Originally Posted By: paulri
Thanks. I come here to learn. Usually, like this thread, what I learn is not nearly what I expected the answer to be, but that's ok....that's all part of the process. The info to trolling ratio is very high here at BITOG, at least I've found it to be so. Makes it easy for me to ask whatever pops into my noggin.
Here's something that I wrote a while back that explains the limitations of the instrumentation used for UOAs when trying to relate wear metals to actual wear:
UOA by ICP is not a proxy for wear. The limits of the instrumentation prevent it from being used as such. It is possible to have "high" wear metals and lower wear than an engine with "low" wear metals. What supporting proof do you have for each data point that is is a valid indicator of wear? An example would be ferrographic particle counts, or measurements of each engine correlating actual wear with the UOA data.
Now, let's talk about the size of those particles that make it into the plasma. You've probably read many times that ICP can see wear particles at about 5 microns. The labs will tell you this also. They are wrong.
The particle size is based on the aerodynamic diameter, not the actual diameter. An ICP is designed to have a hard cut off of 4.5-5.0 microns due to the fact that droplets larger than that destabilize the plasma. That's where the 5 micron figure comes from. The problem is that is the aerodynamic diameter and is based on a spherical droplet of water. Aerodynamic diameter is affected by density and shape. Metals have a higher density than water, therefore smaller particles are required to achieve the same aerodynamic diameter and be allowed to pass through to the plasma.
This is one of the many reasons why "wear metals" do not serve as a good indicator of wear. The ICP only sees a portion of "normal" wear and miss most if not all of the larger particles generated by abnormal and break-in wear. The other is that due to the different densities of the metals the instrument does not see them equally. Given an equal amount and distribution of particle sizes, the ICP could read 4X as much aluminum as lead due to the density difference between the two. An accurate measure of what is in the oil can only be made if the oil undergoes a digestion to put the metals in solution.
The first 5 pages of this presentation cover what I have talked about. The illustration at the top of page 5 shows the relationship visually. They use a material with a density of 4000 kg/m3 for illustration. Aluminum, depending on alloy has a density of about 2700-2800, copper 8940, iron/steel 7850, and lead 11340. Visualize the 4000 kg/m3 circles at half that size and that is roughly the relative size of an iron particle that an ICP can see vs the 4.5 micron water droplet.
Aerodynamic Diameter
Ed
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