1. Particle Counting - Oil Analysis 101 10/7/11 3:34 PM
Particle Counting - Oil Analysis 101
Noria Corporation
Tags: particle counting, oil analysis
Practicing Oil Analysis (7/2002)
This edition of Oil Analysis 101 focuses on particle counting and related standards. Considered by many to be the single
most important test for used oil analysis, whether you are conducting onsite particle counting, or relying on a commercial
lab, there can be no doubt that a host of problems can be quickly and easily determined by monitoring the number and
size-distribution of particles in an oil sample.
The importance of clean oil cannot be over emphasized. Particle count data is an invaluable part of any proactive
condition-monitoring program, from ensuring that abrasive bearing wear is minimized, to determining if a hydraulic fluid is
clean enough for reliable operation.
Particle counting was introduced during the 1970s as a result of the pioneering work on hydraulics and fluid power,
conducted at Oklahoma State University by Dr. E. C. Fitch, father of Noria’s founder and president Jim Fitch, and
coworkers. However, it has only been the last 10 to 15 years that industry, determined to work smarter, has started to fully
realize the importance of fluid cleanliness.
The most common unit of reporting fluid cleanliness is the ISO Code System. This convention is covered under the ISO
standard 4406:99. In this standard, the number of particles in three different size categories, >4 mm, >6 mm and >14 mm
are determined in one milliliter of sample.* ISO 4406:99 states that the number of particles in each size category should be
counted with the absolute count converted to an ISO code, using the ISO range code chart shown in Figure 1. Although
the most common standard, ISO 4406:99, is not the only method by which the cleanliness of an oil sample can be
reported. Others standards include NAS 1638 and MIL-STD 1246C, as well as outdated standards such as the SAE fluid
cleanliness rating system. However, whichever method of reporting is selected, the first step is to count the number of
particles in a volume of fluid.
Click Here to See Figure 1
There are three basic methods that can be used to determine the absolute number of particles in any given sample. These
are outlined below:
Optical Microscopy (ISO 4407)
The original method for determining fluid cleanliness levels was to take a representative portion of the sample and
examine it under an optical microscope. In this procedure, the particles are manually counted, which can then be used to
determine the fluid cleanliness of the bulk sample. While this method may seem outdated, slow and cumbersome, it is still
in use today and considered by many to be the most reliable and accurate method of particle counting, unaffected by
some of the limitations of more modern, automated methods.
Automatic Optical Particle Counting (ISO 11500)
Perhaps the most widely deployed method today for determining fluid cleanliness is to use an automatic optical particle
counter. There are a variety of instruments commercially available to optically count particles; from portable units for onsite
use that cost as little as $15,000, to large, sophisticated lab-based instruments that may cost in excess of $40,000. There
is even a low-cost, online optical particle counter available for under $1,000. However, all instruments, whether they be a
hand-held unit or a full lab instrument use one of two methods, either a white light source, or more commonly today, a
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2. Particle Counting - Oil Analysis 101 10/7/11 3:34 PM
laser.
In a white light instrument, particles pass through the capillary detection zone and create a shadow on a photocell
detector. The drop in voltage produced by the photocell is directly proportional to the size of the shadow and hence the
size of the particle passing through.
In a laser-based instrument, due to the near-parallel nature of the laser beam, light scattering from the unimpeded laser
beam is minimal because it is focused into a beam stop - until a particle passes through the instrument. As the laser
strikes the particle, light scatters and hits the photocell. Just like a white light instrument, the change in voltage across the
photocell is directly related to the size of the particle. In the laser-based instrument, one looks at an increased signal
against what should be a zero background (in theory). Laser optical particle counters are generally considered to be
slightly more accurate and sensitive than white light instruments.
There are several subtleties that must be considered with the automatic optical particle counters. First, for the most part,
particles from used oil samples are not perfectly spherical. This can create problems for optical counters because with the
one dimensional ISO 4406:99 coding scheme, how are particles counted that are 5 microns across the minor axis, but 40
microns across the major axis? To resolve this issue, developers of automatic optical particle counters have devised a
compromise known as the equivalent spherical diameter (Figure 2).
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3. Particle Counting - Oil Analysis 101 10/7/11 3:34 PM
Figure 2. The Spherical Diameter was Developed to Help Resolve
Reporting Errors with Optical Particle Counters
With the equivalent spherical diameter method, a particle is counted in the size-range under which the shadow, or
scattering effect observed would have appeared, if the particle had been a perfect sphere. This allows the average fluid
cleanliness to be estimated, permitting the ISO code to be trended over subsequent samples.
Another concern is the effect of false positives. For example, both air bubbles, and free and emulsified water appear as if
they are a particle using the optical particle counting method. While the effect of air and water can be negated using an
ultrasonic bath and vacuum de-gassing to remove air, and solvent extraction to dissolve free and emulsified water, other
false positives are possible with multiple particle coincidences and additive floc. For this reason, care and attention to
procedural details must be exercised when performing optical particle counts.
