What is oil analysis in industrial condition monitoring?
Oil analysis is a condition monitoring method that tests used lubricant for oil condition, contamination and wear debris to find machine problems early. The definition holds for oil-lubricated industrial machines such as gearboxes, pumps, compressors, turbines and hydraulic systems, when samples come from the same point on a schedule and each result is trended against a baseline.
Oil analysis is performed to determine three conditions: machine mechanical wear, lubricant condition and lubricant contamination (NASA, 2008).
ASTM D5185 determines 22 elements in used and unused lubricating oils, with detectability for most elements in the low mg/kg range (ASTM D5185, 2026).
ISO 4406:2021, edition 4, was last reviewed and confirmed in 2026 and specifies the code that defines the quantity of solid particles in a hydraulic fluid (ISO 4406:2021).
ISO 17359:2018, edition 3, gives guidelines for the general procedures for setting up a condition monitoring program and applies to all machines (ISO 17359:2018).
How does oil analysis work?
Oil analysis works by drawing a small sample of in-service lubricant from a running machine, testing it in a laboratory, and comparing each result with the new oil and with the same machine's earlier samples. NASA describes lubricating oil analysis as a way to monitor and trend emerging conditions, confirm problems identified through other predictive testing, and troubleshoot known problems on in-service machines (NASA, 2008). The oil carries evidence from every wetted surface: additives that have been used up, water or dirt that got into the system, and metal released by wear. A single number says little on its own. The change from baseline is what flags a problem, and ASTM D5185 states that wear metal concentrations can indicate abnormal wear if there are baseline concentration data for comparison (ASTM D5185, 2026). Trending only works when samples are comparable, so the U.S. Department of Energy guide says samples should be taken from an active, low-pressure line ahead of any filtration device, and from the same place in the system each time (US DOE, 2010). In standards terms, oil analysis belongs to tribology-based monitoring: ISO 14830-1:2019 specifies requirements and guidelines for the analysis of lubricating oils, hydraulic fluids, synthetic fluids and greases, and leaves electrical insulating oils and heat transfer oils out of its scope (ISO 14830-1:2019).
What three things does oil analysis measure?
Oil analysis measures three things: the condition of the lubricant, contamination of the lubricant, and wear of the machine (NASA, 2008). The U.S. Department of Energy guide frames the same split as three questions: will the oil still lubricate as designed, has a physical boundary been breached and let contaminants in, and how much are the machine's parts wearing (US DOE, 2010). NASA adds that the three areas are not unrelated, because changes in lubricant condition and contamination, if not corrected, lead to machine wear (NASA, 2008). The table below decides which family of results to read first when an oil analysis report comes back, with the test method behind each result.
Measurement family | Example tests with method number | What a change usually means |
|---|---|---|
Lubricant condition | Kinematic viscosity, ASTM D445, in cSt; acid number, ASTM D974 or D664, in mg KOH/g (NASA, 2008); additive elements by ASTM D5185 | Viscosity 10 percent off baseline in systems under about 19 L (5 US gal): discard or refresh the oil (NASA, 2008). A drop below baseline may mean contamination, probably from the wrong makeup oil (NASA, 2008). Rising acid number: oil oxidation (US DOE, 2010). Additive elements far from specification: possibly the wrong oil (ASTM D5185, 2026) |
Contamination | Water by coulometric Karl Fischer titration, ASTM D6304, from mg/kg to percent level (ASTM D6304, 2025); particle count per mL by size (NASA, 2008), coded to ISO 4406:2021 | Water: condensation, coolant leak or process leak around seals. Silicon: dirt (US DOE, 2010). Boron, sodium or potassium jump: coolant leakage (ASTM D5185, 2026) |
Machine wear | Wear metals by ICP-AES, ASTM D5185, in mg/kg (ASTM D5185, 2026); direct-read and analytical ferrography | Rise in a specific wear metal such as iron, copper, chromium, aluminum, lead, tin or nickel: a particular part is wearing (US DOE, 2010) |
The contamination row is a practical place to start, but a rise in spectrometric wear metals is itself a trigger for analytical ferrography (NASA, 2008). Water or silicon points to a breached physical boundary such as a seal or cooler (US DOE, 2010), and NASA notes that contamination left uncorrected leads to machine wear (NASA, 2008). This suggests a contamination change is often the earliest point at which a plant can act before wear metals rise.
How is an oil analysis sample tested in the lab?
An oil analysis sample is tested in the lab with a routine package of screening tests first, and more detailed tests are added only when a screening result changes. NASA advises that lubricating oil and hydraulic fluid analysis should proceed from simple techniques to more sophisticated ones, used when conditions require more information and the equipment cost or criticality justifies it (NASA, 2008). Commercial laboratories often group several tests into packages that cover lubricant condition, contamination and wear together (NASA, 2008). Elemental analysis by inductively coupled plasma atomic emission spectrometry (ICP-AES) has a known blind spot for larger debris. ASTM D5185 calibrates with oil-soluble metals, and its results are low for particles larger than a few micrometers (ASTM D5185, 2026). NASA puts the spectrometric range at soluble metal and metal particles up to 5 to 10 µm (NASA, 2008). Direct-read ferrography covers the larger debris: it separates ferrous wear particles with a magnet into small (5 to 15 µm) and large (greater than 15 µm) fractions and trends their ratio (NASA, 2008). Analytical ferrography, a microscope examination of the deposited particles, is typically started when direct-read ferrography, spectrometric metals or the particle count change, and it can show wear type such as rubbing, sliding or cutting (NASA, 2008).
