Testing in-service lubricants for determining lubricant and equipment integrity

Feb. 24, 2009
Diagnosing equipment conditions and evaluating in-service lubricant test results includes looking at both the health of the equipment and the integrity of the oil.

Companies with mechanical maintenance needs might be familiar with used oil testing for their equipment, but many have not fully grasped the relevance of and the value incurred from implementing an oil testing program. Companies currently using testing services still have questions regarding test reporting and its applicability. This article provides an overview of the practice of testing in-service lubricants and clarifies how testing and analysis benefits companies with no operating equipment investments.

Diagnosing equipment condition and evaluating in-service lubricant test results includes looking at both the health of the equipment and the integrity of the oil. Trending of the test data over a period of time is a valuable tool to monitor ongoing equipment condition and predict corrective action before interruption to operations or increased maintenance costs occur. Using proper trending analysis will minimize equipment downtime, permit more efficient maintenance scheduling, protect warranty claims and increase equipment resale value.

Predictive maintenance is a key advantage of analyzing in-service lubricants. Scheduled maintenance alone, without using in-service lubricant testing, may occur too frequently or not allow enough coverage between services. Optimizing maintenance schedules leads to reduced costs and a greater return on investment. Companies that take in-service lubricant analysis to another level concentrate on root cause analysis. This allows monitoring contaminants, such as dirt or water, so that root causes are controlled and kept at a level in which resulting detrimental effects are minimized.

Evaluating a used lubricant sample is not always based on a single test parameter. Equipment or lubricant conditions may be flagged based on the range of acceptable limits for multiple parameters. In addition, it may not be necessary to wait for test parameters to reach a condemning or warning limit across the board, as changes in a combination of parameters may also lead to condemning the oil or flagging an equipment condition. Trained diagnosticians must understand equipment application and design, oil formulation and costs involved in draining and replacing in-service oil. The diagnostician must also be trained to recognize the integrity or applicability of the data in order to make judgments regarding the oil’s condition.

When looking at establishing a testing program, consult with a laboratory and your lubricant supplier for the most cost-effective test package relevant to your needs. More is not always better. Acquire information that can be used effectively. Decide how the testing relates to and complements your maintenance needs. Look at tests that meet your particular needs. An important consideration is working with your laboratory to optimize the service in order to obtain the most cost-effective results and create successes that lead to increased equipment reliability. Typical testing programs are comprised of standard routine tests that can meet most service requirements. Laboratories providing your testing service should also have the capability to provide specialized testing and technical services in order to address non-routine needs.

Standard tests


Viscosity is the most important lubricant property measured. Proper viscosity is required to maintain an adequate oil film that will separate and protect moving parts. The selection of a proper viscosity grade is dependent upon temperature, speed and load. Too high of a viscosity will cause wear during start-up and excessive heat during operation. Too low of a viscosity will not create a sufficient oil film to protect moving parts. Viscosity increases due to oxidation and oil degradation, or soot in the case of diesel engines. Viscosity decreases due to contamination with a less viscous product (e.g., different oil, solvent, or fuel) or shearing of the viscosity index improver. Viscosity changes are monitored in conjunction with other test parameters (e.g., fuel dilution, soot and oxidation). Viscosity can also be judged as application-specific. For example, if an excessive mixture of natural gas in the oil is common, higher viscosity grade oil may be used to compensate for anticipated viscosity thinning.

Spectrochemical analysis

Spectrochemical analysis determines the level of dissolved and suspended metal concentrations in an oil sample with a size range of 10 micron or less, which represents metalorganic additives, contaminants and wear. Test results are expressed in parts per million (ppm). This determination is used to trend wear rates in equipment as well as to monitor changes in certain additive and contaminant levels.

Combinations of metals are analyzed to determine which components are wearing. Different equipment designs and makes will have characteristic wear metals as well as limits for wear metal concentrations. Wear metal alarms do not always have to reach an upper limit. A sudden increase in the wear metal concentration over previous sample trends should also generate a warning flag.

