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The practice of transferring suspended particles to the surface of a membrane for analysis has been around for decades. It is perhaps the earliest method for inspecting solid contaminants and wear debris in a used sample of oil. It is of no surprise that these methods have enduring use today. In fact, some are the basis of recently adopted standardized methods by ASTM and ISO.
While membrane-based procedures for preparing particles for analysis can be time consuming and messy (usually involving the use of glassware and solvents), the benefits can be substantial compared to alternative methods. The main advantage is the ability to both quantitatively and qualitatively describe particle contamination, depending on the method used. As in the case of microscopic particle counting, you see what you’re counting and can confirm visually what appears to be a particle. You can also characterize particle type (e.g., dirt, wear debris, rust, fibers, etc.).
Because of the importance of these techniques to a modern and well-engineered oil analysis program, it seemed like a good time to review the options. There are actually eight different methods that are worth mentioning. Most have previously been covered in Noria publications, but a couple have not. We’ve used all but one of these methods in our failure investigation work here at Noria. A narrative of the methods is provided below and a summary that compares and contrasts them is shown in Table 1.
A comparator with a convenient scale is sold for use with fuel samples based on ASTM D2276. It can also be used for oils.
One of the main benefits of this method is its low cost, simplicity and portability. An aliquot of oil is pushed through a 4-5 micron membrane by positive pressure or pulled by vacuum (syringe, sample pump or vacuum pump) after the sample is diluted in solvent. Particles larger than the pore size remain on the membrane’s surface for later inspection. Low power microscopes can be used for this, but often the patch is simply examined without magnification using patch comparators. An excellent comparator with a convenient scale is sold for use with fuel samples based on ASTM D2276. This comparator can be used for oils as well. Because of the amount of fluid filtered in patch testing, the debris is typically too dense on the patch to attempt to count or characterize individual particles.
This method predates today’s popular automatic particle counters by decades and is still supported by standards organizations today (e.g., ISO 4407). Unlike the patch test, far less fluid is pushed through the membrane to enable sufficient spacing between particles so that they can be more easily sized and counted. Typically, particles are sized by the longest cord (longest projected dimension of an irregular-shaped particle). On average, the longest cord is dimensionally about 1.3 times larger than particles measured using the equivalent spherical diameter (optical methods). Understandably, the procedure is unpopular among technicians who are asked to perform the tedious task of counting particles one at a time. However, the ability to obtain both a particle count (including ISO Codes) and a photomicrograph of the particles is inarguably a comprehensive assessment of total solids. Some labs run microscopic particle counts on exception, i.e., only after a high reading from an automatic particle counter.
This method is almost identical to microscopic particle counting, but combines various particle identification and characterization techniques. These include the use of bi-chromatic microscopy, magnetism and assessment of particle color, texture, shape and size. In this sense, it is very similar to analytical ferrography, where particles are magnetically and gravitationally deposited on a glass slide called a ferrogram. However, with patch ferrography, a cellulose nitrate membrane is used, called a filtergram. A clarifying solution is applied to enable both bottom and top lighting to be used during examination. Unlike ferrograms, heat treatment of the particles and chemical microscopy are not practical options. Patch ferrography is particularly popular for samples having non-ferromagnetic wear particles (brass, aluminum, Babbitt for instance). Additionally, unlike ferrograms, particles don’t generally pile or clump (obscuring them from view).
Just as the name implies, gravimetric analysis uses a laboratory scale to weigh particles and sediment on a membrane (typically 0.45 microns). No attempt is made to count, size or visibly characterize the particles. Often, the particles on the membrane are dominated by organic “soft” particles such as resins from oxidation, sludge and additive degradation. Some laboratories use a series of solvent washes of various polarities to extract soluble fractions from the membrane in an effort to approximate composition. Common solvents used for this purpose include pet ether, toluene, trichloromethane and methanol.
Optical imaging methods have been developed that rapidly scan and digitize particles on a membrane for analysis by computer algorithms. As of this writing, there are three such products that are commercially available with varying degrees of capability. These instruments are based on ISO 16232 7/8 and can size particles either by longest cord or equivalent spherical diameter. Additionally, because each particle is individually imaged, they can be categorized by shape and further inspected visually by the analyst if needed. Basically, the PMPI offers the benefits of both microscopic particle count and patch ferrography without the hassles of having to count, size and categorize individual particles manually.