What is oil analysis used for in a plant?
In a plant, oil analysis is used to find developing faults in oil-lubricated machines early, to decide when the oil needs changing, and to catch handling errors. The U.S. Department of Energy guide lists turbines, boiler feed pumps, electrohydraulic control systems, hydraulics, servo valves, gearboxes and roller bearings as typical applications (US DOE, 2010). The same guide reports that trended oil analysis data has revealed contamination introduced at oil change-out, poor flushing after repairs, addition of the wrong lubricant and improper operation (US DOE, 2010). For machines selected for wear monitoring, such as motors of 5.6 kW (7.5 hp) or larger and critical or high-cost machines, with a year or more of condition history, NASA puts routine sampling at quarterly or semi-annual intervals (NASA, 2008). Oil analysis does not pay off everywhere: for small reservoirs of about 3.8 L (1 US gal) or less, NASA notes that oil is usually changed on operating time, because replacing it can cost less than analyzing it (NASA, 2008). Choosing which tests to run is a failure mode question. ASTM D7874 applies failure mode and effect analysis (FMEA) to select in-service lubricant tests, and it is meant to show which tests would be of highest value and which would be ineffective for the failure modes of interest (ASTM D7874, reapproved 2022).
How does oil analysis differ from vibration analysis?
Oil analysis differs from vibration analysis in what it senses: oil analysis reads the chemistry and debris carried by the lubricant, while vibration analysis reads the motion that rotating parts transmit to the machine housing (US DOE, 2010). The comparison below shows where each method is the stronger first choice in the field.
Question | Oil analysis | Vibration analysis |
|---|---|---|
What it senses | Lubricant condition, contamination and wear debris (NASA, 2008) | Amplitude and frequency of machine motion (US DOE, 2010) |
Faults it finds best | Degraded or wrong oil, water, dirt, coolant ingress, wear of oil-wetted parts | Unbalance, misalignment, looseness, resonance, rotor rub, bearing and gear problems (US DOE, 2010) |
What it cannot see directly | Mechanical faults that have not yet produced debris (inferred) | Oil chemistry and contamination (inferred) |
The key row is the second one: unbalance and misalignment are faults for vibration analysis (US DOE, 2010), while only oil analysis reads the lubricant itself. The two methods overlap on wear. NASA pairs wear particle analysis with vibration monitoring when a discrete vibration frequency associated with gears or bearings is present, and advises investigating changes in spectrometric metals with analytical ferrography and correlating them with vibration analysis (NASA, 2008). When the two data sets point to different causes, a lubrication or reliability specialist should review them together; Lubinpla can organize such a case from your own documents and site history and return one cited document for review.
Related terms
ISO 4406 code: the cleanliness code that expresses the quantity of solid particles in a hydraulic fluid, used for particle count results.
ICP-AES: inductively coupled plasma atomic emission spectrometry, the elemental test behind most wear metal and additive results.
Ferrography: magnetic separation and examination of ferrous wear particles, in direct-read and analytical forms.
FMEA: failure mode and effect analysis, the method ASTM D7874 uses to match oil tests to failure modes.
Tribology-based monitoring: the condition monitoring family, covered by ISO 14830-1, that includes oil, hydraulic fluid and grease analysis.
Working a case like this? Lubinpla works it from your own documents and site history, step by step with your approval. Free to start, no card.
Sources checked 2026-10-13.
Sources
NASA. Reliability-Centered Maintenance Guide for Facilities and Collateral Equipment. September 2008. https://www.nasa.gov/wp-content/uploads/2023/06/nasa-rcmguide.pdf
U.S. Department of Energy, Federal Energy Management Program (prepared by Pacific Northwest National Laboratory). Operations and Maintenance Best Practices: A Guide to Achieving Operational Efficiency, Release 3.0 (PNNL-19634). 2010. https://www.pnnl.gov/main/publications/external/technical_reports/PNNL-19634.pdf
ASTM International. ASTM D5185-26, Standard Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). 2026. https://store.astm.org/d5185-26.html
ASTM International. ASTM D6304-25, Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration. 2025. https://store.astm.org/d6304-25.html
ASTM International. ASTM D7874-13(2022), Standard Guide for Applying Failure Mode and Effect Analysis (FMEA) to In-Service Lubricant Testing. Reapproved 2022. https://store.astm.org/d7874-13r22.html
ISO. ISO 17359:2018, Condition monitoring and diagnostics of machines, General guidelines. Edition 3, 2018, confirmed 2023. https://committee.iso.org/standard/71194.html
ISO. ISO 14830-1:2019, Condition monitoring and diagnostics of machine systems, Tribology-based monitoring and diagnostics, Part 1: General requirements and guidelines. Edition 1, 2019, confirmed 2025. https://committee.iso.org/standard/69304.html
ISO. ISO 4406:2021, Hydraulic fluid power, Fluids, Method for coding the level of contamination by solid particles. Edition 4, 2021, confirmed 2026. https://committee.iso.org/standard/79716.html
Written by the Lubinpla team from the sources listed and checked against them before publication.