Jim,
I have to agree with you regarding "oil analysis haveing its place", and the fact that one has to look at the real value of the data by determining trends rather than in the accuracy of any one individual test.
Regarding oil analysis, in general there are two different classes of analytical tests, those that measure the physical properties of the oil, and those that measure the level of contamination.
Physical properties are a good indication of the condition of the oil, and are often used to determine oil drain intervals. Some of the most common physical property tests are: viscosity, total acid number (TAN) and total base number (TBN), as you have mentioned.
To explain a bit more regarding the physical properties of oil analysis, look at the Viscosity, since it is one of the most important Physical properties of a lubricant.
Kinematic Viscosity (ASTM D-445) determined at 40 Deg. C and/or 100 Deg. C is a measure of the flow rate of an oil in relation to time, and is expressed in centistokes (cSt) 1 square mm/1 second = 1 cSt. This data is used to assign an SAE grade to an oil. Example: 40 grade oil has to fall between 12. 50 cSt @100 C minimum, to 16. 29 cSt @ 100 C maximum. Normally a 25% increase in viscosity is a warning that the oil is reaching the end of its useful life.
Now look at some other tests that will determine if the used engine oil is suitable for continued use.
TAN (ASTM D974) determines the level of acidity by mixing in an indicator solution and then adding potassium hydroxide (KOH) until the solution changes color. The acidity is expressed as the milligrams of KOH required to neutralize a gram of oil (mgKOH/g). TBN (ASTM D2896) determines the level of alkalinity in an oil, which indicates the ability of the oil to continue to neutralize corrosive acids. The test measures the change in electrical conductivity. A higher TBN oil is considered better in neutralizing acids than a lower TBN oil. It is best to measure the change in TBN from new oil of the same type and brand you are using. The TBN of engine oil may be obtained from data sheets, or measured by analyzing a sample of new oil. Some manufacturers, such as Amsoil put the TBN number on the labels. A 50% reduction in TBN is a warning that the additives are becoming depleted and an oil change should be considered.
Common contamination tests include: water content, fuel dilution, and dirt ingestion and wear metals analysis. Water contamination can usually be detected visually, but a water content test (ASTM D1744)) is sometimes used as well. Fuel dilution is serious in that it can significantly reduce oil viscosity and increase engine wear. Since most engine oils gradually increase in viscosity over their useful life, a noticeable reduction in viscosity is a strong indication of fuel dilution.
And then there are the "wear metals" that you are most familiar with.
Wear metals are metals used in the manufacture of the engine that will wear in normal use, such as Iron (Fe), Chromium (Cr), Lead (Pb), Copper (Cu), Tin (Sn), Aluminum (Al), Nickel (Ni), Silver (Ag). Analysis of the types and levels of wear metals can be used to determine which engine components are wearing and if the level of wear is becoming critical. Most tests measure wear metal levels spectrographically. The most common is emission spectroscopy. In this procedure a small oil sample is burned in a high temperature flame, and the equipment detects different levels of light emitted. The equipment is calibrated to simultaneously measure the emitted light from as many as 18 different wear metals and contaminants.
Another wear metal test called the atomic absorption analysis will provide the greatest level of accuracy for each element examined, but is more costly as well as time consuming as it requires one pass through the machine for each element tested.
Either of these two wear metal tests are expressed in (PPM) Parts Per Million by weight.
Then there is dirt.
Dirt is probably the most common engine oil contaminent, and high levels can lead to excessive engine wear. The most effective way to detect dust or dirt contamination is to monitor silicon levels by spectrochemical analysis, though some tests can indicate total solids by centrifugal separation or filtering through a fine membrane filter. Contamination levels will vary according to the type of engine and the application, with off-highway equipment often having the highest levels. Again, it is important to measure the change in silicon or solids levels, rather than look at any individual analysis.
Additive levels may also be measured with spectrographic metals analysis. Normal metals analysis will detect the levels of zinc, phosphorous, calcium and barium, which are common elements in most additive packages. A 50% reduction in parts-per-million of these elements indicates the oil should be changed.
And there are more data that can be looked at, such as Fuel Dilution, water, soot, and solids, but these will most likely effect TBN, Viscosity, and wear metals.
All of the above mentioned tests in my mind would then determine *IF* the oil should be changed.
You can go to
www.noria.com or
www.analystsinc.com for some good informatoin regarding oil analysis.
Hope this helps,
Wayne
amsoilman