Comprehensive limits and interpretation guide for lubricating oil and transformer oil analysis
| Property | Unit | Normal | Caution | Critical | Notes |
|---|---|---|---|---|---|
| Viscosity @ 40°C | cSt | ±5% of new oil | ±10% of new oil | >10% change | Compare to baseline; ISO VG grade dependent |
| Total Acid Number (TAN) | mg KOH/g | < 0.2 | 0.2 - 0.5 | > 0.5 | Indicates oxidation; compare to new oil baseline |
| Moisture Content | ppm | < 100 | 100 - 250 | > 250 | Water contamination; hydraulics more sensitive |
| Oxidation Inhibitor (DBPC) | % | > 50% remaining | 25-50% remaining | < 25% remaining | Antioxidant depletion tracking |
| RPVOT (Oxidation Stability) | Minutes | > 75% of new | 50-75% of new | < 50% of new | Rotating Pressure Vessel Oxidation Test |
| Varnish Potential (MPC) | ΔE | < 15 | 15 - 30 | > 30 | Membrane Patch Colorimetry; varnish precursors |
Wear metals indicate component degradation. Compare results to established baselines and trend over time. A sudden increase (>25% from previous sample) warrants investigation regardless of absolute value.
| Element | Symbol | Normal | Caution | Critical | Primary Sources |
|---|---|---|---|---|---|
| FeIron | Wear | < 30 | 30 - 75 | > 75 | Gears, bearings, shafts, cylinders |
| CrChromium | Wear | < 5 | 5 - 15 | > 15 | Rings, liners, exhaust valves, stainless steel |
| PbLead | Wear | < 10 | 10 - 30 | > 30 | Bearings (babbitt), solder |
| CuCopper | Wear | < 20 | 20 - 50 | > 50 | Bushings, thrust washers, oil coolers |
| SnTin | Wear | < 5 | 5 - 15 | > 15 | Bearings (babbitt overlay), bronze |
| AlAluminum | Wear | < 15 | 15 - 30 | > 30 | Pistons, bearings, housings, pumps |
| NiNickel | Wear | < 5 | 5 - 10 | > 10 | Valves, high-temp alloys, bearings |
| AgSilver | Wear | < 1 | 1 - 3 | > 3 | Silver-plated bearings, wrist pins |
| Element | Symbol | Normal | Caution | Critical | Primary Sources |
|---|---|---|---|---|---|
| SiSilicon | Contaminant | < 10 | 10 - 25 | > 25 | Dirt ingression, seals, antifoam additive |
| BBoron | Contaminant | < 5 | 5 - 15 | > 15 | Coolant leak (antifreeze), some additives |
| NaSodium | Contaminant | < 10 | 10 - 25 | > 25 | Coolant leak, salt contamination, grease |
| MgMagnesium | Contaminant | < 5 | 5 - 15 | > 15 | Additives, seawater, some alloys |
| CaCalcium | Contaminant | < 10 | 10 - 30 | > 30 | Detergent additive, grease, hard water |
| KPotassium | Contaminant | < 5 | 5 - 15 | > 15 | Coolant leak, seawater |
Additive levels should be compared to new oil baseline. Significant depletion (>50%) indicates the oil may need replacement or top-up with fresh oil. Cross-contamination with different oil types will show unexpected additive patterns.
| Element | Symbol | Function | Action Level | Notes |
|---|---|---|---|---|
| ZnZinc | Additive | Anti-wear (ZDDP) | < 50% of new oil | Often paired with phosphorus; EP additive |
| PPhosphorus | Additive | Anti-wear (ZDDP) | < 50% of new oil | Part of ZDDP package; EP additive |
| MoMolybdenum | Additive | Friction modifier | Monitor trend | Solid lubricant; may also indicate wear |
| BaBarium | Additive | Detergent/dispersant | Monitor trend | Rust inhibitor; less common today |
ISO 4406 cleanliness code consists of three numbers representing particle counts at >4μm, >6μm, and >14μm sizes per mL.
>4μm / >6μm / >14μm
| Code | Particles/mL (min) | Particles/mL (max) |
|---|---|---|
| 13 | 40 | 80 |
| 14 | 80 | 160 |
| 15 | 160 | 320 |
| 16 | 320 | 640 |
| 17 | 640 | 1,300 |
| 18 | 1,300 | 2,500 |
| 19 | 2,500 | 5,000 |
| 20 | 5,000 | 10,000 |
| 21 | 10,000 | 20,000 |
| Application | Target ISO Code | Filter Rating (β10) | Notes |
|---|---|---|---|
| Servo Valves | 14/12/9 | 3μm | Most sensitive; tight clearances |
| Proportional Valves | 15/13/10 | 5μm | High precision hydraulics |
| Vane & Piston Pumps | 16/14/11 | 10μm | Industrial hydraulics |
| Gear Pumps | 17/15/12 | 15μm | Less sensitive |
| Steam Turbines | 18/16/13 | 10μm | Typical power generation |
| Industrial Gearboxes | 18/16/14 | 25μm | Circulating systems |
| Property | Unit | Good | Marginal | Poor | Test Method |
|---|---|---|---|---|---|
| Dielectric Strength | kV | > 30 | 25 - 30 | < 25 | ASTM D1816 / D877 |
| Power Factor @ 25°C | % | < 0.5 | 0.5 - 2.0 | > 2.0 | ASTM D924 |
| Interfacial Tension | mN/m (dynes/cm) | > 32 | 25 - 32 | < 25 | ASTM D971 |
| Acid Number (Neutralization) | mg KOH/g | < 0.10 | 0.10 - 0.20 | > 0.20 | ASTM D974 |
| Moisture Content | ppm | < 20 | 20 - 35 | > 35 | ASTM D1533 |
| Color | ASTM | < 2.0 | 2.0 - 3.5 | > 3.5 | ASTM D1500 |
| Oxidation Inhibitor | % DBPC | > 0.15 | 0.08 - 0.15 | < 0.08 | ASTM D2668/D4768 |
| Specific Gravity @ 20°C | - | 0.86 - 0.90 | - | Outside range | ASTM D1298 |
DGA interpretation requires considering gas ratios, rates of change (gassing rates), and transformer history. Single sample values should be compared against IEEE condition categories and historical trends.
