Sentinel Synthetic Lubricants


Practical Guide to Oil Analysis

The practice of oil analysis has drastically changed from its original inception in the railroad industry.  In today’s computer and information age, oil analysis has evolved into a mandatory tool in your reliability-centered maintenance program.

As a predictive maintenance tool, oil analysis is used to uncover, isolate and offer solutions for abnormal lubricant and machine conditions.  These abnormalities, if left unchecked, usually result in extensive, sometimes catastrophic damage causing lost production, extensive repair costs, and even operator accidents.

The goal of a world-class oil analysis program is to increase the reliability and availability of your machinery, while minimizing maintenance costs associated with oil change-out, labor, repairs, and downtime.  Accomplishing your goal takes time, training and patience.  However, the results are dramatic and the documented savings in cost avoidance are significant!  We have seen savings of up to 35 or 40 % in savings for less use of spare parts.

Why Analyze Used Lubricants

There are three aspects of oil analysis: Lubricant Condition, Contaminants and Machine Wear.  Let see each one of them.

Lubricant Condition:  The assessment of the lubricant condition reveals whether the system fluid is healthy and fit for further service, or is ready for a change.

Contaminants:  Ingressed contaminants from the surrounding environment in the form of dirt, water and process contamination are the leading cause of machine degradation and failure.  Increased contamination alerts you to take action in order to save the oil and avoid unnecessary machine wear.

Machine Wear:  An unhealthy machine generates wear particles at an exponential rate.  The detection and analysis of these particles assist in making critical maintenance decisions.  Remember, healthy and clean oil lead to the minimization of machine wear.

What Are Lubricants?

Industrial oils are specially designed fluids composed of a base oil and a compliment of additives.  The Base Oil performs the following functions:

  • Form a fluid film between moving parts in order to reduce friction and wear.
  • Carry away contaminants to the filter.
  • Remove heat generated within the machine.

Additives are chemical compounds added to the base oil to significantly enhance the performance characteristics of the lubricant oil or to reduce the negative ones.  Typical enhanced properties include:

  • Oxidation stability.
  • Wear protection.
  • Corrosion inhibition.

Functions Of A Lubricant?

It goes from cooling to cleaning to lubricating, it is the absolute key to keeping your equipment running.  Let’s see how it does it:

Lubricate:  By introducing a film between moving parts, opposing friction surfaces are separated and allowed to move freely without any interlocking of the asperities at the metal surface.  By physically separating the moving parts, friction is greatly reduced.  The result is less wear generated and less energy required to perform the work.

Cool:  Lubricants absorb the heat generated at the friction surface and carry it away to a reservoir where it is allowed to cool before returning to service.  Oil coolers and heat exchangers are sometime used to more efficiently disperse heat.  Lubricants are an excellent dissipater of heat.

Clean:  Oil picks up solid contaminants and moves them away from the contact zone.  The contaminants can then be removed by filtration or settling in the reservoir.  Many oils have detergent characteristics to hold tiny dirt and soot particles in suspension and help prevent sludge and varnish in a system.

Protect:  Lubricants coat component surfaces providing a barrier against moisture.  The presence of moisture in the air causes oxidation, eventually leading to corrosion.  Rust occurs when steel surfaces are attacked by moisture.  Corrosion occurs when a metal surface is attacked by acids, a byproduct of oxidation.  Oils can be fortified with alkaline reserves to counter the corrosive contaminants.

Seal:  Many lubricants form a viscous seal to keep contaminants out of a component.  Greases form physical barriers to protect against dirt and water ingress.

Transmit Power:  Hydraulic systems use lubricants to protect sliding, contacting surfaces and as a source of fluid power.  Fluid under pressure actuates moving parts.

How Do Lube Oils Fail?

Contamination, degradation, or the loss of specific properties, provided or not, by the additives.


  • External Sources:  Dirt, water, and process related liquid or materials.
  • Internal Sources:  Machine wear and degradation byproducts.

Oil Degradation: 

  • Oxidation: What is it?  Atmospheric oxygen combines with hydrocarbon molecules.  The hotter the oil and the greater exposure to air, the faster the oxidation proceeds.  The initial byproducts of oxidation are sludges and varnishes.  However, further oxidation converts these byproducts into carboxylic acids.  These acids aggressively attack and corrode many machine component surfaces.

