Remember your elementary chemistry? Lubricants are nothing but a mixture of long-chain hydrocarbons that somebody dug out of the ground and some specialty chemicals. Everyone knows that you can’t destroy hydrocarbon molecules by grinding them between gears and mashing them between steel plates. Hydrocarbon molecules are immune to physical attack. So, what can go wrong with the durable molecules in your oil? What can justify spending money on oil analysis?
Nevertheless, 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.
The shipping department very carefully packed the bottle to minimize the transit problems then dispatched the carton to your favorite lab. A week later, the lab sent you a brand new, clean sample bottle, another blank label, and a sheet of paper filled with numbers they claim represents your previous sample.
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?
The big picture
Like any other fluid, oil has certain physical and chemical properties. It’s the specific combination of these properties that prompts people to use oil for lubrication. Unlike the physical gnashing of gears, oxidation is a chemical process that does break those durable molecules into smaller pieces to alter the lubricant’s properties. Therefore, oil analysis should address the intrinsic properties of the lubricant itself.
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. Because these fluids are not so good at minimizing friction, they are considered to be contaminants. Oil analysis should address the fluid 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. This mechanical action induces stress that exceeds the metal’s elastic limit and microscopic metal pieces break off. The oil carries these pieces away so more stress can break off more pieces. Therefore, oil analysis should address the bits and pieces of solid contaminants.
Oil analysis report forms 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.
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, it’s easy to measure, but it doesn’t really tell the whole story.
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 tech. for best results, always measure the viscosity at the same temperature on the same viscometer operated by the same technician. Because there are so many variables that affect the trend data, the smartest thing to do is to buy your own viscometer and train someone to use it in-house.
Measuring viscosity at some 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 and has both a lower and upper limit. The specs on the lubricated machine should indicate the proper viscosity to be used. Find an oil that matches it.
Don’t count on your viscometer to give you readings shown on the label on the oil drum or on the oil spec sheet. 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
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. One caution though – clean oil has a measurable acid number because the lab test responds to some of the antioxidants and antiwear additives as if they were acid contaminants. This is called the interference effect.
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.
Take action just before hitting the bottom of the curve. Replace the oil completely or refortify it either with concentrated additives or with a partial replacement.
If you miss the bottom, don’t risk getting into trouble. Even if you drain the sump and replace the oil, acid in the residual oil rapidly depletes the additives in the new oil. The best action to take if you miss the bottom of the curve is to drain the sump and lines, then clean them well before adding new oil.
Total base number
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. It’s the fuel’s sulfur content that forms sulfuric acid during the combustion process. In the past few years,the sulfur content of diesel fuel has been very low. This means, for example, that some over-the-road engines operate for 20,000 miles and reduce the total base number from an initial value of 8 down to a final 6. in any case, if you monitor this variable, then you should be trending using a baseline measurement on the clean oil.