Ghost Particles
I've been travelling and spending a lot of time talking about the issues with the traditional methods of optical particle counting. Specifically I've done a number of presentations 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” that are detected in the oil and are related to insoluble oxidative by-products and/or additives present in the lubricant
- Soft (or Ghost ) particles can cause laser light-scattering in optical particle counting instruments that lead 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 particle count results as a result of soft particle effects. Our prefered solvent is 25% isopropanol and 75% toluene. The isopropanol masks water (up to 2% with just a 1:1 dilution), and 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 a 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 increased particle 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 were having on the apparent particle counts of new / unused fluids. One lubricant manaufacturer 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 agressive filtration program for the new fluid and were still struggling to meet their ISO targets for this fluid in their machines. They asked the oil company why they were not able to meet their cleanliness targets with this ATF even after agressive 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. Then they added oil additives one by one and tested the particle count at each stage. The friction modifiers and the anti-foam additives had a dramatic effect on the apparent cleanliness levels of the oil. 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 agressive 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. Considering that this was new unused oil the decrease in silicon was an indication of 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 was that the calcium level also decreased by close to 15% indicating that some of the detergent had also been stripped from the oil during filtration.
These studies revealed an alarming issue with new fluids going into OEM machinery. That very likely, OEMs, using traditional particle counting instruments and results were employing agressive filtration to achieve their oil cleanliness targets unaware that the high ISO Codes on the new oil was due to additive effects, and also 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 the method ASTM D7647 wherein samples are diluted with a masking solvent prior to particle counting. The CINRG instruments automatically perform the sample dilution so you can trust the particl count data you’re collecting, ensuring you're only responding to legitimate contamination issues.
By using ASTM D7647, our particle counting instruments are capable of measuring accurate particle counts in FRFs, including water/glycol, and aqueous-based hydraulics 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 and results.