Executive Summary
Fluid analysis tests, including oil analysis and grease analysis, are essential to avoiding equipment downtime and ultimately saving money. According to readers, a variety of test methods are under-appreciated, often due to the costs associated with running multiple tests. The value of each individual test depends on the particular application—and expert advice from a reliability professional is a valuable investment in choosing the correct tests.

Q.1. Which oil analysis tests do you feel are most under-appreciated, and why?
Turbine oil oxidation stability test (TOST) for hydraulic fluids—many brands promote this, and some do not and argue that a TOST test is specific to turbine oils. This is not entirely accurate because the oxidative stability of hydraulic fluids is as important as the additive content.

Particle count, Fourier transform infrared spectroscopy (FTIR)/high-performance liquid chromatography (HPLC), viscosity.
 
Material identification analysis because most people aren’t interested in knowing the source of the particulate. However, many times it’s the only way to get to the root of the problem and prevent future issues. 
 
It seems to me that the measurement of sediments and the consequences that their presence can cause in the systems has been very neglected, to the extent that many companies do not even review the condition of the filter elements, which allows us to see the great detachment we have toward this parameter.

Particle count size distribution, benchmark new oil analysis.

Oxidation. It is key in setting oil drain intervals.

Membrane patch colorimetry (MPC) as it’s a measure of the risk of varnishing and not well understood.

Oxidation by FTIR and perhaps particle count.

Quench oil testing for quenching oils. These tests are essential to maintain the product’s performance and integrity.

Particle counts; generally they are an added cost, and most customers want the lowest cost option.
 
Online oil debris monitoring. It is expensive, may require machinery modification and additional qualified personnel.

Viscosity at both 40℃ and 100℃. It is common for only one to be requested. This makes it difficult to define the viscosity index (VI) and means that a very simple first check is no longer possible. 
 
ISO 4406 cleanliness testing—not enough end-users understand the significance of not only quantity but morphology of the wear debris generated in equipment components. Sampling from primary, secondary and even tertiary sampling ports becomes supremely important when identifying wear particle origins, component degradation, scheduling filter swaps (when applicable) and scheduling equipment downtime for repairs.

Antifreeze analysis. Forgotten part of the engine until steam is flying. 

Analysis of elemental content via inductively coupled plasma (ICP). Some standard tests for elemental content don’t look at the whole additive package chemistry.

FTIR oxidation/nitration on gasoline engine oil samples are overlooked. Combining these results with viscosity, base and additives can give a good indication on whether the oil is being overworked.

Water contamination, operators hardly see benefits from the water control (ppm contamination).

ISO cleanliness. I still see a lot of customers who don’t understand the importance.

I think most oil tests are appropriately appreciated. Acid number might be more paid attention to.

Metals and viscosity because they are taken for granted. Both tell you so much about the oil, although when values are off one does need to look deeper.

Condition monitoring.
 
I think it might be demulsibility testing, especially for industrial applications. Poor water separation can lead to all kinds of issues leading to reduced machine life. It can occur slowly over time and may not be noticed until some damage may have already occurred. Also, it is something that can be easily checked onsite by using a spoon, pliers (or heat resistant gloves) and a lighter (safety first). Additionally, water in an engine oil could signal a coolant leak as well—the engine killer.

MPC. I think that all significant hydraulic and circulating systems need to include routine MPC. You need to have an MPC trend to understand where the oil is at in the oxidation lifecycle. A stand-alone MPC value of 20-30 does not tell anything about whether the oil will varnish, has varnished or is at a stable state with respect to varnish potential. Need the trend.

Ferrography is under-appreciated. I feel people are not understanding its importance, and I also feel people are not correctly interpreting it for their equipment.
 
ASTM D7647/ISO 11500 particle counts (typically not used enough because of additional cost). Also MPC for varnish—sludge potential in hydraulic oils (once again added testing cost).

Rotational disc electrode—this testing can detect larger particles than ICP and provides more useful data to a customer.

Thermal oil. Due to ignorance.
 
Particle count—particle contamination is a leading source of accelerated wear. However, faulty sampling techniques often result in artificially high particle count numbers, so training is essential.

