Turbine Engineer asked Khash:

MPC measures color intensity but Varnish can be not colorful but present How to do the analysis of MPC test correctly ? MPC near Zero is the best ?

Khash answered:

Yes. This is one of the biggest misunderstandings about the MPC test (ASTM D7843).

MPC does not directly measure varnish mass.
It measures the color intensity of membrane deposits formed after filtering degraded oil under controlled conditions.

So your statement is technically correct:

Oil can contain significant varnish potential even when the patch is not very colorful.

Especially in:

• Light-colored oxidation products
• Early-stage soluble varnish
• Certain Group II / Group III turbine oils
• Amine-rich degradation products
• Very fine transparent lacquer-type deposits
• Some phosphate ester degradation products

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What MPC Actually Measures

The MPC test measures:

[
\Delta E = \sqrt{(\Delta L)^2 + (\Delta a)^2 + (\Delta b)^2}
]

\Delta E = \sqrt{(\Delta L)^2 + (\Delta a)^2 + (\Delta b)^2}

Where:

• ΔL = darkness/lightness shift
• Δa = red ↔ green shift
• Δb = yellow ↔ blue shift

The membrane patch is analyzed by spectrophotometry.

So MPC is fundamentally:

• A colorimetric index
• Not a direct gravimetric varnish measurement
• Not a direct mass measurement
• Not a chemistry-specific test

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Why MPC Can Miss “Invisible” Varnish

Some degradation products:

• are nearly transparent,
• have weak chromophores,
• or do not strongly change patch color.

But they can still:

• increase oil polarity,
• plate onto metal surfaces,
• create servo sticking,
• form bearing lacquer,
• reduce heat transfer,
• or destabilize additives.

This is why:

• low MPC ≠ absolutely no varnish risk
• high MPC ≠ all deposits are severe

---

The Correct Way to Analyze MPC

1. Never Analyze MPC Alone

MPC without context is dangerous.

Always correlate with:

• RULER / LSV
• TAN (ASTM D664)
• Oxidation
• Remaining antioxidant life
• Patch appearance
• Operating temperature
• Electrostatic discharge history
• Water contamination
• Insolubles
• Servo valve behavior
• Bearing temperatures

---

2. Look at the Patch — Not Only the Number

The patch itself is extremely important.

Black / dark brown patches

Usually indicate:

• carbonaceous degradation
• microdieseling
• hot spots
• electrostatic discharge
• severe oxidation

Yellow / orange patches

Usually indicate:

• soft varnish precursors
• oxidation byproducts
• soluble degradation materials

Uneven patch

Can indicate:

• contamination
• water influence
• localized precipitation

Very light patch with operational problems

Possible:

• transparent varnish chemistry
• dissolved precursors
• early-stage oxidation

This is why patch photography is critical.

---

3. Sampling Temperature Matters

This is MASSIVELY misunderstood.

If oil cools before sampling:

• soluble varnish can precipitate elsewhere,
• or remain dissolved differently.

Best practice:

• sample hot
• turbulent zone
• before filters if possible
• stable operating conditions

Hot turbine oil contains more soluble varnish precursors.

---

4. Oil Type Matters

Group II/III oils:

• are naturally lighter,
• often produce lighter-colored degradation products.

So:

• two oils with identical varnish risk
  may produce different MPC appearance.

This is why trend analysis is more important than absolute comparison between brands.

---

5. Trending Is More Important Than One Result

Example:

Month	MPC
January	8
February	14
March	22

This trend is more meaningful than a single MPC = 22.

Rapid increase usually indicates:

• antioxidant depletion
• oxidation acceleration
• thermal stress
• electrostatic activity
• contamination ingress

---

Is MPC Near Zero the Best?

In general:

• lower MPC is better,
• but “zero” is not always realistic or necessary.

Typical interpretation:

MPC	General Interpretation
0–10	Excellent
10–20	Monitor closely
20–30	Elevated varnish potential
>30	High risk
>40	Severe risk

But this depends on:

• OEM philosophy
• oil type
• turbine criticality
• system design
• operating temperature

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Very Important Reality

A perfectly clean oil system can still have:

• MPC = 5–10

because:

• some oxidation products always exist,
• additives themselves may contribute slight color,
• trace degradation is unavoidable.

So the goal is NOT:

“absolute zero”

The goal is:

• stable trend,
• controlled oxidation,
• no deposit formation,
• healthy antioxidant system,
• reliable equipment operation.

---

Most Advanced Way to Interpret MPC

The best engineers combine:

1. MPC value
2. Patch appearance
3. ΔL / Δa / Δb individually
4. RULER trend
5. TAN trend
6. Bearing temperatures
7. Servo valve behavior
8. Oil age
9. Filtration history
10. Operating conditions

That is real varnish diagnostics.

Not just:

“MPC = 18 therefore oil is good/bad.”

That approach is too simplistic for modern turbomachinery reliability.