Some lubricant formulations have been found to restrict flow through fine filters (one to three microns), leading to premature plugging. The problem most commonly associated with insoluble additives include degraded additives from in-service exposures such as heat and water (hydrolysis). To gauge this characteristic, filterability testing is sometimes performed on the new oil. The test is conducted by passing dry or wet oil through a membrane until a certain terminal pressure drop is reached. The volume of oil that passed relative to the surface area and micron size of the membrane quantifies the filterability rating (there are several variations of the method).
There are three particle counters on the market that estimate particle size and concentration based on membrane pore blockage. The instruments use either constant pressure oil flow through the membrane and measure flow decay or use constant flow and measure pressure rise. The profile of the flow decay or pressure rise is used to estimate particle counts. The membranes used by these instruments can be used over and over. As with optical particle counters, no information is provided on the shape or composition of the particles. However, unlike optical counters, pore blockage instruments can be used with dark oils and oils with emulsified water. They have performance limitations as well.
Often called a varnish potential test, MPC uses a 0.45 micron membrane to collect organic and inorganic insoluble oil suspensions. The apparatus is identical to a patch test kit, but the sample preparation is different. Afterwards, the membrane is analyzed by spectrophotometry to get a CIE LAB ?E value (color scale). Optionally, the membrane can be weighed and the gravimetric value reported. Additive floc, oxide-insolubles, carbon resins and similar soft contaminants all contribute to the MPC reading. The procedure is gaining popularity with the proliferation of varnish and sludge problems in recent years. An ASTM standard is pending approval.
So that’s a short summary of membrane-based particle analysis methods. Many of these methods can be used onsite in addition to commercial laboratories. The basic patch test kit is small and inexpensive enough to be brought to the field. When used in the correct application, they can all enhance value from a condition monitoring program.
About the Author Jim Fitch has a wealth of “in the trenches” experience in lubrication, oil analysis, tribology and machinery failure investigations. Over the past two decades, he has presented hundreds of courses on these subjects. Jim has published more than 200 technical articles, papers and publications. He serves as a U.S. delegate to the ISO tribology and oil analysis working group. Since 2002, he has been director and board member of the International Council for Machinery Lubrication. He is the CEO and a co-founder of Noria Corporation. Contact Jim at email@example.com.
Dr. Ernest C. Fitch, Ph.D., a pioneer in developing hundreds of test procedures relating to tribology, contamination control and fluid power, died March 16, 2011, at the age of 86 at his home in Stillwater, Okla.
His son, Jim Fitch, is the CEO and co-founder of Noria Corporation.
Dr. Fitch received his Bachelor of Science in engineering in 1950 and Master of Science in mechanical engineering in 1951, both from Oklahoma State University. He received his Ph.D. in Engineering Science in 1964 from the University of Oklahoma.
He began his engineering career as a journeyman machinist before entering the military in World War II. After receiving his master’s degree he set out to gain practical experience by taking jobs with Jersey Production Research, Boeing Aircraft, Deere and Company, Cincinnati Milacron and Cessna Fluid Power.
During his 35 years on the faculty of Oklahoma State University, he advised more than 100 doctoral and master degree students and countless undergraduate students. In order to provide a hands-on research opportunity for his students, he started a contract research center in 1956 that became the Fluid Power Research Center (FPRC). At least 160 industrial companies and governmental agencies sponsored research that provided financial support for his students.
For 18 years Dr. Fitch headed teams of research engineers in developing hundreds of test procedures relating to tribology, contamination control, and fluid power. Many of these procedures have since become national and international standards. He served as a member and chairman of half a dozen standards committees including SAE, ANSI, NFPA and ISO.
During his career, Dr. Fitch co-founded several companies providing services relating to hydraulics, tribology and contamination control. He also served on more than 250 consulting projects, nearly 200 court cases and has written more than 210 technical articles and 20 books. He was been awarded 16 U.S. and 15 foreign patents and from the 1960s through the 1980s, he has served as editor-in-chief of three international technical journals.
Dr. Fitch was Emeritus Professor of Mechanical and Aerospace Engineering at OSU. He has received 15 major honors and awards from state, national and international professional organizations.