Additive Depletion: 

  • Additives are consumed or chemically changed while performing their function.  The performance characteristics of the lubricant are altered and the enhanced properties are wiped out.

What Does Oil Analysis Measure?

Physical and chemical properties of the oil, contamination, and machine wear.  Let’ see how:

Lubricating Oil Properties:  Uncover contamination or degradation by trending rates of change in selected lube properties.

  • Fourier Transform Infrared (FT-IR): Degradation byproducts (oxidation, nitration, sulfate).  External contaminants (water, glycol, fuel, soot).
  • Viscosity:  A physical property.
  • Karl Fischer Water:  Contamination.
  • Acid Number (AN): Degradation.
  • Particle Counting:  Both contamination and wear debris.

Wear Mechanisms

Type                                                               Cause

Abrasive Wear           Hard particles between or embedded in adjacent moving surfaces.

Adhesive Wear          Metal to metal contact due to overheating or insufficient lubrication.

Fatigue Wear            Repeated stress on friction surface leading to micro cracks and spalling.

Corrosive Wear         Water or chemical contamination.

Erosive Wear             Particles and high fluid velocity.


Measure of a lubricant’s resistance to flow at a specific temperature.

Operating Principle:  Measured using a viscometer.  The sample is introduced into a “U” shaped calibrated glass tube, submerged in a constant temperature bath.  The oil is warmed to a desired temperature of 40 C or 100 C and allowed to flow freely via gravity down the tube and up the other side.  The number of seconds the oil takes to flow through the calibrated section is measured.  The viscosity in centistokes (cSt) is the flow time (seconds) multiplied by the tube constant.

Significance:  Viscosity is measured at 40 C for industrial applications and 100 C for engine oil applications.  Viscosity for industrial oils is classified using the ISOVG (International Standard Organization Viscosity Grade) system that is the average viscosity at 40 C.  Viscosity for engine oils is classified according to SAE (Society of Automotive Engineers).  Viscosity is the most important physical property of oil.  Viscosity determination provides a specific number to compare to the recommended oil in service.  An abnormal viscosity (+ or – 10 %) is usually indicative of a problem.

An increase in viscosity may indicate:

  • Increasing suspended solid material such as wear particles, contamination, or soot
  • Additions of a higher viscosity oil
  • Lubricant oxidation

A decrease in viscosity may indicate:

  • Contamination from water, fuels, or process fluid
  • Additions of lower viscosity oil
  • Additive shear

Advantages:  Quickly detects the addition of a wrong oil.  Quick and inexpensive to run.  Best measurement of oil serviceability. 

Application:  All industrial lubricants.

ISP Spectroscopy

Measures the concentration of wear metals, contaminant metals and additive metals in a lubricant.

Operating Principle:  A diluted sample of oil is atomized by inert gas (argon) to form an aerosol.  This is magnetically induced to form a plasma at 9,000 C.  The high temperature causes metal ions to take on energy and release new energy in the form of photons.  A spectrum with different wavelengths is created for each element.  The instrument quantifies the amount of energy emitted and determines the concentration in parts per million (ppm) of 20 elements present in the sample.

Significance:  The Inductively Coupled Plasma (ICP) Spectrometer measures and quantifies the elements associated with wear, contamination, and additives.  This information assists decision makers in determining the oil and machine condition.  The following list outlines the specific elements detected and possible sources of the element.

Advantages:  Very repeatable, proven technology.

Disadvantages:  Cannot detect particles greater than 7 microns in size.  Level of additive elements not necessarily indicative of additive package depletion.

Application:  All industrial lubricants.


Elements                                Possible Sources

Iron                 (Fe)     Shafts, Gears, Housings, Piston Rings, Cylinder Walls
Copper                        (Cu)    Brass/Bronze Alloys, Bearings, Bushings, Thrust Washers
Lead                (Pb)     Bearings, Anti-Wear Gear
Tin                  (Sn)    Bearing Alloys, Bearing Cages, Solder
Aluminum       (Al)     Pumps, Thrust Washers, Pistons
Chromium       (Cr)     Roller Bearings, Piston Rings, Cylinder Walls
Nickel              (Ni)     Pumps, Gear Plating, Valves
Titanium         (Ti)     Exotic Alloys
Silver               (Ag)     Some Bearings
Magnesium     (Mg)    Detergent Additive, Coolant Additive
Silicon             (Si)     Dirt, Defoamant Additive
Boron              (B)       Anti-corrosion in Coolants
Sodium            (Na)    Detergent Additive, Coolant Additive
Barium                        (Ba)     Rust and Corrosion Inhibitor
Calcium           (Ca)    Detergent/Dispersant Additive
Phosphorus     (P)       Anti-wear Additive, EP Gear Additive
Potassium       (K)      Coolant Additive
Molybdenum  (Mo)    EP Additive
Zinc                 (Zn)     Anti-wear Additive