Samples requiring spectrochemical analysis with large metal particulates would normally require special preparation in order to account for the large particles that do not stay in suspension or are not detected by the spectrometer.


One way of monitoring oxidation in oil is by infrared analysis (FTIR), which is reported in absorbance per centimeter. Test results are based on differences in the used oil chemistry detected by infrared analysis when compared to new, unused oil.

Values for oxidation represent a relative level of oil degradation constituents. Evaluation of a lubricant’s oxidation level is best compared with and complements other measurement parameters of oil degradation. It is not necessarily considered a condemning parameter by itself but is a good trending tool when compared to other test results, such as viscosity, acid number and other oxidation tests.

Nitration and sulfation

Like oxidation, nitration and sulfation in oil can be determined by infrared analysis (FTIR) and are also reported in absorbance per centimeter. Test results are based on differences in the used oil chemistry detected by infrared analysis when compared to new, unused oil.

Values for nitration and sulfation, which are forms of oxidation due to nitrogen oxide (NOx) and sulphur oxide (Sox) compounds respectively, represent a relative level of oil degradation constituents. Nitration can be caused by oil mixture with natural gas during engine combustion, oil mixture with exhaust gases in diesel engines, or localized high pressure or hot spots in industrial applications. Sulfation tends to increase in used oil as the alkalinity reserve decreases in used oil. Evaluations of a lubricant’s nitration and sulfation levels are best compared with and are complementary to other measurement parameters of oil degradation, such as viscosity, acid number and other oxidation tests. They are not necessarily considered condemning parameters by themselves but are good trending tools when compared to other test results. In the future, as the use of ultra low sulfur fuel becomes more prevalent, sulfation will probably become less of a relevant test parameter for diesel engines.

Acid number

Acid number is oil formulation- and application-specific. The lubricant’s initial acid number is an indicator of the presence of some additive compounding, such as rust inhibitors. Used lubricating oils in service will show an increase in the acid number as the oil begins to age, experiences oxidation and degrades. Many oxidation byproducts are acidic in nature or eventually degrade to acids. The measurement of the increase in the acid number over the initial new oil value is used in conjunction with other test parameters to monitor the service life of a lubricant.

Base number

The base number measures the level of alkalinity reserve present in engine oil. The alkalinity reserve (base number) neutralizes acidic compounds that are created during the combustion process. Without this neutralization capacity, acidic contaminants will corrode engine parts and cause oil degradation, leading to further engine wear. All engine oils start out with an initial level for base number, per product specifications. As the oil ages, the base number drops as the alkalinity reserve is depleted. This is a direct measurement of the service life of engine oil.

Water by Karl Fisher titration

This is a method for accurate measurement of the level of water contamination within lubricating oil. This method is more accurate than other common methods and can quantify water content to as low as 10 parts per million. Some water-sensitive bearings can experience reduced service life with regular exposure to levels as low as 200 ppm.


A qualitative test used to help monitor the general condition of the lubricant while in service, this test complements other parameters used to determine lubricant service life, such as acid number, viscosity and oxidation number.

Particle count

A particle count measures the number of particles present in oil within different micron size ranges and applies a cleanliness rating to the test results. The test results are used to monitor the level of particulate contamination in a lubricant for systems that require a certain cleanliness level in order to operate properly, minimize equipment wear, or meet warranty requirements.

Oxidation by rotating pressure vessel oxidation test (RPVOT)

RPVOT measures the amount of oxidation inhibitor remaining in turbine and power generation fluids in order to monitor the overall service life. This test expands upon other monitoring tools, such as acid number, color, viscosity and oxidation number.

Copper strip corrosion

This test measures general corrosiveness of a lubricant and its reactability to yellow metals, such as babbitt material containing copper, bronze alloy, or brass alloy.

