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The application of particle counting for the monitoring of lubricating oil and hydraulic fluids has increased dramatically in the past ten years. This has been largely due to the many field studies that have associated oil contamination levels to machine wear rates. So too, changing particle count trends have been found to be instrumental in detecting incipient levels of abnormal wear debris, regardless of the cause.
While the importance of particle counting in oil analysis is indeed established, some organizations in the past have experienced erratic data and have become despondent to its usefulness. The reasons are frequently traced to reoccurring problems relating to sampling procedures and sample preparation. Two important factors, often misunderstood in terms of impact of trends, are bottle cleanliness and sample agitation.
Because of the extreme oil cleanliness levels now commonly set as targets by many organizations, even the slightest dirt originating in a sample bottle can have an adverse effect on the precision of the particle count. The extent of the effect can be represented as a signal-to-noise ratio (SNR). The SNR is calculated in the following way:
For instance, it is common to represent bottle contamination levels as the number of particles greater than ten microns per milliliter. This is the ten-micron count. If the target cleanliness for a hydraulic fluid is 50 particles (greater than ten microns) and the bottle is cleaner than 10 particles then we have a 5:1 SNR. This means that the noise (original particles in the bottle) can influence the particle count precision by roughly 20%. If, on the other hand, the bottle was cleaner than one particle, a 50:1 SNR would result. In such case the bottle contamination would then have insignificant influence on the particle count trends. The cleaner the target the more difficult it is to maintain high SNR's. Where possible, a SNR of at least 5:1 should be achieved.
Recently a study was conducted by Fluid Engineering Services (FES) on the cleanliness of sample bottles commonly distributed (or used) by suppliers and laboratories. Fifteen bottles were included in the study: eight glass, four PET plastic (transparent), & four polyethylene plastic (opaque). The bottles varied in size from 35 ml to 500 ml. Four of the bottles were provided by bottle suppliers, five were bottles from plant laboratories, and six bottles came from commercial laboratories. Four of the bottles supplied by commercial laboratories were research grade ultraclean glass bottles. The cleanliness testing was done according to the procedure outlined in ISO 3722 and summarized below:
It can be seen from the cleanliness data (Table 1) reported by FES that there is considerable variance between the sample containers. Using ISO 4406 to represent concentration, the data ranged from ISO 5/3 to a high of 16/13. Applying a signal-to-noise ratio of 5:1 to the high value it is seen that this bottle should not be used with oils having target cleanliness levels below an ISO 18/15. When research grade ultraclean bottles are removed from the group, the average ISO Code of the remaining bottles is approximately a 14/11. With a SNR of 5:1, the minimum target cleanliness level for this group is approximately a 16/13.
Users of sample bottles should both define and communicate their quality (cleanliness) expectations and be prepared to pay a premium where higher cleanliness is required. While the designations of Clean, Superclean, and Ultraclean (see Table 1) do not refer to any published standard, they do provide a convenient and meaningful means of describing bottle cleanliness levels. Bottle suppliers and laboratories that do not inspect or report sample bottle cleanliness may need to be avoided where precise particle counts and wear metal concentrations are needed.
An important step in preparing an oil sample for analysis of insoluble suspensions (dirt, wear debris, additive floc, water, soot, etc.) is to agitate it violently. When done properly, a uniform dispersion is achieved and the sample is returned to its original state. This is important because particles and other insolubles can settle rapidly and cling tightly to container surfaces. For instance, it only takes about 2.8 minutes for a 20-micron babbitt particle to settle ½ inch in a 22 cSt oil (Figure 1).
Because of the adherent forces of very small particles to the surfaces of sample bottles, considerable energy is often needed to ensure complete resuspension. This is assisted greatly when sample bottles are not filled completely, leaving ullage (the head space between the oil level and the cap). Table 2 shows the differences between particle counts on three identical samples. All three samples were taken from an homogenous bulk mixture of hydraulic fluid and AC Fine Test Dust (ACFTD). The samples were allowed to sit on a shelf for one week. Sample 1 was counted with no agitation. Sample 2 was agitated violently by hand for five minutes. Sample 3 was agitated using a paint shaker for five minutes (Figure 2).
It is obvious from the data that proper agitation is necessary to achieve accurate analysis of particles and other insolubles. The unshaken sample had only 1.75% of the reference dust remaining in suspension. The hand agitation procedure was successful in resuspending only 77.6% of the dust larger than 10 microns. However, the paint shaker achieved a 98% resuspension of the original reference test dust in the fluid.
Users of oil analysis should discuss agitation procedures with laboratories including the hardware they employ. The standard ISO 4402 describes the preparation of calibration fluids for automatic particle counters. There is discussion of how to prepare the fluid for particle counting, including the importance of violent agitation. This preparation procedure has direct application with used oil analysis as well.
This article describes two risk areas that frequently lead to erroneous oil analysis data–bottle cleanliness and agitation. When it comes to oil sampling and sample preparation, there are many links in the integrity chain. If precision oil analysis is to be expected, no weak links can be tolerated. Future issues of Practicing Oil Analysis will look further into the best practices that build integrity and precision in oil sampling and sample preparation.
For information on getting bottle cleanliness tested to ISO 3722, contact Fluid Engineering Services, Inc. at: 405/743-4337 (USA).