What is the difference between elemental spectroscopy and particle counting in oil analysis?

Elemental spectroscopy (ICP-OES per ASTM D5185) measures dissolved and fine-suspended metal concentrations in ppm — each element indicating a specific component source (iron: steel bearings and cylinder liners; copper: bronze bushings; silicon: silica ingress; calcium/zinc/phosphorus: additive package). It measures only dissolved metals and particles below 5–10 µm detectable by spectroscopy. Particle counting (APC, ISO 11171) measures all particles ≥4 µm regardless of composition, expressed as ISO 4406 codes. The two methods are complementary: rising APC codes indicate total contamination increase; rising metal values identify the wear source. A system with high APC code but low metallic elements suggests external contamination ingress (silica from seals or breathers) rather than component wear.

What parameters does FTIR analysis measure in used oil, and what failure modes does each indicate?

Fourier Transform Infrared Spectroscopy (FTIR, ASTM E2412) identifies functional groups in oil by their infrared absorption wavelengths. Key parameters: Oxidation (1,710 cm-1 carbonyl peak) — oil thermal degradation, indicates oil oxidation rate and remaining oxidation inhibitor reserve. Nitration (1,630 cm-1) — products of blow-by combustion gas contamination in lube oil. Sulfation (1,170 cm-1) — sulfur combustion byproducts, elevated with high-sulfur fuel or EGR contamination. Water (3,450 cm-1) — free or dissolved water, confirmed by Karl Fischer test if elevated. Glycol (950–1,070 cm-1) — coolant contamination from head gasket or oil cooler failure. Fuel dilution is detected by viscosity comparison (ASTM D445) rather than FTIR. Each parameter has alarm thresholds — for example, oxidation increase of 25 absorbance units above baseline triggers oil change recommendation.

What is the Total Base Number (TBN), and how is it used to determine oil change intervals?

Total Base Number (TBN, ASTM D2896 or D4739) measures the alkaline reserve in engine oil available to neutralise acidic combustion byproducts (sulfuric acid from high-sulfur fuel, nitric acid from blow-by gases, organic acids from oil oxidation). Fresh API CK-4 engine oil for modern diesel engines typically has TBN of 12–14 mg KOH/g. As the oil neutralises acids, TBN depletes. The oil change trigger point is typically when TBN falls to 50% of new oil TBN (6–7 mg KOH/g for 12–14 mg/g new oil) or when Total Acid Number (TAN, ASTM D664) rises to equal or exceed remaining TBN. High-sulfur fuels (above 500 ppm sulfur, common in emerging markets) deplete TBN 3–5× faster than ULSD fuel (<15 ppm sulfur), requiring shorter drain intervals or higher TBN specification oils.

How is particle counting used to monitor filter element performance during service?

Particle counting upstream and downstream of a filter element in service tracks actual Beta ratio under real operating conditions — which may differ significantly from laboratory ISO 16889 measurements due to pressure pulsations, temperature cycling, and actual contamination particle composition. If downstream APC readings rise at constant upstream concentration, filter efficiency has declined — possible causes include bypass valve opening (element restriction exceeds cracking pressure), end-cap seal failure, or media collapse. Sequential samples from the downstream sampling point at consistent operating conditions, compared against the baseline established at commissioning, identify trending before failure. An APC code increase of 2 ISO units or more in consecutive samples without corresponding change in system operating conditions indicates filter integrity investigation is required.

What sampling interval is recommended for oil analysis in high-severity hydraulic systems?

High-severity hydraulic systems — those with contamination-sensitive proportional or servo valves, operating in dusty environments, or subjected to high thermal cycling — should be sampled for oil analysis at 250-hour intervals minimum, or monthly for continuously operating systems. Systems with established stable baseline analysis may extend to 500-hour intervals once three consecutive analyses within specification confirm system stability. Severe-duty markers (sustained operating pressure above 80% of rated, frequent cold starts, or confirmed contamination events) should trigger unscheduled sampling. ISO 4413:2011 requires at least one fluid sample analysis per planned maintenance interval, with the result evaluated against the OEM-specified cleanliness target code before the system is returned to service after maintenance.