Rust test

This test measures the rust-preventative capabilities of an industrial lubricant and the performance of the remaining active rust inhibitor present in used oil.

Foaming characteristics

A foaming characteristics test measures the tendency for foaming to occur while a lubricant is in service. Excessive foaming can lead to lubrication and power transmission problems. The test also measures the stability of the foam once it has formed and gauges how easily it dissipates.

Water separability or demulsibility

The water separability test measures the ability for a lubricant to shed water and prevent an emulsion from forming. Poor water separability inhibits the proper performance of industrial lubricants.

Wear particle analysis — LaserNet Fines

LaserNet Fines instrumentation is able to determine different types of wear particles, such as fatigue, sliding and cutting wear. These specific wear particles are quantified according to size ranges above 20 microns. The test helps demonstrate wear patterns and measures wear occurrence not always detected by the standard spectrochemical analysis and particle count. The test also provides a particle count cleanliness rating along with the wear data.

Wear particle analysis — Direct Reading Ferrography

Direct Reading Ferrography is a trending tool used to monitor the relative level of ferrous wear material within an oil sample. The number obtained by the instrument is not an absolute number and is not quantified by weight, percent, or ppm. It quantifies ferrous wear particles in relative concentration levels. The higher the test result number determined by the DR Ferrograph, the higher concentration of ferrous wear material in the oil. Increase in the DR Ferrograph reading over time shows an increase in ferrous wear particle concentration. A sudden increase in the numbers can predict abnormal wear conditions or impending catastrophic failure. DR Ferrography quantifies particles into two sizes ranges: density large (DL) and density small (DS). The large density particles are in the size range of 5 micron and greater; the small density particles are in the range of 1-2 micron.

Wear particle analysis — Analytical Ferrography

Analytical Ferrography examines wear metal morphology in order to determine types of wear occurrence and the composition of wear material. It is performed by a trained technician looking at a prepared ferrogram slide and examined via a computer-aided microscope. Analytical Ferrography is an excellent failure analysis method. Alloy composition, oxides, contaminants, wear particles size and concentration, as well as wear mechanism can be determined.

Wear particle analysis Particle Quantifier Index (PQI)

PQI monitors the relative level of ferrous particles. As with Direct Reading Ferrography, it is an excellent trending tool. Over time, the level of ferrous particles attributed to wear generation can be trended and reported in relative concentrations ranging from light to heavy. A sudden increase in the index reading on a particular piece of equipment would indicate a change in wear particle generation and suggest a prompt inspection of components. Many of the ferrous wear particles detected by PQI may be created by an accelerated wear condition that may not have progressed enough to be picked up by the spectrochemical analysis.

Varnish precursor determination (varnish potential)

Industrial lubricants that are in service for a number of years or are exposed to stress from pressure, heat, or moisture can produce varnish deposits on working surfaces. These hardened varnish deposits start out as soluble gums suspended in the working lubricant then change to an insoluble material as their composition changes and the molecular weight increases. A series of tests are available for measuring and predicting the elevated presence of varnish precursors in a lubricant that can lead to excessive varnish deposits.

Compatibility testing

Industrial oil formulations can vary widely for the same application. The differences in base groups and additive compounding between two oils can cause incompatibility responses that will create insoluble precipitates, soaps, deposits and degradation of additive compounding. When mixing or changing out industrial oils from different suppliers or different formulations, tests may be required to verify compatibility.

Other tests

Other routine tests can include soot and insolubles in used oils, detection of coolant contamination, dirt detection, microscopic analysis of particles and fuel dilution in used engine oils.

Sampling of lubricating systems

Good sampling procedures are important in order to obtain reliable test data. Timely submission to the laboratory after taking a sample is imperative to receiving current equipment condition information. Complete and accurate information is important to proper sample processing and receiving the most value out of a testing program. Documenting procedures is recommended to ensure that proper and timely sampling occurs, that proper information is entered and that samples are shipped to the testing laboratory on time. Testing laboratories can provide a variety of sampling supplies, generally at an added cost. Good service providers are always willing to work with end users to develop sampling procedures and provide material applicable to an end user’s needs.