Vanadium        (V)       Turbine Blades



Crackle Test

Quick screen to determine if a sample contains moisture.

Operating Principle:  A drop of oil is placed on a hot plate that has been heated to approximately 600 F.  The sample drop bubbles, spits, cracks or pops when moisture is present.  When moisture is detected, a Karl Fischer water test is performed.

Significance:  A crackle test is a good screening test to use to determine if a sample contains moisture.

Advantages:  This is a very low cost test.  It is a good way to determine the need for further moisture analysis.

Disadvantages:  The crackle test can only detect moisture greater than 0.05% (500 ppm).  A sample with entrained gas often results with false positive results.

Applications:  All lubricants that are non-water based.

Karl Fisher Water

Quantifies the amount of water in the lubricant.

Operating Principle:  A reagent is titrated into a measured amount of sample and reacts with the OH molecules present in the moisture.  This depolarizes an electrode and determines the titration end point.  Results are reported as either % water or ppm (1% = 10,000 ppm).

Significance:  Water seriously damages the lubricant properties of oil and promotes component corrosion.  Increased water concentrations indicate possible condensation, coolant leaks, or process leaks around the seals.

Advantages:  Accurate to 0.001%.  Quantifies both emulsified and free water.

Disadvantages:  Sulfur, acetones and ketones can sometimes trigger erroneous readings.

Applications:  All lubricants, especially effective in systems sensitive to water.

Form of Water in Oil:

  • Free water (emulsified or in droplets).
  • Dissolved water.

Water Contamination Causes:

  • Fluid breakdown, such as additive precipitation and oil oxidation.
  • Reduced lubricating film thickness.
  • Corrosion.
  • Accelerated metal surface fatigue.

Sources of Water Contamination:

  • Heat exchanger leaks.
  • Seal leaks.
  • Condensation of humid air.
  • Inadequately sealed reservoir covers.


Measures the ability of a lubricant to separate from water.

Operating Principle:  Combine 40 ml of distilled water with 40 ml of oil in a graduated cylinder.  Place in a constant temperature bath and stir for 5 minutes.  The amount of oil separation is recorded at 5 minutes intervals over a period of 60 minutes.  Failure is considered an emulsion layer greater than 3ml at the end of the test.  The results are reported as such: [40-40-0(60)].


Rotating Pressure Vessel Oxidation Test

Significance:  Measures the resistance of oil to oxidation when subjected to accelerated oxidation in a sealed chamber filled with pure oxygen under pressure and at elevated temperatures.  This is influenced by the quantity and type of antioxidants, the presence of natural inhibitors in the base oil, and the resilience of the base oil to oxidation.  As a lubricant absorbs oxygen, pressure in the sealed chamber drops.  The results of this test are reported as the time (minutes) until the pressure drops to a predetermined level.

Rust Test

Rust preventing characteristics of oil in the presence of water.

Operating Principle:  A portion of the oil is placed in a beaker along with water and a polished steel rod.  The beaker is then immersed in a heated bath and is stirred for 4 hours.  At the end of the 4 hours, the steels rod is inspected for the presence of rust / corrosion.

Significance:  Evaluates the ability of the lubricant to prevent the corrosion of ferrous parts should water become mixed with the oil.

Application:  Turbines or any other machine where there is a concern of corrosion with the presence of water.

Foam Test

Measures the foaming tendency of a lubricant.

Operating Principle:  Air is forced through a diffuser within a portion of oil creating foam.  After 5 minutes of blowing, the amount of foam is recorded.   Then, the sample is observed for the clearing of generated foam.  Then either time of full dissipation is recorded or amount of foam remaining after 10 minutes.

