Why Particle Counting
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.
Higher levels of particulates in the sample may indicate machine health issues, a high rate of external particulate ingression or filter inefficiency. High levels of particulates can lead to accelerated machine wear due to abrasive conditions. Maintaining lower levels of particulates can increase the operational life of lubricated equipment.
Applications: Systems that rely on clean fluid for proper operation.
A critical factor to particle counting is how and where the sample is obtained – for samples that will include the particle count test it is recommended that samples be obtained from a turbulent zone such as on an elbow of the return line.
Historically, the light extinction principle enjoyed success due to its reference in the ISO standards. As the technique was readily available, a number of suppliers provided units for either portable or laboratory applications. Because of its flexibility in sample size (usually requiring less than 20 milliliters), light extinction is the preferred technique for most laboratories.
However, many laboratories offer a particle count only on hydraulic and turbine fluids, and where conditions permit. There are several reasons for this, namely:
Large, visible wear debris particles can block the sensor orifice (approximately 100 microns in size).
Multiphase hydraulic fluids (water-based fluids, for example) are difficult, if not impossible, to count successfully.
Samples require de-aeration after re-agitation, or careful resuspension of the particles to avoid aeration. Aerated samples, such as in flushing fluids, cause counting errors on online instruments.
Wet or saturated samples cause counting errors due to the water droplets present. This can be overcome by a method of solvent dilution as prescribed by oil analyzers (CSI’s 5200 Minilab is one such tool).
Dark oils or diesel engine oils heavily laden with soot may cause difficulties.
In all of these cases, these problems can be overcome. However, this comes at a significant cost in terms of time and materials. Most laboratories offer a package of tests and generally question the value of particle counting when asked specifically to perform this test. Instead, most laboratories and users use light blockage for its perceived ease of use regarding sample volumes and its compliance with ISO standard references.
Pore Blockage Particle Counting
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.
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.
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