What is viscosity measurement and why is it a primary oil condition indicator?

Kinematic viscosity (ASTM D445) measures the time for a fixed volume of oil to flow through a calibrated capillary at 40°C and 100°C. ISO VG grade number is the nominal kinematic viscosity at 40°C in cSt. Oil viscosity changes indicate: fuel dilution (large viscosity decrease, >1 cSt below specification at 40°C); glycol contamination (can cause viscosity increase through emulsion formation); severe oxidation (viscosity increase >20% from new oil baseline); or viscosity improver shear degradation (viscosity decrease in multigrade oils, indicating polymer additive breakdown). A viscosity change of ±15% from new oil specification at 40°C is a typical alarm threshold in oil analysis programs. Viscosity must be measured on a well-mixed sample at controlled temperature — field refractometer measurements are inadequate for precision viscosity assessment.

What is wear particle analysis, and how does analytical ferrography differ from spectrometric analysis?

Analytical ferrography (ASTM D7690) separates wear particles from oil magnetically and deposits them on a glass slide for microscopic examination by size, shape, morphology, and composition. It detects particles from 1 µm to 1,000 µm — capturing the large particles (>10 µm) that spectrometric analysis misses. Particle morphology provides wear mode identification: flat, smooth particles (2–5 µm) indicate normal rubbing wear; cutting particles (curved ribbons) indicate abrasive wear from hard particles; spherical particles (1–5 µm) indicate rolling element fatigue spalling; dark particles (oxides) indicate high-temperature wear events. ICP spectrometry is more quantitative for ppm-level metal analysis but size-limited to particles below 10 µm. Combining both methods provides both quantitative elemental data and qualitative wear mode diagnosis.

How is glycol contamination in engine oil diagnosed and quantified?

Glycol contamination from coolant ingress (head gasket failure, oil cooler leak, cylinder liner crack) is detected by: FTIR analysis showing absorption at 950–1,070 cm-1; elevated potassium (K) by spectrometry — coolant inhibitors contain potassium borates or silicates; elevated sodium (Na) from coolant corrosion inhibitors; and physical observation of oil sample as milky or gel-like. Quantitative glycol measurement uses the refractometer method or ASTM D2982 (HPLC). Glycol at concentrations above 0.1% (1,000 ppm) causes oil emulsion formation that prevents hydrodynamic film formation at bearing journals. Above 0.5% glycol, engine oil viscosity can increase 5–10×, and glycol-oil complexes form gel-like deposits that block oil galleries. Confirmed glycol contamination requires immediate oil change, system flush, and repair of the coolant ingress source before engine restart.

What is the significance of silicon in used oil spectrometric analysis?

Silicon (Si) in used oil spectrometry has two possible sources requiring differentiation: (1) Airborne silica dust ingress — external contamination from failed air filter seals or bypass events, indicating poor filtration protection. Silicon as silica (SiO2) is highly abrasive (hardness 7 Mohs) and the primary cause of abrasive cylinder liner and bearing wear. (2) Silicone sealant or coolant contamination — silicone gasket compounds or silicone-based coolant antifoam additives contribute Si to oil as a non-abrasive organosilicon compound. Differentiation is achieved by cross-referencing with magnesium (Mg) — coolant silicate inhibitors contain both Si and Mg. Elevated Si without elevated Mg indicates airborne silica ingress. Alarm thresholds for Si in diesel engine oil are 20–30 ppm above baseline for airborne contamination, or any detectable Si if new fill oil Si was zero and no silicone sealants were used during the last service.

What reporting format and trending methodology is used in an effective oil analysis program?

An effective oil analysis program uses trend reporting — tracking each parameter over consecutive samples on a time-series plot — rather than single-point threshold comparison. Trend analysis detects gradual degradation before alarm thresholds are reached: a slow increase in Fe from 20 to 35 to 50 ppm over three consecutive samples indicates progressive bearing wear, while a single 50 ppm reading in isolation is ambiguous. Reporting formats include: individual parameter time-series charts; rate-of-change alerts (flagging when a parameter increases by more than X ppm per 250 hours); a traffic-light status summary (green/amber/red for each parameter group); and a machine health score derived from weighted combination of all parameter statuses. Sample labels must include machine ID, compartment, oil type, fill date, hours since fill, and total machine hours — without this context, trend analysis is impossible.

