If cleaner motor oil extends engine service life, why aren’t labs reporting particle counts?
Anyone who has worked in an oil analysis lab knows the challenges of counting and sizing particles in soot-laden diesel engine oil. Historically, most labs have chosen not to attempt particle counts with black oils. However, there is increasing user interest to have such numbers included in oil analysis reports, and some labs have already begun to comply using various instruments and methodologies.
The increasing interest in contaminant monitoring of crankcase oils is driven in part by the many studies that have reported the positive effects gained from improved filtration on engine service life. One such study by General Motors documented that upgraded filter performance reduced engine wear by 50 percent. Statistics like this are hard to trivialize. In condition monitoring, any property that is important to machine reliability and life extension should be a property that is measured, assuming it is technically and economically feasible. Of course, this is the point of considerable consternation by the oil labs when it comes to crankcase oils.
How does one perform practical particle counts on opaque fluids, achieving precision and cost efficiency at the same time? Before we examine the specific challenges of particle count methodologies, let’s review the various ways to assess the presence of destructive particles (other than soot) in used motor oils. These are listed in Table 1.
Elemental Analysis of Silicon – Related standards are ASTM D5185 and D6595). Dirt concentration is estimated based on the measured elemental concentration of silicon (assumed to be silica), a prominent component of dirt.
Inexpensive. No additional testing is required. No interferences from soot or water. Quantitative.
Does not count or size particles. No ISO Code is given. Elemental spectrometers have maximum particle size detection limits in the range of 5 to 7 microns. Many solid particles are not composed of silicon and are larger than 7 microns.
|Total Insolubles (gravimetric or volumetric) – Related standards are ASTM D893-97, D4055-97 and D4898-90. Particles and other insolubles are removed and measured (weight or volume) by centrifuges of membrane filtration. Coagulants and solvents may be used to characterize/differentiate dispersed soot from other particles.||
Can measure particle concentrations, soot and soft particle suspensions collectively to less than 0.5 micron. Different membrane sizes can be use to approximate particle concentrations above defined sizes, such as 5 microns and 20 microns.
Does not count particles above specific sizes. No ISO Code is given. No information is given on particle shape, hardness, or composition (unless the sediment is tested subsequently with XRF spectrometry).
|Microscopic Particle Count - Related standard ISO 4407. Oil is passed through a membrane to exclude particles above a certain size. The particles left on the surface of the membrane are then counted by size microscopically.||
Using suitable rinse fluids, interferences from soot can generally be avoided. Actual count of solid particles above 4 microns can be obtained. Results can be reported as an ISO Code, typically just two range numbers. Shape and color of particles can be characterized visually. Unusual particles can be further examined for composition using SEM-EDX, etc.
||Labor intensive, tedious and costly. Reported results may vary between technicians.|
|Particle Count by Pore Blockage Method – Related standard BS 3406, pending ISO standard. Calibrated screen is used to approximate particle counts above certain sizes by pressure rise or flow decay methods.||As an automatic particle count method, samples can be processed relatively quickly without need to dehydrate or deaerate. Generally not influenced by oil color, soot or water contamination. Calibration standards traceable to NIST can be used. Measures hard particles only. Can estimate particle counts and ISO Codes (two range numbers only). Cannot size and count particles across a broad range of sizes.||Cannot give 3-range number ISO Codes. Does not provide information on particle composition, shape or color.|
|Automatic Optical Particle Count Method – Related standards are ISO 11500 and ISO 11171. Various similar methods but typically particles are counted by light blockage from particles passing in front of a laser beam. The interrupted laser beam is measured using a photodiode.||Can count particles across a broad spectrum. Automatic configurations enable relatively high laboratory processing speed. Can give a 3-range number ISO Code. Calibration standards traceable to NIST can be used. Widely used and accepted method.||Unless special sample preparation procedures are employed, interferences from water, soot and some oil additives may result. Even a small amount of dispersed soot can saturate the particle counter. Does not provide information on particle composition, shape or color. Soft semisolid particles are counted along with hard rigid particles.|
From Table 1, it is seen that only two methods provide an automatic method to determine particle counts (pore blockage and light blockage). Nonetheless, a few labs remain committed to the tedious microscopic counting method, primarily for the benefit of being able to characterize the shape and composition of observed particles. There are a few labs that operate both optical and pore blockage counters in tandem. Typically, all water-based and opaque fluids are processed through the pore blockage instrument while all others go through the automatic optical particle counter.
As a practical matter, more than 90 percent of commercial labs use automatic optical particle counters only. To perform particle counts on engine oils, these labs are faced with the option of either buying a pore blockage counter or preparing the sample in such a way to facilitate the use of the optical particle counter. Following are some suggestions on sample preparation methods that can enable an optical particle counter to be used successfully with diesel engine oils.
Particle Resuspension Method: This involves passing the used motor oil, diluted with a thin solvent, through a square-weave, back-flushable screen (four microns). A low-viscosity, superclean hydraulic fluid (for example, MIL-H-5606B) is then passed in the reverse direction through the screen to resuspend the particles (excluding soot particles) for subsequent counting by the optical particle counter (after agitation, sonification and deaeration).
Motor Oil Dilution Method: Oil samples are screened for dispersancy first by using the blotter spot method (might as well report a dispersancy value with the oil analysis data while you’re at it). If the oil has good dispersancy, a small amount of the sample is diluted 10:1 with fresh, superclean and dry SAE 20 motor oil. The soot particles will be finely divided and typically less than one-half micron. Most of the emulsified water (if any) will also become solubilized in the motor oil diluent.
The diluted sample is then ready for processing with the optical particle counter. Do not report the four-micron ISO range number. The dispersed and diluted soot particles effectively fall below the radar screen of the optical particle counter. For oils that have lost dispersancy, particle counts will need to be determined using another method (such as pore blockage or microscopic methods).
Solvent Cut-back Method: Some labs have reported success introducing a cocktail of solvents into a fraction of the oil sample, then violently agitating to disperse and thin out the soot before particle counting. Beware that some solvents will actually coagulate the soot - the opposite of what you want to achieve. The objective is to reduce the concentration of the soot (avoiding small particle coincidence/saturation errors) and keep the soot unagglomerated (dispersed). An individual, dispersed soot particle is typically less than 0.1 micron and too small to be counted by the optical particle counter. Agglomerated soot can range more than five microns.
All oil analysis technologies have unique imperfections. These include problems associated with calibration, precision, repeatability, interferences, fluid compatibility, false negatives and others. However when these technologies, including the sample preparation method are used properly, valuable results can usually be obtained.
It is my opinion that routine particle counting of in-service diesel engine oils will eventually occur on a broad scale, especially for off-road equipment. Increasingly, I’ve asked owners of fleet equipment to request that particle counting be included in their diesel engine oil test slate. We’ve already seen excellent case studies that validate the importance of this practice. Making laboratories and users alike feel comfortable with this valuable metric is more than a quest, it’s a mandate.