Significance:  The tendency of lubricants to foam can cause serious issues in systems with high-speed operations.  Not only can foam cause inadequate lubrication but also other problems such as overflowing reservoirs.

Base Number

Measures the reserve alkalinity in a lubricant.

Operating Principle:  A weighed amount of sample in titration solvent is titrated with a hydrochloric acid solution to a definite end point.

Significance:  The amount of reserve alkalinity in a lubricant is critical for certain oils.  Often oil is fortified with alkaline additives to combat acid formation.  The TBN is at its highest as a new oil and decreases with service.

Applications:  Motor oils.

Fuel Dilution

Measures the amount of fuel (%) present in an engine oil.

Operating Principle:  The instrument samples the headspace above the sample for fuel vapors.  A pump inside of the test equipment draws the vapors across a sensor where absorbed hydrocarbons are measured in percent fuel present.

Significance:  Fuel dilution in engine oils is measured by this process and returns a value in percent fuel dilution.  Excessive fuel dilution can cause a drastic drop in viscosity, which may lead to increased wear.

Application:  Diesel and Gasoline Engines.

Acid Number

Measures the acidity of a lubricant.

Operating Principle:  A weighed amount of sample in titration solvent is titrated with a potassium hydroxide solution to a definite end point.

Description:  Organic acids, a byproduct of oil oxidation, degrade oil properties and lead to corrosion of the internal components.  The AN is lowest as a virgin oil and can gradually increase with use.  High acid levels are typically caused by oil oxidation.

Advantages:  A sudden rise in acidic number is an alarm for an oil change.

Applications:  All lubricating systems where extended drain intervals are considered.  Limited applications for combustion engines.

Particle Count

Measures the size and quantity of particles in a lubricant.

Operating Principle:  Light Blockage Principle.  A known volume of oil (5ml) is injected through a sampling cell.  On one side of the cell is a beam of laser light and on the other side is a detector.  As particles pass through the cell, they block the beam and cast a shadow on the detector.  The drop in light intensity received by the detector is proportional to the size of particles blocking the light beam.  Both the number and the size of particles are measured.

Operating Principle:  Fluid Flow Decay Principle.  Oil is passed through a screen of known mesh size (10 micron) and the time taken to plug the screen is determined.  The instrument then calculates the distribution in the other predetermined size ranges by extrapolation.

Significance:  Optical particle counters use the light blockage method and are particularly effective in clean systems such as turbines and hydraulics.  However, this method yields inaccurate results in the presence of water or air bubbles.  Pore blockage particle counters are based on the fluid flow decay principle.  Their data is not affected by air bubbles or water.

Advantages:  Excellent for “clean” systems (turbines, hydraulics, etc.).  Limits provided by equipment manufacturers determine filter efficiency.

Disadvantages:  Abnormal wear can be masked on systems with routinely high levels of particulate matter.  Does not determine what TYPE of debris is in sample.

Applications:  Use whenever equipment manufacturer provides recommended lubricant cleanliness levels.  Turbines, Boiler Feed Pumps, EHC Systems, Hydraulics, Servo Valves.  Any system where oil cleanliness is directly related to longer lubricant life, decreased equipment wear or improved equipment performance.

Sources of Contamination:

Built in contaminants:  This is residual contamination remaining in a system during construction or assembly.

External ingression:  This is contamination that enters the system from outside.  Possible sources include:  during an oil fill, leaving breathers off, leaving covers off the reservoir, or faulty seals.

Internally generated:  This is wear debris normally caused by the two sources above.  If you can control built-in and ingressed contamination, wear debris will be generated at significantly lower levels.

Target Cleanliness


Pressure Range

                                                            >2500              1500-2500                   <1500
Servo Valve                                       14/12/10            15/13/11                      16/14/12
Proportional Valve                            15/13/11            16/14/12                      17/15/12
Fixed Piston Pump                           17/15/12            17/16/13                      18/16/14
Vane Pump                                       17/16/13            18/16/14                      19/17/14
Pressure Control Valve                     17/16/13            18/16/14                      19/17/14
Gear Pump                                        17/16/13            18/16/14                      19/17/14
Roller Bearing System                                                                                    16/14/12
Journal Bearings                                                                                              18/16/14

Insight Cleanliness Code

The insight cleanliness code references a three-digit code that represents the cumulative number of particles greater than 4, 6, and 14 microns in one milliliter of the fluid.  The number of particles at each size range is cross-referenced to the following table to locate the ISO contamination code.  The code is written as three numbers with a slash, “/”, between them.  For example: 21/19/15.  The first number represents the code number at 4 micron, the second number is the code number at 6 micron, and the third number is the code number at 14 micron.  The code referred to is ISO 4406:99.

