Correct Oil Sampling
Oil analysis is only as good as the sample. Poor sampling technique introduces contamination and invalidates results. Correct method:
- Sample from a live sampling port in the mid-stream of the oil flow (not from the drain plug or at the bottom of the sump)
- Sample when the machine is at normal operating temperature and running (or immediately after shutdown — within 15 minutes)
- Flush the sample port with 3–5 mL of system oil before taking the sample
- Use clean, dry sample bottles — never reuse bottles
- Label clearly: machine ID, location, oil type, oil hours, machine hours since last oil change
- Send to lab within 5 days; store samples below 30°C
Lubricant Condition Tests
| Test | What it measures | Action trigger |
|---|---|---|
| Kinematic viscosity (mm²/s at 40°C and 100°C) | Oil thickness — change indicates oxidation, contamination, or wrong oil | ±15% from new oil spec |
| Total Acid Number (TAN, mg KOH/g) | Acid build-up from oxidation of mineral oils | TAN increase >1.5× new oil value or >2 mg KOH/g |
| Total Base Number (TBN, mg KOH/g) | Remaining alkaline additive reserve (engine and marine oils) | TBN below 50% of new oil value |
| Oxidation (FTIR, absorbance units) | Oxidative degradation — heat aging, free radical chain reactions | >20 A/cm (varies by oil type) |
| Nitration (FTIR) | Combustion by-product — indicates blow-by in engines | >20 A/cm |
Contamination Tests
| Test | What it measures | Action limit |
|---|---|---|
| Water content (Karl Fischer, ppm) | Free and dissolved water in oil | >200 ppm hydraulic; >500 ppm gearbox; >0.1% engine |
| ISO 4406 particle count (cleanliness code) | Particle contamination by size (>4, >6, >14 µm) | Depends on system — e.g. hydraulics: ISO 17/15/12 typical target |
| Fuel dilution (%) | Fuel in engine oil — indicates injector leak or blowby | >0.5% (minor); >2% (significant — immediate action) |
| Soot (%) | Incomplete combustion products | >1% — investigate engine combustion condition |
| Coolant (glycol detection) | Antifreeze ingress via head gasket or cooler leak | Any positive result — stop machine; identify source |
Wear Metal Analysis
ICP (Inductively Coupled Plasma) spectroscopy measures dissolved and colloidal wear metals in ppm. Key metals and their source components:
| Metal | Source component | Concern threshold (gearbox, ppm) |
|---|---|---|
| Iron (Fe) | Gears, shafts, bearings, casing | >150 ppm (watch); >500 ppm (action) |
| Copper (Cu) | Bronze gears, bearing cages, cooler tubes | >100 ppm (watch) |
| Lead (Pb) | Bearing overlays, solders | >50 ppm (action) |
| Aluminium (Al) | Aluminium casing, seals, pistons | >50 ppm |
| Silicon (Si) | Ingested dust (soil contamination); seal material | >25 ppm (contamination indicator) |
| Chrome (Cr) | Rings, chrome-plated components | >20 ppm |
Note: ICP only detects particles <5–10 µm. Large wear particles (indicating severe wear) may be present but not detected by ICP. Supplement with particle count (ISO 4406) and ferrography or filter analysis for larger particle detection.
Trend Analysis
Never make decisions from a single sample. Trends reveal accelerating wear. Key trend signals:
- Sudden increase in Fe or Cu in consecutive samples — accelerating wear, investigate urgently
- Slowly rising Si — ingress contamination through worn seals or air filtration failure
- Viscosity creep upward — oxidation; consider reducing change interval
- Water appearing — find the source immediately (cooler, seal, condensation)