In-use quench oil analysis. In preservation of metal analysis, for instance heavy duty engine oils, tracking wear metals is pretty well understood to be cheap insurance. Quench oil baths can be like tar pits, and the customer often desires antiquated testing. Yet the heat transfer dynamics of the quench oils do change, affecting the metallurgical properties of the finished parts. The only real answer, in my experience, is to run cooling curve testing at some frequency. Unfortunately this is demanding with respect to sample size, equipment cost and footprint and labor. As a result, some operations simply don’t test at all.

FTIR, understanding the oxidation state of the oil will aid in remaining useful life, as long as there is sufficient filtration.

TOST as it is not as widely known, and the industry has been following rotating pressure vessel oxidation test (RPVOT) for a number of years (even though it is outdated and doesn’t have a good repeatability score).

Infrared (IR) analysis is commonly used to detect contamination in lubricants. However, people often avoid examining the full IR spectrum due to the large number of peaks and the need to overlay it with a reference oil for comparison. While IR analysis may not identify the specific contaminant, it can indicate the presence of something that should not be there. It is a quick, easy and low-volume test that helps identify potential issues requiring further investigation.

X-ray fluorescence (XRF), insufficient education.

Wear metal analysis using mass spectroscopy.
 
Ferrography. Because of limited availability and skill requirements for analysis.

Acid number and base number. These provide a good long-term evaluation of how hard the lubricant has been pushed throughout its service life.

FTIR, because it can be very complex and is lube type specific.

All the test parameters.

MPC and RULER.

Elemental analysis—there is no clear information about the condition of the additives, and limits for contaminants vary from one lab to the other.

All oil analysis tests and results are important to me.

MPC varnish, oxidation stability.

Grease analysis. This also mirrors most operators’ lack of knowledge about grease applications.

FTIR analysis—the test takes very little sample to run and produces a wide range of results, such as the presence of contaminants like water and soot, oxidation and nitration, additive types, additive degradation, grease thickener type and base oil type. It gives the most bang for the buck with very little sample required.

Analytical ferrography. Main issue is cost and lack of awareness of impact.

The particle quantifier (PQ) index is underestimated due to the ISO 4406 code, but we must disambiguate that they are different topics.

Water and viscosity.

Gas chromatography mass spectrometry (GC-MS) and liquid chromatography high-resolution mass spectrometry (LC-HRMS) analyses for in-depth understanding of root causes, early detection of degradation onset, both contributing to effective counter measure actions and to improvements of lubricant formulations. Also surface analyses by X-ray photoelectron spectroscopy (XPS) are important for in-depth understanding of lubrication mechanisms and to design effective lubricants.

Crackle test, if you need a quick field test before sending a sample in for full analysis.

Oxidation/nitration.

Blotter spot test. Gives a lot of information for very little cost but does not lend itself to automation by the big labs.

Ferrous particle quantifier index (PQI) and also debris analysis in general. From a reliability engineering perspective debris contamination is known to be the major source of equipment failure and downtime. It is important to understand the root cause and source of the debris to mitigate future failures.

Different oil analysis test laboratories provide different test results. Therefore, the problem is not in under-appreciated tests, the problem is in finding the good quality oil analysis test laboratory.

Particle count, cleanliness of the lubricant directly affects the life of your lubricated equipment.

H2O because the water affects the metal and starts the rust, particle count because the particles affect the metal surface, and viscosity because the variation in viscosity could affect the machine that is using the oil.
 
For oil analysis, a system that can analyze the main items on time without taking a long time is necessary. For this purpose, instead of competing with individual systems, I think it is effective to use a method that keeps the threshold value of the oil within a certain range and then understands what is happening when it deviates from that range.
 
Water content as even ppm traces of water in oils can be a prelude to initiating failure, solids in greases.
 
Analytical methods are diverse, and some are less commonly used despite their potential benefits. Now, I will introduce two methods that are highly valuable but are less commonly used: 1) Automatic vacuum distillation, ASTM D1160, and 2) refractive index, ASTM D1218. For example, vacuum distillation is highly effective in identifying oil components, particularly for Newtonian fluids, as it reveals whether the composition is wide or narrow. Another method is refractive index measurement, which is quick, easy and helpful for identifying compound types. Additionally, there are other lesser-known methods that can provide valuable insights in specific applications.
 