Ferrous Wear Concentration

Measures the amount of ferrous wear in a lubricant.

Operating Principle:  A wear particle analyzer quantifies the amount of ferrous material present in a sample of fluid.  A measured amount of sample is inserted into the analyzer and the amount of ferrous material is determined by change in magnetic flux. This change is then converted into ferrous concentration in parts per million.  Instead of using a light sensor to measure particles and report a unitless number, this instrument measures concentration and reports the results in parts per million.  Using this method, there are no interferences with non-ferrous particles.

Significance:  This test gives a direct measure of the amount of ferrous wear metals present in a sample.  Trending of ferrous concentration reveals change in the wear mode of the system.

Advantages:  Excellent trending devise for “dirty” systems such as large splash lubricated gearboxes.  No particle size limitation.

Disadvantages:  Does not detect non-ferrous particles.

Applications:  Gearboxes, Anti-friction bearings.

Analytical Ferrography

Allows analyst to visually examine wear particles present in a sample.

Operating Principle:  To create a ferrogram, a sample of oil is passed over a glass slide.  The slide rests on a magnetic tape that attracts ferrous wear particles in the oil onto the surface of the slide.  The ferrous wear particles line up in rows with the largest particles forming rows at the top of the ferrogram.  Non-ferrous particles are easily detected because they deposit randomly across the slide.

Significance:  A trained analyst visually determines the type and severity of wear deposited onto the substrate by using a high magnifying microscope.  The particles are readily identified and classified according to size, shape, and metallurgy.

Advantages:  Best method for determining severity and type of wear present.  There are no particle size or metallurgy limitations.  Wear can be documented by digital photography.

Disadvantages:  Subjective results dependent upon the Analyst.  The test is time consuming, labor intensive, and, therefore, expensive.

Applications:  Best used when other test methods indicate possible problems.

Classifying Wear

Rubbing Wear.

Description:  Ferrous particles, less than 30 microns in size.  Some Sources:  Rubbing wear is typically found in both reciprocating and non-reciprocating units.

Comments:  On a ferrogram the particles tend to align in chains.  Normal ferrous wear can be categorized as low alloy, cast iron and high alloy steel.

Severe Wear.

Description:  Metallic particles greater than 30 microns.  Fatigue or component overload that cause larger pieces of wear to detach creates severe wear. 

Comments:  Severe wear is a definite sign of abnormal running conditions.

Sliding Wear.

Description:  Metallic particles, both normal and severe, with sliding striations along one or more surfaces.  Sliding wear can be created when two parts of a machine scrape together.

Comments:  Sliding striations are often a good clue as to what part of the machine is causing wear.


Description:  Thin, smooth particles, which appear to have been rolled flat.  Roller bearings, areas where high-pressure angled or lateral contact occurs.

Comments:  Wear created by extraneous particle if the laminar has small holes or indents.

Cutting Wear.

Description:  Shaved metal particles that look like wood shavings from a lathe.  Seen in sleeve bearings and shaft couples.  Abrasives embedded in soft bearing or burrs on hardened metals create these wear particles. 

Comments:  Worm drives have a tendency to create this type of particle.  When seen indicates abnormal wear.

Dark Metal Oxide Wear.

Description:  Grey to black chunks with a semi-metallic appearance and mottled edges.      Some sources:  Breakdown of boundary film, excessive operating temperatures, and lubricant oxidation.

Comments:  The darker the color, the more severe the oxidation of the particle.


Description:  A relatively smooth particle.  Spheres are created in bearing fatigue cracks, typically roller bearings.

Comments:  Spheres are often precursors of bearing spalls.  A large increase in quantity is indicative of imminent spalling.

Non-Ferrous Metal Wear.

Description:  Any metallic particle that is not ferrous.  Most common include aluminum, copper alloy, chrome, and Babbitt.  Non-ferrous wear can be created by the machines or as additive packages in the lubricant.

