At some point, someone convinced you that sending an oil sample to a lab for analysis is a smart thing to do. After some discussion, you dispatched your best technician to collect the sample. The technician used the best possible sampling techniques and properly labeled the bottle. A few days later you receive the results and can clearly read the numbers and charts on the report.
Now, I ask you, what are you going to do with that sheet of numbers? What does it all mean? How are you going to convert this presumably important information into some action that justifies both the lab cost and in-house cost charged against your department? Essentially what I want to understand is WHY?
The big picture
If you remember elementary chemistry, you know that lubricants are nothing but a mixture of long-chain hydrocarbons that somebody dug out of the ground mixed together with some specialty chemicals designed to improve its properties. What is great for maintainers is that you can’t simply destroy hydrocarbon molecules by grinding them between gears and mashing them between steel plates. Hydrocarbon molecules are immune to physical attack making them perfect for lubricating industrial machinery. These molecules do break down over time and through oxidation.
Oil analysis should address the fluid contaminants.
The sump and system from which your technician drew that sample constitutes a container. There are other fluids besides oil that can reside in that same container. These fluids are the enemy of your critical equipment and are considered contamination.
Oil analysis should address the bits and pieces of solid contaminants.
The only reason a machine requires a lubricant is that somewhere in its internal workings are some pieces of metal and other materials rubbing or banging against each other. This mechanical action induces stress that exceeds the metal’s elastic limit and metal pieces break off and incorporate into the mix of lubricant and moving metal. The oil carries these pieces away so more stress can break off more pieces.
Oil analysis reports present a lot of information covering the properties of the oil, the fluid contaminants mixed in the oil and the solids suspended in the oil.
Let’s get specific on some of the terms and metrics that can have an impact on your lubricants.
The specific gravity is nothing more than the mass per unit volume referenced to a standard temperature. Oil contaminated by solvents, for example, has a specific gravity different from that of clean oil. It’s a nice number to know, however it’s typically not a tell-all and may simply indicate that additional investigation is needed to see a bigger picture.
The most important characteristic of your lubricant. Viscosity is a measure of the oil’s relative resistance to flow. Its value depends on the temperature at which viscosity is measured, the viscometer used for the measurement and the technique of the lab technician. For best results, always measure the viscosity at the same temperature on the same viscometer operated by the same technician. Since this may be impossible to do on-site, many companies outsource this measurement to insure total consistency.
Measuring viscosity at a constant room temperature yields the greatest resolution. At elevated temperatures, the difference between the viscosity of degraded oil and good oil is too close to differentiate. The correct viscosity is application dependent, temperature dependent, and has both a lower and upper limit. The specs on the lubricated machine should indicate the proper viscosity to be used. Using a lubricant that fits squarely in this range is critical to the longevity of the equipment.
Because lubricants are mixed on a per batch basis, you simply cannot count on your viscometer to give you readings shown on the label on the oil drum or on the oil spec sheet. To be sure your lubricant is in spec, measure the viscosity to get a baseline value for the current lot of clean oil. Get another baseline reading each time you start drawing from a different load of oil.
Total acid number (TAN) and Total base number(TBN)
This measurement, in milligrams of potassium hydroxide per gram of oil, is related to the level of oxidation present in the oil – more oxidation, higher acid number. Get a baseline reading on a sample of clean oil.
As the additives deplete over time, the reported acid number drops, goes through a minimum, then rises. It drops because the additives are being depleted. The curve bottoms out when they are gone. It rises again because there’s now genuine acidic material in your oil.
Acid buildup in oil is a bad thing. Chemistry being what it is, bases react with acids to neutralize them. The total base number is an aggregate measure of the concentration of components in the oil that counteract acid buildup. It’s measured in equivalent milligrams of potassium hydroxide per gram of oil.
This test is generally performed only for oils taken from internal combustion engine crankcases.
There seems to be some controversy about changing oil on the basis of total base number. Some proponents advocate changing oil when the total base number depletes to 50% of the total base number of the virgin stock. Others advocate waiting until the total base number drops to somewhere around 2 mg. KOH/g. My advice – be conservative. Oil is cheap, machinery isn’t.
Fourier Transform Infrared Spectroscopy (FTIR) is a versatile way to determine contamination levels and additive depletion in oil. Infrared radiation interacts with materials on a molecular level, not the atomic level.
The spectrometer measures the infrared light passing through the sample. The spectrograph is nothing more than a curve of the intensity of the transmitted IR as a function of wavelength.
This analytical method monitors the depleting concentration of antioxidants and anti-wear components. It also measures increasing levels of contamination from moisture, oxidation products and dissolved metals. If the sample is from an internal combustion engine, it also measures ethylene glycol and soot levels.
Particle count is a critical test in proper oil analysis that reveals the type of wear debris in the oil and its particle size distribution. Keep an eye on how the distribution curve shifts over time. If you want to gain the most value from this test, this data should be tracked regularly and trended.
Wear particle monitoring depends on the magnetic properties of the particles. Solids in oil is bad. Particle count is a direct measure of the degraded pieces of gear, bearing, and/or other metals which have disconnected from the machine and are loose in your lubricant. Since particle count isn’t the only critical factor in lubrication degradation use this as part of a comprehensive series of tests vs. a tell-all.
Analytical ferrography is another form of wear particle analysis. This approach identifies the composition of the solid chunks in your oil sample. The objective is to identify the solid pieces seen in the microscope eyepiece. Getting meaningful results as the identity of wear particles, their sizes, counts, size distribution and shape is a matter of experience and training.
Because this test is a visual inspection, experienced technicians can identify nonmetallic and other nonmagnetic wear particles. Because analytical ferrography is a direct look at the particle, it provides more objective information about the machine from which the oil was taken. The test is neither inexpensive nor quick because there’s a great deal of handling and expertise in testing this way. Ferrography can tell you what pieces of your equipment are wearing and at what rate which makes this test best for verification testing.
So, what now?
There are more test available – for instance, the entire series of ASTM standards for measuring oil. Practical oil analysis depends on the lubricant and the application. The key to making sense of oil analysis is understanding that no single test can give an absolute clear picture.
To make your results the best they can be, you must be consistent, get a representative sample, and provide trending for each of the tests that relate to your application and equipment.
Alarming test results
From time to time your results may be so out of limit that it doesn’t make sense. Good labs will re-test when this happens to make sure it was not an error in the test equipment or process. What is highly alarming is getting this result even after the sample is re-tested. The recommendation would be first get another sample to the lab as soon as possible to have that second sample re-tested as well. If the results are identical or nearly identical you should consult with the lab to discuss what may be going on in the equipment. That expertise isn’t something that people just know, it comes from thousands of samples from similar equipment in other machines and environments. Your ferrography results don’t lie, having someone be able to explain the result in a way that makes sense is the true value of the program in the first place.