  • Use safe sample extraction methods
  • Use consistent practices
  • Use optimal sampling points and choose a representative location
  • Minimize sources of garbage data
  • Never sample directly from a filter
  • Take samples before the oil is changed and before adding make-up oil
  • Have the oil warmed up and thoroughly mixed before sampling
  • Do not overfill sample bottles, but ensure there is enough sample to perform all tests
  • Allow samples to cool before sealing container lids
  • In-line sample ports for circulating systems are best
  • Sample in areas of turbulence, such as bends, and avoid straight sections of pipe
  • Always use clean sample containers and equipment
  • Use well-sealed containers so they don’t leak after replacing the container cap
  • Provide required sample information
  • Keep sample bottles sealed and clean for sending to the lab
  • Keep paperwork clean
  • Establish procedures for forwarding samples to the laboratory
  • Send samples immediately to the testing lab to ensure applicable test results

Circulating system: in-line sample

Use in-line sampling ports whenever possible. The best in-line samples are obtained from the return line after the oil has passed through the operating equipment but before entry on the return side of the reservoir. This sample will contain the most information relating to the equipment health. Taking a sample after a filter only removes information.

Flushing sample ports and any extraction devices is recommended. Clean the area around sampling ports before use. Flush at least three times the volume of the sample port to ensure a representative sample.

Fill the sample bottles approximately 80% full. Too little sample may not provide enough for adequate testing; too much may cause leakage while in transport. Allow the new sample to cool slightly before replacing the bottle cap. Seal it tightly. Re-using the insert needle and tubing for sampling other systems is not recommended.

Circulating system: reservoir sample

Obtain a sample from the reservoir only if an in-line sample is unobtainable. Fill ports or breathers are usually the best access for entering a reservoir from the top with a pump and sampling tube. If possible, sampling from the inlet side of the reservoir is recommended.

Clean the area before opening sampling access.

Do not sample from the drain port.

Do not obtain samples from the very top or the very bottom of the fluid level. Aim for 1/3 to 2/3 from the surface of the fluid.

CAUTION: Some reservoirs operate under pressure. Remove access covers carefully.


Non-circulating systems for some rotating equipment, such as gears and some bearings, use sump lubrication. A sampling port at the proper fluid level located on the sump casing provides the best means for obtaining a sample. Fill ports can sometimes provide access for entering a sump reservoir with a pump and sampling tube from the top, if the tube path is unimpeded. Avoid drawing samples directly from the bottom of the sump.

If sampling from a drain port, flush the drain port thoroughly before obtaining a sample. Also, if sampling from a drain port during the oil change, sample halfway through the drain.

Clean area before opening sampling access.

Clean systems requiring particle count

Always take extra precaution to prevent contamination of the oil during the sampling process.

Clean all entryways before opening access.

If using a sample port valve and pump, clean the area around the valve. Flush the sample port valve access and sampling equipment into an oil waste bottle. Flushing three times the volume of the port valve access and sampling equipment is recommended.

Samples can be contaminated with particulate during the sampling process without being visible to the eye. Using shop towels or any other wiping material that creates lint is not recommended.

Use dedicated sampling equipment and bottles for particle count on clean systems. The use of certified clean bottles is recommended.

Engine samples: using in-line sampling ports

The engine oil should be warm and circulating. Leave the engine idling during the sampling procedure. Locate the sampling port that is fitted to the engine block. The sampling valve should be located before the oil filter. Clean the area around the sampling port.

Flush at least three times the volume of the sample port to ensure a representative sample. Fill the sample bottle 80% to 85% full. Allow the new sample to cool slightly before replacing the bottle cap. Seal tightly.