FTIR. It is a relatively new addition to the test suite. Most oil labs in Australia simply report results by the direct joint oil analysis program (JOAP) method but are oblivious to the impact of ester interference in the oxidation region. This impacts the expanding use of synthetic products. Most labs in Australia are competent in routine testing but their diagnosis is poor (little evidence that they understand the performance requirements of the lube or the machines).

Proximate and ultimate analysis test, because they are less related to tribology.

Particle counting. Many people don’t understand how to interpret the test appropriately.

Critical load. It is difficult to measure true values at friction nodes.

Grease analysis as the sample is much more complicated to take.

Refrigeration compressor oil tests. Many just make a visual check to see if the oil looks clear. With little contaminants in refrigeration systems, it seems all good, but viscosity could be way off.

Mini-rotary viscometer (MRV) for engine/motor oils, additive and wear elements.

Chromatography (liquid or gas) to quantify the different molecules.

Onsite particle counting with portable devices: underused, possibly through initial costs. Gives immediate insight into particulate issues and can give water content, oil quality, viscosity and density readings as well, depending on the device used.

ISO 4406 particle count.

MPC. This test is not a routine test but provides critical information.

Additive metals by ICP and comparison with new oil reference can provide valuable information on lubricant mix, wrong product identification and oil degradation.

Boiling point curve (BPC) analysis. Contains a host of under-utilized information on the characteristics of hydrocarbons in general and what one can learn from the results.

The particle count. A quote from lubricators: “A dirty environment is always going to be dirty.”

FTIR and RULER, they’re probably under-appreciated because they’re more difficult to interpret and require more data points to create meaningful analysis. Where things like viscosity, particle count and ICP can give you a snapshot of the current oil sample condition, some tests do require much more context, so they’re less utilized.

Not oil, but coolant tests. They are rarely done and can prevent failures just as well as oil sampling.

Corrosion tests for various metals.

Air release testing—an important test to detect presence of polar contamination in turbine and compressor lubricants. 
 
ISO 4405. It is easy to process, and it gives so much of information with microscopic examination. Soluble varnish content can as well been detected.
 
Particle analyses of used oil, ASTM D4898. Mostly only a number is requested of weight increase of filter used. But information on the nature of the particles is lost.

What percentage of end-users do you feel are doing oil analysis?
0%-20%         16%
21%-40%         41%
41%-60%         31%
61%-80%         9%
81%-100% 3%
Based on an informal poll sent to 15,000 TLT readers.

Turbines oil analysis.

Heat transfer oil analysis will be the most under-appreciated. Most maintenance people never agree to try once.

That would depend on the source of the used oil. Engine oil compared to hydraulic oil, as an example, would necessitate a different look and test to determine if the most under-appreciated.

Prolonged worked stability of greases (penetration change after 100,000 stroke) is rarely done, but it is a very important parameter showing the shear stability of the grease.

Density at 15℃ as indicative of composition with respect to aromatic, naphthenic, isoparaffin and n-paraffin content when taken into context with other parameters.

Q.2. Are there any “non-standard” oil analysis tests that you feel should be more regularly used? And in what applications?

Crackle test for water.

Centrifuge test for turbine oil deposits.

Digest versus direct spray ICP.
 
For applications with extended operating times above 100℃ and the resulting oxidative stress on the fluid, assessment of the fluid’s remaining useful life would be beneficial. ASTM D6971 linear sweep voltammetry comes to mind.
 
Varnish potential for hydraulic systems.

Yes, sweep voltammetry!

Analysis of particle morphology is one area that I believe would be highly beneficial if it were more prevalent. The type of particulate contamination present in a system is easily as important as the amount of contamination present.
 
Pressure differential scanning calorimetry (PDSC) in applications where heat and water are present.

Microbial analysis, especially for water-based metalworking fluids. As we know, once you get bacteria or fungi in a system, depending when it is determined, it can be a real problem to get rid of. It usually requires a specialized cleaning procedure including a dispersant-type chemistry to dislodge growth from the walls of the sump and the piping of the system, then a full flush of the system. This requires a halt in production and time and resources better spent elsewhere but becomes a necessity for a bad case to take care of the problem (usually) and for large centralized systems can be a plant manager’s nightmare.

Ferrography should be performed in regular sampling intervals in oil and grease lubricated bearings to avoid catastrophic failures.
 
Moisture testing by relative humidity. This can be done on any type of sample, and it provides effective answers that the crackle test could never do.