Comments:  Non-ferrous metallic wear can be across the entire length of a ferrogram.  These particles will not be aligning with the ferrous wear chains.


Description:  Dirt, sand and other silica particulate.  Contaminants can enter into a system by a variety of ways: poor seals, incorrectly installed breather, during oil change, etc.

Comments:  Some can appear like crystals.  Contaminants are easily identified by using only the transmitted light source on the microscope.


Description:  Fibers are threadlike material made of asbestos, paper, glass or a synthetic material.  Most common source is fiber material.   Could be from machine housing, cleaning rages, or air contaminant.

Comments:  A small amount of fibrous material is common.

Red Oxide.

Description:  Iron oxides or rust.  It appears as orange/red in color.  Red oxides are produced when moisture enters into the system.  Water does not have to be present when red oxides are seen, as they are often difficult to filter out of the oil.

Comments:  Red oxides are not necessarily magnetic like ferrous wear.  Alpha hematite is paramagnetic and will be found on all regions of a ferrogram.

Varnishing Potential

Detect the onset of varnishing problems in turbines and hydraulics.

Varnish is formed when degradation by-products come out of solution and form soft contaminants, which can agglomerate and form varnish deposits.  Varnish is detrimental to the performance of rotating equipment, particularly gas turbines.  Several condition-monitoring tests have been developed which can be used to gauge the varnish potential of a lubricant.

Ultra Centrifuge

Operating Principle: As an oil sample is spun at 17,000 rpm in the ultra centrifuge the soft contaminant oxidation by-products, which have a higher molecular weight than the oil, will be forced to the bottom of the centrifuge tube.

Significance:  The amount of deposited contaminants can be visually compared to a scale to quantify the level of contaminants present in the oil.

Membrane Patch Colorimetry (MPC)

Operating Principle:  Insoluble deposits are extracted from the sample using a membrane patch.  The color of the patch is analyzed using a spectrophotometer.  Results are reported as delta E value in the CIE LAB scale.

Significance:  The delta E value can be trended and used to monitor oil condition with regard to varnish potential.


Operating Principle:  Antioxidants are removed from the oil by mixing sample with a solvent.  The dissolved antioxidants are then measured using linear sweep voltammetry.

Significance:  By comparing the levels of antioxidants in the used oil sample to the levels present in a virgin reference sample of the same lubricant the remaining useful life of the used oil can be estimated.

Filter Debris Analysis

Operating Principle:  The FDA instrument is a self-contained unit, which employs automated method for filter washing to extract all debris from the filter with high repeatability and reproducibility.  A used filter is placed in the system wash chamber and all debris is removed from the filter using a combination of fluid and pressurized air.  The wash fluid carrying the filter debris passes through an on line sensor, which quantifies and sizes the amount of ferrous debris.  The fluid then runs through a filter patch where the sample of debris is captured for further metallurgical analysis by X-Ray Fluorescence (XRF).  XRF analysis provides the percentage elemental composition of the sample, which can be correlated to the wear debris of interest.

Significance:  In traditional oil analysis, the only particles available for analysis are those circulating in the oil or immediately released in the oil prior to sampling.  Given the fine filtration used in rotating equipment today to produce longer life cycles, 95% of the wear debris, which could provide useful insight into machine condition, is caught in the filter and never end up in an oil sample.  It is typically discarded with the filter.  Increasingly, fine filtration is making conventional monitoring techniques less effective at providing reliable indication of machinery component wear.  FDA captures this lost information and identifies the specific components that are wearing, providing improved diagnostic and prognostic information about impending failures.

Sampling Methods

Non-Pressurized Valves:  Install valves upstream of any filter in order to capture wear particles generated by the machine.  Make sure the valve is clean and adequately flushed.

Pressurized Valves:  Use a vacuum pump with appropriate tubing.  Make sure to use new tubing for each sample in order to avoid cross contamination.  Cut the tubing to the same length each time you sample.  Try to avoid scraping the sides or the bottom of the tank or reservoir.

Ball Valves:  The least desired method of sample acquisition.  Make sure you drain plenty of oil before you collect your sample.  The sludge, particles, and water that settle to the bottom of a tank or reservoir provide poor results.

Conclusion:  As you have seen, there are many types and forms of analysis but I want to impress upon you that the results will only depend on how well the sample has been taken!!