Pore Blockage Particle Counting (BS3406)
The pore blockage method is a widely used method of obtaining an automatic particle count. In this method, a volume of
fluid is passed through a mesh screen with a clearly defined pore size, commonly 10 microns. There are two instrument-
types that use this method.
One instrument measures the flow decay across the membrane as it becomes plugged while pressure is held constant,
first with particles greater than 10 microns, and later by smaller particles as the larger particles plug the screen. The
second measures the rise in differential pressure across the screen while the flow rate is held constant as it becomes
plugged with particles. Both instruments are tied to a software algorithm, which turns the time-dependent flow decay or
pressure rise into an ISO cleanliness rating according to ISO 4406:99.
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While pore block particle counters do not suffer the same problems as optical particle counters with respect to false
positive caused by air, water, dark fluid, etc., they do not have the same dynamic range as an optical particle counter, and
because the particle size distribution is roughly estimated, are dependent on the accuracy of the algorithm to accurately
report ISO fluid cleanliness codes according to ISO 4406:99. Nevertheless, they accurately report the aggregate
concentration of particulates in the oil, and in certain situations, particularly dark fluids such as diesel engine oils and other
heavily contaminated oils, pore block particle counting does offer advantages.
Why Particle Counting?
For many who are considering setting up their own onsite oil analysis program, the first question they ask is “What
instrument should I purchase first?” While there are many fine onsite instruments available in the oil analysis marketplace,
there can be little doubt that the ability to quantify an oil’s fluid cleanliness using particle counting is the most valuable tool
in any proactive maintenance program.
So, whether you are looking for early signs of bearing wear, ensuring that a hydraulic fluid is good for service, or
evaluating the performance of a filter in the field, particle counting offers a plethora of information for what is by
comparison precious little money.
* The ISO code 4406 changed in 1999 to reflect changes in the calibration fluids used to calibrate particle count
instruments. For more details on these changes, refer to “How the New ISO Particle Count Standard Will Affect You,” by
Leonard Bench in the July-August 2000 issue of Practicing Oil Analysis magazine.
Particle Counters Offer a Multitude of
Uses and Benefits
Verifies filter performance
Permits “on-condition” laboratory oil
analysis
Confirms corrective maintenance
Verifies pump condition
Identifies high engine blowby conditions
Verifies stored lubricant cleanliness
Identifies changing atmospheric
contamination
Confirms systems flushing
Identifies botched machine repairs
Troubleshoots and isolates problems
Identifies the need for ferrographic analysis
Determines timing of filter cart usage
Determines optimum filter change point
Identifies new filter defects
Verifies centrifuge performance
Detects high corrosive wear
Monitors machine break-in
Identifies abrasive wear conditions
Verifies bearing condition
Confirms target cleanliness levels are
achieved
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Verifies breather condition
Identifies cold-start wiper seal performance
Verifies effectiveness of filter selection
Oil quality n Identifies abnormal gear wear
Identifies use of dirty top-up containers
Verifies shaft seal exclusion performance
Recognizes filter bypass condition
Determines new oil cleanliness
Identifies cavitation conditions
Identifies oil film failure problems
Tips to Ensure Accurate Particle Counting:
1. Make sure the sample is taken from the correct location
using the correct sampling procedure. Take steps to
minimize sample contamination during sample collection and
be sure to leave headspace in the bottle.
2. Ensure the sample bottle is at least 2 ISO range codes
cleaner than the desired lowest levels of ISO fluid
cleanliness (Read “The influence of Bottle Cleanliness and
Sample Agitation on Particle Count Trends,” by James Fitch
in the March/April 1999 issue of Practicing Oil Analysis
magazine).
3. Dilute viscous fluids and heavily contaminated samples
using prefiltered reagent grade kerosene prior to particle
counting (remember to recalculate the original particle count
concentrations using the appropriate dilution factor). Prefilter
the diluent if its cleanliness is in question.
4. Test for free and emulsified water using a crackle test or
other moisture screening method. If water is present, the
water may be vacuum dehydrated or treated by mixing the
oil 50:50 with a tonic comprised of three parts toluene and
one part isopropyl alcohol.
5. If more than a few minutes pass between sampling and
testing, resuspend the particles by agitating the sample for
five minutes (longer for viscous fluids) in a paint shaker.
6. Ensure that the effects due to air bubbles are negated by
using vacuum ultrasonic bath and/or a vacuum de-greasing -
preferably both, starting with the ultrasonic bath (optical
particle counters).
7. Don’t leave your onsite particle counter idle on the shelf -
make the most of this valuable asset each and every day.
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