, breathers, seal ingress, or thermal degradation sources.
If both MPC and patch weight are high, a more aggressive oil conditioning strategy may be required.
6. Patch weight helps explain filter plugging risk
A turbine oil with increasing patch weight may indicate increasing suspended or semi-insoluble material in the oil.
This can be related to:
Filter plugging
Servo valve sticking
Control valve instability
Bearing deposit formation
Cooler fouling
Tank bottom sludge
Heat exchanger deposit accumulation
Reduced oil cleanliness stability
Poor response to normal particulate filtration
The patch weight adds a physical dimension to the report.
It answers:
How much material is being captured?
While the MPC value answers:
How strongly colored is the captured material?
Both are valuable.
7. Patch photo helps validate the patch weight
Patch weight alone can also be misleading if there is no photo.
For example, high patch weight may come from:
Dirt contamination
Fibers
Rust particles
Water-related sludge
Seal material
Carbonaceous particles
Sampling bottle contamination
Improper sample preparation
Actual varnish/sludge deposits
Without the patch image, you cannot judge what kind of material contributed to the weight.
This is why patch weight and patch photo should be reported together.
The photo gives visual context to the mass.
8. Patch photo protects against wrong conclusions
Imagine a report showing:
MPC = 12
The number looks acceptable.
But the patch photo shows black spots and uneven contamination.
This may not be a high varnish potential result, but it is still not a normal-looking patch.
Another example:
MPC = 45
The number looks serious.
But the patch photo shows an unusual ring pattern or edge effect, which may require retesting or cautious interpretation.
Without the photo, the user cannot challenge the result or ask the correct technical questions.
9. Patch photo is critical for trending
For turbine oils, one single MPC result is less powerful than a trend.
A good report should allow comparison of patch appearance over time:
Year 1: light yellow
Year 2: amber
Year 3: brown
Year 4: dark brown center
Year 5: dark brown with black particles
This visual trend is very powerful. It shows how the oil degradation fingerprint is changing.
Even if the MPC value increases slowly, the patch photo may show early signs of abnormal deposit behavior.
The report should ideally include the latest patch photo and, for critical machines, a historical comparison.
10. Patch weight improves trending even more
If patch weight is trended, it can help identify whether the oil is accumulating more physical deposit material.
Example trend:
| Date | MPC Value | Patch Weight | Patch Appearance |
|---|---|---|---|
| Jan | 12 | Low | Light amber |
| Apr | 21 | Slight increase | Amber |
| Jul | 34 | Moderate increase | Brown center |
| Oct | 48 | High | Dark brown / black specks |
This tells a stronger story than MPC alone.
It shows that varnish potential is increasing and physical deposit loading may also be increasing.
11. Suggested report format
A high-quality MPC section in a turbine oil report should include:
| Item | Why it matters |
|---|---|
| MPC value | Numerical varnish potential severity |
| MPC patch photo | Visual fingerprint of degradation/contamination |
| Patch weight, if available | Physical mass of captured material |
| Patch appearance description | Uniform, ringed, speckled, dark center, etc. |
| Comparison with previous patch | Visual and numerical trend |
| Interpretation | What the result likely means |
| Recommendation | Monitoring, filtration, root cause checks, or action |
12. Suggested wording for a professional report
You can use this wording:
The MPC result should not be interpreted only as a numerical value. The membrane patch photograph is an essential part of the result because it provides visual evidence of the deposit pattern, color intensity, particulate contamination, and test quality. Where available, patch weight should also be reported because it gives an additional indication of the physical mass of material captured on the membrane. MPC value, patch appearance, and patch weight together provide a more complete understanding of varnish potential, deposit-forming tendency, contamination influence, and oil degradation severity.
13. Important technical caution
Patch weight should not automatically be called “varnish weight.”
This is very important.
The material on the patch may include:
Oxidation by-products
Varnish precursors
Soft sludge
Dirt
Carbonaceous particles
Rust
Fibers
Additive residue
External contamination
Thermal degradation products
So the correct wording is better as:
Patch weight
Membrane deposit weight
Mass of captured material on the patch
Not always:
Varnish weight
Because not everything captured on the membrane is necessarily varnish.
14. Khash final statement
For critical turbine oils, the MPC report should not only show a value.
It should show the number, the photo, and, where possible, the patch weight.
The MPC value tells us the severity of color-forming degradation products.
The patch photo shows the fingerprint.
The patch weight shows the physical deposit loading.
Together, they help the reliability engineer understand whether the oil is simply changing color, actively forming deposits, carrying contamination, or moving toward a real varnish-related reliability risk.
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