Knowledge Center›Engineering›Oil Analysis Methods

Engineering · 10 min

Oil Analysis Methods

Q: How is particle counting used to monitor filter element performance during service?

Particle counting upstream and downstream of a filter element in service tracks actual Beta ratio under real operating conditions — which may differ significantly from laboratory ISO 16889 measurements due to pressure pulsations, temperature cycling, and actual contamination particle composition. If downstream APC readings rise at constant upstream concentration, filter efficiency has declined — possible causes include bypass valve opening (element restriction exceeds cracking pressure), end-cap seal failure, or media collapse. Sequential samples from the downstream sampling point at consistent operating conditions, compared against the baseline established at commissioning, identify trending before failure. An APC code increase of 2 ISO units or more in consecutive samples without corresponding change in system operating conditions indicates filter integrity investigation is required.

Q: What sampling interval is recommended for oil analysis in high-severity hydraulic systems?

High-severity hydraulic systems — those with contamination-sensitive proportional or servo valves, operating in dusty environments, or subjected to high thermal cycling — should be sampled for oil analysis at 250-hour intervals minimum, or monthly for continuously operating systems. Systems with established stable baseline analysis may extend to 500-hour intervals once three consecutive analyses within specification confirm system stability. Severe-duty markers (sustained operating pressure above 80% of rated, frequent cold starts, or confirmed contamination events) should trigger unscheduled sampling. ISO 4413:2011 requires at least one fluid sample analysis per planned maintenance interval, with the result evaluated against the OEM-specified cleanliness target code before the system is returned to service after maintenance.

Q: What is viscosity measurement and why is it a primary oil condition indicator?

Kinematic viscosity (ASTM D445) measures the time for a fixed volume of oil to flow through a calibrated capillary at 40°C and 100°C. ISO VG grade number is the nominal kinematic viscosity at 40°C in cSt. Oil viscosity changes indicate: fuel dilution (large viscosity decrease, >1 cSt below specification at 40°C); glycol contamination (can cause viscosity increase through emulsion formation); severe oxidation (viscosity increase >20% from new oil baseline); or viscosity improver shear degradation (viscosity decrease in multigrade oils, indicating polymer additive breakdown). A viscosity change of ±15% from new oil specification at 40°C is a typical alarm threshold in oil analysis programs. Viscosity must be measured on a well-mixed sample at controlled temperature — field refractometer measurements are inadequate for precision viscosity assessment.

ICP Spectroscopy, TAN/TBN, Viscosity, and Wear Metal Trending

Oil analysis is the systematic measurement of physical and chemical properties of in-service lubricating oil to detect component wear, monitor oil degradation, and identify contamination before damage occurs. When applied to a defined sampling schedule, oil analysis provides a leading indicator of failure — typically identifying problems 200–500 operating hours before mechanical failure, allowing planned intervention rather than unplanned breakdown.

ASTM D5185

ICP standard

ASTM D664 (mgKOH/g)

TAN standard

≤50% of new oil TBN

TBN service limit

ASTM D445 at 40°C and 100°C

Viscosity standard

01 /

ICP Spectrometric Wear Metal Analysis

Inductively Coupled Plasma (ICP) spectrometry is the standard method for measuring dissolved and sub-20 µm particle wear metals in engine and hydraulic oils. ASTM D5185 (Multi-Element Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry) is the reference method. A plasma torch at approximately 10,000 K atomises and excites elements in the oil sample; emission lines at element-specific wavelengths are measured simultaneously. Reportable elements and their diagnostic significance: iron (Fe) — cylinder liners, camshaft, crankshaft wear; chromium (Cr) — piston ring, cylinder liner wear; aluminium (Al) — piston crown, bearing housing, gear case wear; copper (Cu) — bearing overlay wear (Cu–Pb, Cu–Sn overlays), bronze bushing wear; lead (Pb) — bearing overlay failure; silicon (Si) — airborne dust ingestion (silica) OR antifreeze coolant (silicone additive); sodium (Na) — coolant ingress (sodium silicate antifreeze); boron (B) — coolant ingress (sodium borate antifreeze). ICP reliably detects particles below 7–10 µm; larger wear particles (fatigue spalling, chip wear) are not efficiently dissolved or atomised and require particle counting or analytical ferrography for detection.