| Gas | Formula | Condition 1 (Normal) |
Condition 2 (Caution) |
Condition 3 (Warning) |
Primary Fault Indication |
|---|---|---|---|---|---|
| Hydrogen | H₂ | < 100 | 100 - 700 | > 700 | Partial discharge, arcing, corona |
| Methane | CH₄ | < 75 | 75 - 125 | > 125 | Low-temp thermal fault (150-300°C) |
| Ethane | C₂H₆ | < 65 | 65 - 100 | > 100 | Low-temp thermal fault (150-300°C) |
| Ethylene | C₂H₄ | < 50 | 50 - 100 | > 100 | High-temp thermal fault (>300°C) |
| Acetylene | C₂H₂ | < 1 | 1 - 9 | > 9 | Arcing, very high temp (>700°C) |
| Carbon Monoxide | CO | < 350 | 350 - 570 | > 570 | Cellulose (paper) degradation |
| Carbon Dioxide | CO₂ | < 2,500 | 2,500 - 4,000 | > 4,000 | Cellulose degradation, aging |
| Total Dissolved Combustible Gas (TDCG) | - | < 720 | 720 - 1,920 | > 1,920 | Sum of all combustible gases |
| Ratio | Indicates |
|---|---|
| C₂H₂/C₂H₄ | Arcing vs thermal fault |
| CH₄/H₂ | Fault temperature |
| C₂H₄/C₂H₆ | Thermal fault severity |
| CO₂/CO | Paper aging rate (normal: 5-10) |
| PD | Partial Discharge |
| D1 | Low Energy Discharge |
| D2 | High Energy Discharge |
| T1 | Thermal <300°C |
| T2 | Thermal 300-700°C |
| T3 | Thermal >700°C |
| DT | Mix of Thermal & Electrical |
Furans are byproducts of cellulose (paper insulation) degradation. 2-FAL (2-Furfuraldehyde) is the most significant indicator. Furan levels correlate with remaining paper life and Degree of Polymerization (DP).
| Furan Compound | Abbreviation | Normal (ppb) | Caution (ppb) | Critical (ppb) |
|---|---|---|---|---|
| 2-Furfuraldehyde | 2-FAL | < 100 | 100 - 1,000 | > 1,000 |
| 5-Hydroxymethyl-2-furfural | 5-HMF | < 100 | 100 - 500 | > 500 |
| Furfuryl Alcohol | 2-FOL | < 100 | 100 - 500 | > 500 |
| 2-Acetylfuran | 2-ACF | < 100 | 100 - 500 | > 500 |
| 5-Methyl-2-furfural | 5-MEF | < 100 | 100 - 500 | > 500 |
| 2-FAL (ppb) | Estimated DP | Paper Condition | Estimated Life |
|---|---|---|---|
| < 100 | > 700 | Good | > 20 years |
| 100 - 1,000 | 450 - 700 | Moderate Aging | 10-20 years |
| 1,000 - 2,500 | 250 - 450 | Significant Aging | 5-10 years |
| > 2,500 | < 250 | Severe Degradation | End of life approaching |
Download comprehensive IEEE transformer oil analysis guidelines
Download IEEE C57.104 Reference Guide (PDF)* Place your IEEE document in the downloads folder and update the link
| Test Type | Container |
|---|---|
| Elemental Analysis / Wear Metals | Clean plastic or glass bottle |
| Particle Count | Pre-cleaned bottle (low particle) |
| Moisture / Karl Fischer | Glass syringe or sealed bottle |
| DGA (Transformers) | Glass syringe, no air bubbles |
| Dielectric Strength | Clean 1L glass container |
| Equipment Type | Criticality: High | Criticality: Medium | Criticality: Low |
|---|---|---|---|
| Steam/Gas Turbines | Monthly | Quarterly | Semi-annually |
| Hydraulic Systems | Monthly | Quarterly | Semi-annually |
| Gearboxes | Quarterly | Semi-annually | Annually |
| Compressors | Monthly | Quarterly | Semi-annually |
| Power Transformers (>10MVA) | Quarterly | Semi-annually | Annually |
| Distribution Transformers | Annually | Annually | Every 2-3 years |
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