Engine samples: using a sampling pump and siphon tube through the dipstick tube

Use an adequate length of new siphon tubing when using a sampling pump. The tubing should be long enough to reach the oil level in the sump through the dipstick without touching bottom. The oil should be warm (at least 110°F) and circulating just prior to obtaining a sample.

Shut off the engine and immediately insert the tubing into the oil reservoir through the oil dipstick tube. Lower the open end of the sample tubing to approximately one-quarter to one-half vertical depth from the top of the oil level in the sump. Do not lower the tubing to the bottom of the sump.

Draw oil into the sample bottle using the sampling pump. Fill sample bottle 80% to 85% full. Allow excess oil in the tubing to drain back into the engine. Discard the used tubing. Allow the new sample to cool slightly before replacing the bottle cap. Seal it tightly.

Engine samples: taking engine oil samples through the oil pan drain plug

Samples from the oil pan drain plug should only be used if no other sampling procedure is available. Samples are to be taken before the oil is changed and prior to adding make-up oil to the sump. Remember to take a sample after the engine has been running and the oil is warm (at least 110°F) and well mixed.

Immediately shut off the engine and clean around the oil pan drain plug. Remove the drain plug and begin draining the warm engine oil from the oil pan into a waste receptacle. Obtain a sample during mid-drain as the oil flows out of the oil pan by holding the sample bottle up to the stream of oil and filling the bottle 80% to 85% full.

CAUTION: The oil may be hot and can cause burns. Wear protective gloves. Do not take the oil sample at the start of the drain flow.

Wipe off the outside of the sample bottle. Allow the new sample to cool slightly before replacing the bottle cap. Seal it tightly.

Transformers and specialized equipment

Consult maintenance engineers and a laboratory technical department.

Analysis reporting

One of the biggest challenges to obtaining the maximum benefit from testing in-service operating equipment lubricants is getting the data back where it can be effectively used and follow-up actions can be scheduled. Clear instructions to the testing laboratory and in-house personnel for report routing is important. The data needs to go where it can be used effectively and needs to be received in a timely manner.

Data reports can be delivered in several formats, but electronic, Web-based reporting provides significant advantages for record keeping, data management and maintenance integration. Companies with multiple sites, large equipment or fleet inventories are able to manage information much more effectively using electronic reporting (as opposed to sorting through multitudes of paper reports).

A Web-based reporting service can effectively incorporate the use of in-service lubricant analysis with other maintenance processes.

Effective follow-up by end users can maximize return on investment. Part of the service provided by the testing laboratory should allow for accessible communication between the end user and the lab. Sometimes, information provided on a test report can be enhanced tenfold after a conversation with the testing laboratory. Positive results require follow up on detrimental trends before they manifest into larger problems. A well-tuned analysis program is expected to complement other maintenance practices. It is good practice to document and build on successes in order to realize improved maintenance, operating efficiency and cost savings.

Return on investment

Everything we’ve discussed thus far comes down to one thing: cost savings that improve your bottom line. End users who have documented their experiences have attributed cost savings of $25,000 or more to a single oil analysis on a single piece of equipment. Though the numbers below are considered conservative savings per detected incident, they signify the return on investment end users can realize through testing in-service lubricants.

*Condition parameter Average savings per incident
Wear only detected $2,600
Coolant contamination $2,800
Dirt detected $2,900
Water detecte $2,900
Fuel dilution detected $4,600
Improper viscosity $2,300
High engine soot $2,300
High insolubles $1,000
*Based on ALS Staveley Services customer feedback survey of 227 incidences from April 2004 to July 2008.

As stated above, these cost savings are considered conservative. However, they do demonstrate the return on investment that in-service lubricant analysis can provide. In-service lubricant analysis has proven to be a cost-effective predictive maintenance tool that will enhance equipment reliability and produce cost savings. This may explain why so many Fortune 500 companies with significant investments in operating equipment use lubricant testing services in one form or another.

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