Liner, crankshaft, camshaft wear

Iron (Fe)

Dust ingestion OR coolant (context dependent)

Silicon (Si)

Effective to ~7–10 µm diameter

ICP particle size limit

02 /

Total Acid Number and Total Base Number

Total Acid Number (TAN) measures the concentration of all acidic components in oil — oxidation products, acidic combustion by-products, and depleted additive degradation products. ASTM D664 (Potentiometric Titration Method) expresses TAN in mg KOH per gram of oil (mgKOH/g). New engine oil TAN typically ranges 0.3–1.5 mgKOH/g. A rising TAN trend indicates accelerating oxidation or acid contamination. Alert limit: TAN ≥ 2× new oil value or ≥ 2.0 mgKOH/g depending on OEM specification. Total Base Number (TBN) measures the alkaline reserve of the oil — primarily the detergent and dispersant additives that neutralise combustion acids. ASTM D2896 (Potentiometric Perchloric Acid Titration Method). New diesel engine oil TBN: 10–30 mgKOH/g depending on application (mining diesel at 30+ TBN for high-sulphur fuel; on-road diesel at 10–14 TBN with low-sulphur ULSD). As TBN depletes during service, TAN rises. Service limit is typically TBN ≤ 50% of new oil TBN, or TAN approaching TBN. TBN:TAN ratio ≥ 1 is the minimum criterion for maintaining protective alkaline buffering.

ASTM D664 (mgKOH/g)

TAN test method

ASTM D2896 (mgKOH/g)

TBN test method

≤50% of new oil TBN, or TBN ≤ TAN

Service limit (TBN)

03 /

Kinematic Viscosity

Kinematic viscosity is the fundamental physical property governing oil film thickness, pump efficiency, and bearing protection. ASTM D445 (Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids) measures viscosity by timing oil flow through a calibrated capillary tube at 40°C and 100°C. Results in centistokes (cSt = mm²/s). Viscosity index (ASTM D2270) characterises viscosity-temperature sensitivity: high VI oils maintain viscosity better across temperature range. Viscosity decrease in service (shear thinning): VI improvers in multigrade oils are high-molecular-weight polymers that shear into shorter chains under mechanical stress, reducing viscosity permanently. Alert: viscosity ≥20% below or ≥30% above new oil value at 40°C. Viscosity increase: oxidation increases molecular weight of base oil and creates sludge precursors. Fuel dilution from injector blow-by reduces viscosity acutely (confirmed by flash point test ASTM D92: fuel-diluted oil flash point drops below 160°C). Coolant ingress from head gasket failure increases water content and, after emulsification, apparent viscosity.

ASTM D445 at 40°C and 100°C

Test standard

≥20% below new oil spec

Viscosity decrease alert

Flash point <160°C (ASTM D92)

Fuel dilution indicator

04 /

FTIR Spectroscopy for Oil Condition

Fourier Transform Infrared spectroscopy (FTIR) identifies molecular species in used oil by comparing absorption spectra to a reference spectrum of new (unused) oil of the same grade. Absorbance peaks at characteristic wavenumbers indicate specific degradation products: oxidation products (1,700–1,760 cm⁻¹, carbonyl stretch); nitration products (1,620 cm⁻¹, nitrile bands from combustion gas oxidation of nitrogen); soot/combustion particulates (2,000 cm⁻¹ baseline elevation); water (3,400 cm⁻¹, OH stretch — used for water detection when coulometric KFT is not available); glycol coolant (866 cm⁻¹, ethylene glycol signature). FTIR is reported in Absorbance Units (AU) relative to the new oil baseline. Alert limits vary by OEM: oxidation ≥ 25 AU and nitration ≥ 25 AU are common fleet trigger values for SAE 15W-40 diesel engine oil. FTIR is a rapid, cost-effective screening technique — a complete spectrum is generated in under 2 minutes. Confirmation of specific contaminants identified by FTIR is done by dedicated test methods (D664 for oxidation acids, D6304 for water).

