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Particle Count Testing Equipment

There are numbers of PC instruments in service. Most involve some form of light blockage or scattering, involving a laser source. It is even possible to execute a count manually by filtering the oil with a very fine membrane and viewing the deposited materials with a high-powered microscope, but such a method is today considered quite tedious and slow. Electronic counters ‘assume’ that all particles detected are round. The objective is to discern the longest chord (the diameter) of the particle and classify/count it accordingly.


Reporting Units: The cleanliness of a hydraulic or similar system, in oil analysis terms, is measured via the lube oil particle count. The de facto PC analysis features results at six (6) particle diameter size ranges: 4µ, 6µ, 14µm 21µ, 38µ and 70µ. An “ISO Cleanliness Rating Number” is derived from the results of the three smallest sizes, therefore the minimum PC test should furnish results at 4µ, 6µ, 14µm.

Below is a typical table covering industry-standard cleanliness ranges and terminology, based on ISO 4406 (1999). This table applies to all six ranges but is referenced primarily for the three smaller particle chord (diameter) sizes of 4µ, 6µ and 14µ, respectively.

ISO 4406 Cleanliness Code

Number of Particles Per mL
More ThanUp to and IncludingRange Number (R)

The ISO Cleanliness Code consists of three numbers corresponding to the particle concentrations for those three sizes. Example:

Particle SizeParticles/mlISO Number

19/18/16ISO Cleanliness Code

General Application Information:
PC is most widely utilized for hydraulic systems, arguably the most sensitive of all oil-wetted systems in terms of the necessity for good contamination control. Unlike gearsets, which frequently operate without the benefit of online filtration, hydraulic systems virtually depend on sufficiently ‘clean’ lubricants to perform properly.

Cleanliness in hydraulic oils is vital because clearances for moving parts (spools, cylinder rods, axial pistons, etc.) are at ten-thousandths of an inch tolerance. Particles entering such clearances can wreak havoc and quickly render a system unable to function properly, posing both production loss and safety hazard threats.

Hydraulic systems feature the most sophisticated filtration found on a multi-component piece of equipment, such as an earth-moving machine. In a plant the component might be a stamping or moulding machine. On an offshore drilling rig there are multiple hydraulic systems, some quite sophisticated, such as found on azimuth thrusters, all requiring ‘clean’ oil to operate at best efficiency and effectiveness.

Other applications for PC can include numbers of component types, such as gas turbines, steam turbines, compressors, gearsets, etc., as appropriate.

Summing up the efficacy of PC testing, especially for hydraulic systems, ‘dirty’ oil is considered to be the primary cause of hydraulic and similar system failures, whether the failure is valve malfunction/sticking, or excessive abrasive wear and subsequent fatigue, or loss of function. PC testing is the only effective way to monitor lube cleanliness toward maximizing life and value of the machine.

Metals analysis, particular for wear elements, is obviously a means of assessing damage more specifically (PC does not identify the compositional aspect of particles, only that they exist in progressive size ranges). If the wear level or rate is sufficiently low (acceptable) it may be that flushing the system, or using offline filtration, sometimes called portable or kidney filtration, can rid the lube of excessive particles, mitigating the abnormal amounts of metal thereafter. If one is able to achieve that effect, the essential parallel piece of maintenance to perform is to determine the mechanism by which the particles got into the system and make necessary repairs or adjustments, so as to prevent recurrence. In assessing contamination scenarios, SP and PC can work together at times:

  • If high Si is shown in SP results the preponderance of excessive particles should probably be suspected to be ‘abrasives’, ‘dirt’, ‘sand’, etc. Oil handling and storage should be on the list of considerations as to cause, along with compromised filters and breathers and, where applicable, sealing gaskets. Be thorough
  • If low Si is shown in SP this, unfortunately, does NOT rule out abrasives, as 3-5µ particles and greater are not readily detected by SP with consistency, yet, such particles can cause untold wear damage as they are able to pass into critical clearances. One must still be wary of abrasives, but one must also consider the possibility of Si-based gaskets and o-rings, contributing particles as they are compromised and crumble

Although there is no ‘standard’ for the three larger particle sizes routinely determined with PC, it is important to treat excessive amounts (as statistically determined) with some degree of importance. Most of the time, particularly when the ISO Cleanliness Level is reasonable, larger particles will represent contamination induced by carelessness, e.g., unclean transfer vessels and piping, open drums of fresh oil, and similar.

One very important application of PC is to inspect fresh oils when delivered, and before dispensing. It is not unusual for delivered oils to be ‘contaminated’ with excessive particulates. Neither is it unusual for stored oils to get contaminated over time owing to poor site conditions or carelessness.

Additional PC considerations:

  • When laser counters are used, most are thwarted by the presence of significant water, wherein the count is unusually high or, in any event, not credible. Alternative approaches may have to be taken to get a valid count, if possible
  • Diligence must be observed in collecting and testing samples for particle count. It is easy to contaminate a sample container by opening it prematurely, or by using ‘dirty’ sampling equipment, particularly if the environment where the machine resides is dusty or otherwise potentially compromised. Similarly, lab technicians need to take proper care when opening the sample container for testing purposes (much of particle counting performed occurs in laminar flow ‘clean rooms’ to minimize incidental, non-representative particle accumulation in the sample)
  • Thorough agitation of the sample should occur just prior to performing the test in order to maximize particles homogeneity when the sample is introduced into the measurement path


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