Ghost Particles
I've been travelling and spending a lot of time talking about the issues with traditional methods of optical particle counting — specifically presenting on what the industry calls Ghost Particles. Ghost particles are essentially soft particles that skew particle-counting results. Soft particles include water, insoluble products such as varnish precursors, and certain oil additives — most notably foam inhibitors and friction modifiers.
The main points of my presentation are as follows:
- Soft (or ghost) particles are non-abrasive “particles” detected in oil that are related to insoluble oxidative by-products and/or additives present in the lubricant.
- Soft particles can cause laser light-scattering in optical particle-counting instruments, leading to erroneously high ISO cleanliness codes.
- Optical particle analyzers cannot distinguish between these soft (ghost) particles and true abrasive contaminants like fibers, dust, and dirt.
- By using the dilution method for particle counting (ASTM D7647) and the proper solvent, particle-count results are more reproducible and repeatable.
Several years ago a new ASTM method was released — ASTM D7647 — which allows for the dilution of samples for particle counting with a masking solvent to prevent erroneous results from soft-particle effects. Our preferred solvent is 25% isopropanol and 75% toluene. The isopropanol masks water (up to 2% with just a 1:1 dilution); the toluene re-solubilizes any contribution from varnish precursors or large molecular additives present in the oil.
For commercial oil-analysis laboratories this method has been very well received. Dark and opaque samples, as well as samples with water contamination, would typically have to be filtered through a Millipore patch and counted under a microscope — at much greater cost in both consumables and technician time. The CS-APC-22M is capable of measuring dark or opaque samples and samples containing significant amounts of water without the inflated counts that plague other particle-counting instruments.
Oil cleanliness and filtration
One of the biggest issues we uncovered in this research was the effect that some specific oil additives have on the apparent particle counts of new / unused fluids. One lubricant manufacturer was fielding customer concerns over higher than previously seen ISO particle counts for fresh ATF (automatic transmission fluid). The customer was testing the cleanliness of the ATF as received and getting very high ISO Cleanliness Codes (ISO 4406:1997 of 23/20/17). As a result, the OEM implemented an aggressive filtration program for the new fluid — and was still struggling to meet their ISO targets in their machines. They asked the oil company why they couldn't meet their cleanliness targets even after aggressive filtration.
The oil company began by doing particle counts of the fluid at each blending stage, starting with the base oil. The base oil had an ISO cleanliness of 19/15/10. They then added oil additives one by one and tested at each stage. The friction modifiers and the anti-foam additives had a dramatic effect on apparent cleanliness. By the time the oil was treated with these two additives, the apparent ISO 4406 increased from 19/15/10 to 22/20/15.
Next the oil company set out to study the effects of aggressive filtration on the new unused ATF. A laboratory test stand was set up to simulate both 3µm and 1µm filtration in the field. The results were alarming. At 3µm filtration the silicon level decreased from 6.7 ppm to 4.4 ppm — since this was new unused oil, that decrease indicated removal of the anti-foam package, which is silicone-based. At 1µm filtration over 70% of the anti-foam treat had been removed. More startling: the calcium level also decreased by close to 15%, indicating that some of the detergent had been stripped from the oil during filtration.
These studies revealed an alarming issue with new fluids going into OEM machinery: very likely, OEMs using traditional particle-counting instruments were employing aggressive filtration to achieve their cleanliness targets — unaware that the high ISO codes on the new oil were due to additive effects, and unaware that their filtration efforts were literally stripping additives from the oil.
The CINRG difference
Traditional optical particle counters using older methods often overestimate particle counts in such complex fluids, leading to unnecessary alarms or maintenance interventions. CINRG particle-counting instruments use ASTM D7647, in which samples are diluted with a masking solvent prior to particle counting. The CINRG instruments automatically perform the dilution, so you can trust the data you're collecting and respond only to legitimate contamination issues.
By using ASTM D7647, our particle-counting instruments are capable of accurate counts in FRFs, including water/glycol and aqueous-based hydraulic fluids — something that has been impossible with traditional particle counters. In fact our instruments can be used to conduct particle counts for phosphate-ester (PE) aviation fluids, polyalkylene glycol (PAG) gear oils, diesel engine oils (with up to 2% soot), diesel fuel, and just about any other sample that commercial oil laboratories process.
Laboratories love our particle-counting instruments for many reasons including their high level of automation, ease of customization, ease of integration into any in-house LIMS, and the fact that the results files include not only the typical ISO 4406 results, but SAE AS4059 results and methods to produce the out-dated but still used NAS 1638 cleanliness rating.