1,700–1,760 cm⁻¹ (carbonyl)

Oxidation peak

866 cm⁻¹

Glycol coolant peak

≥25 AU (relative to new oil)

Oxidation alert limit

05 /

Particle Counting in Used Oil

ISO 4406 particle counting applied to used engine or hydraulic oil provides early detection of increased internal wear before ICP spectroscopy can detect it (ICP is limited to particles ≤7–10 µm; particles from surface fatigue and spalling are typically 25–100 µm). Automatic particle counters calibrated per ISO 11171 count particles at ≥4 µm, ≥6 µm, and ≥14 µm. Establishing a baseline ISO code for each specific machine type (e.g., ISO 17/15/12 for a healthy diesel engine lube circuit) allows deviation detection — a shift of two code units above baseline indicates a doubling in particle generation rate. When particle counts rise and ICP shows no corresponding elemental increase, the source particles are larger than 10 µm and represent fatigue or spalling events. Combined ICP + particle counting covers the full size distribution and provides complementary diagnostic information: ICP for chronic sub-surface wear, particle counting for acute mechanical events.

≤7–10 µm reliably detected

ICP particle size limit

25–100 µm (ICP misses these)

Fatigue particle size

≥2 code-unit rise from individual baseline

Alert threshold

06 /

Sampling Protocols and Trending

Oil analysis value is maximised by consistent sampling protocols that enable trend detection across multiple data points. Key protocol requirements: sample at consistent engine/machine hours from the same sampling point each interval; use pre-cleaned sample tubes or syringes with a dedicated sampling valve (not the drain plug); sample at operating temperature with the system at normal load; fill sample bottles to 3/4 capacity to allow mixing without aeration. Recommended intervals: engine oil in mining trucks, every 250 hours; engine oil in on-road diesel, every 500 hours; hydraulic oil in construction equipment, every 250–500 hours; turbine oil, every 1,000–2,000 hours. A single sample is a measurement; three or more samples from the same machine are a trend. Alert limits for a single sample are less meaningful than a rising trend — an iron reading of 80 ppm that was 15 ppm previously indicates a 5× wear rate acceleration, even if 80 ppm has not crossed the absolute alert limit.

COMMON ENGINEERING MISTAKES

Interpreting single-sample wear metal concentrations as definitive wear rate indicators without rate-of-change context. A high silicon reading may reflect recent seal replacement (run-in contamination), not accelerating abrasive wear — trends over 3+ samples at known drain intervals distinguish the two.

Not adjusting ICP-OES results for oil drain interval when comparing samples taken at different intervals. A 500-hour iron reading of 80 ppm represents higher wear rate than a 250-hour reading of 80 ppm — normalize to mg/hour or mg/L for valid comparisons.

Relying solely on ICP spectroscopy to detect advanced bearing fatigue. ICP reliably detects particles below 5–8 µm — spalling fatigue produces large wear particles (20–100+ µm) that are under-reported by ICP. Particle count or ferrographic analysis is required for complete wear assessment.

ENGINEERING REFERENCES

STANDARD

ASTM D5185-19, Standard Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)

Primary method for elemental metal analysis in used oil for wear particle monitoring.

STANDARD

ASTM E2412-10, Standard Practice for Condition Monitoring of Used Lubricants by Trend Analysis Using Fourier Transform Infrared (FT-IR) Spectrometry

FTIR method for monitoring oxidation, nitration, sulfation, water, and glycol contamination in used lubricating oil.

STANDARD

ASTM D2896-15, Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration

TBN measurement method used to track alkaline reserve depletion and guide oil change timing.

HANDBOOK

Fitch, J.C. and Troyer, D., Oil Analysis Basics, Noria Corporation, 2010

Comprehensive reference covering sampling procedures, parameter interpretation, alarm setting methodology, and trending analysis for industrial oil analysis programs.

FREQUENTLY ASKED QUESTIONS