What percentage of end-users do you feel are doing grease analysis?
0%-20%         81%
21%-40%         13%
41%-60%         5%
61%-80%         1%
81%-100% 0%
Based on an informal poll sent to 15,000 TLT readers.
Oil compatibility.
 
Flash point tests for oils in high temperature applications.

Currently there is a huge problem with hybrid corrosion. Unlike 100% internal combustion engine (ICE vehicles), a hybrid’s ICE runs intermittently and usually does not reach temperatures to drive the water out, which is a product of combustion. OEMs are quietly dealing with major engine issues from water contamination, discovered at the dealerships before the cars are sold! Engine oil (EO) manufacturers and EO additive companies need to wake up. Various procedures already exist to measure the rust protection and water separation of oils that are mixed/contaminated with water. Compared to some of the existing EO qualifications, their inclusion is extremely necessary.

Viscosity should always be done at both 40°C and 100°C to identify any shear effects, even in monograde products.
 
TOST for turbine oil applications as it gives a more accurate representation of the performance of the oil.

Two-dimensional gas chromatography (2D GC) is not widely available, but it offers enhanced capabilities for analyzing lubricants for contaminants and impurities. Additionally, it can help determine if your lubricant supplier has altered their formulation. The improved resolution allows you to detect changes in the base oil composition and subtle variations in additive composition that may not be visible with IR or standard has chromatography (GC) methods. It is a valuable test to consider when conducting detailed investigative analysis of a lubricant-related failure.

Elemental analysis, for rotating machinery.

Metal in the oil.

Photo of the patch membrane.
 
Particle count for < 4 micron size including sub micron.

I think PQ for engines should be a standard as ICP is not always the answer. Though PQ is relatively inexpensive to run, the labs treat it as a specialty test and thus are missing out on some potential data points.

Most labs will add non-standard tests on a by-request basis.

An overall infrared measurement (i.e., not the oxidation or nitration measures alone, but an overview of the complete spectra). When I ran an analysis lab (long ago), this was routinely used to confirm oil type and look for major changes.

Greases analysis should be used more frequently with an appropriate focus.

Aeration especially in hydraulic applications.

High-pressure and shear dependent viscosity changes under application relevant conditions, as they are essential for load carrying capability in demanding applications, e.g., gears, roller bearings and journal bearings.

Oil oxidation (deposition) tests. Putting a piece of metal in the test oil and heating it in an oven at elevated temperature for a week can prevent many issues.
 
Foaming characteristics of lubricating oils.

Actually, when we test automatic transmission fluid (ATF) we try creating “non-standard” tests based on the modification of the standard ones with adjusting the test parameters and test conditions for the specific ATF tests conditions.

Amount of iron.

Crackle test for water in oils, thin film visualizations of lubricating greases.
 
For problem solving, we have had significant success with adding XRF and XRD to the filtergram.

RULER to determine antioxidant depletion and varnish potential rating (VPR) in hydraulic systems.

Express tests “motor check-up.” Objective information on oil condition without laboratory tests.

Foaming tests/air release tests for hydraulic oils. When using IR analyses, it should be an IR of the fresh oil from the machine for comparison available.

For turbine and hydraulic oils, soluble MPC and insoluble MPC tests should be used more regularly and needs to have an ASTM method.

IR, UV and, in general spectroscopic analysis, providing a “fingerprint” containing a lot of useful information.

Insoluble testing. Too many different options; a single test method standard should be adopted.

Yes, but different in applications.

Gelation test, especially for hydraulic oils.

Antioxidant analysis on greases, as part of preventive maintenance strategies on medium/high temperature bearing applications.
 
Foam test, demulsability on hydraulic oils as these properties change as the hydraulic oil deteriorates.
 
Smell and filtration.

Tackiness measurement of the tacky lubricants such as slideway oils, anti-splash coning oils. This is a non-standard finger test, but I believe it should be standardized. 

Editor’s Note: Sounding Board is based on an informal poll sent to 15,000 TLT readers. Views expressed are those of the respondents and do not reflect the opinions of the Society of Tribologists and Lubrication Engineers. STLE does not vouch for the technical accuracy of opinions expressed in Sounding Board, nor does inclusion of a comment represent an endorsement of the technology by STLE.