Hypothetical Question About Method Of UOA's

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This came across my mind the other day about taking UOA's. On my Harley, there is a 'system' you can buy to run all the old oil out of the system during an oil change. It involves the use of a filter block and is a screw on 'cap' that fits where the oil filter is, and simply reroutes the oil right back into the engine, unfiltered. The basic premise of this kit is to remove the 1/2 quart of oil oil left on the dry sump system, for those that cant stand the thought of 1/2 quart of dirty oil mixing with new oil. Anyhow, it got me to thinking. If one wants to do a UOA, are the results not skewed a bit by the oil filter removing particles out of the oil? Would using something like the 'fake filter' cap actually give you a more realistic engine metal portion of the test due to no filtering of the oil?
 

wwillson

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I don't believe a UOA can detect fakes the size an oil filter can remove. So if the 'fake cap' allows some metal flakes into the sample, I don't think it would make any difference in the UOA.
 
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In general if a particle can be filtered out of the oil it’s not going to show up on a standard UOA. There is a maximum particle size for ICP detection. When I ran UOA in college if you wanted to use a spectrographic means to “detect” larger particles in the oil you had to perform an acid digestion first.
 
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In general if a particle can be filtered out of the oil it’s not going to show up on a standard UOA. There is a maximum particle size for ICP detection. When I ran UOA in college if you wanted to use a spectrographic means to “detect” larger particles in the oil you had to perform an acid digestion first.
So is it safe to assume that a UOA tests for particles less than around 20-30 microns as that seems to be what the average filter catches?
 
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So is it safe to assume that a UOA tests for particles less than around 20-30 microns as that seems to be what the average filter catches?
Nearly any filter will remove particles much smaller than that but at a low(er) efficiency.

Particle decomposition in the ICP plasma is complicated. There were some posts here in the past by individuals more knowledgeable than me that indicate the particle morphology is important as well. Maybe search out some of those posts.
 
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Here are the posts by member edhackett that I was referring to. The middle reference has some good information about interpreting the results from a UOA.

 
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I asked Dave @High Performance Lubricants what is the largest particle their ICP can detect, he said it's about 20 microns.
Right, but if you read edhackett's posts it's not as clear cut as an absolute particle size. It's been a while since I performed a spectrographic analysis but I agree with what Ed posted. What HPL told you may be the largest it can detect but that may not apply to all elements in all samples.
 

wwillson

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Right, but if you read edhackett's posts it's not as clear cut as an absolute particle size. It's been a while since I performed a spectrographic analysis but I agree with what Ed posted. What HPL told you may be the largest it can detect but that may not apply to all elements in all samples.
Correct, I should have added "around" 20 microns.
 
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I asked Dave @High Performance Lubricants what is the largest particle their ICP can detect, he said it's about 20 microns.
ICP(Inductively Coupled Plasma) spectrometry is designed to measure elements in solution, not particles in suspension. Droplets larger than about 4.5 microns destabilize the plasma. The proper operation of the instrument depends on the spray chamber removing "particles" larger than the 4.5-5 micron target range. The finer the droplets the nebulizer produces and the higher the efficiency of the spray chamber system at removing the larger droplets, the better the detection limit and relative standard deviation of the instrument. Manufacturers go to great lengths to send those 4.5+ micron drops down the drain tube instead of into the plasma.

The size of the particle is it's aerodynamic diameter, which is affected by density and shape. The 4.5 micron size is based on spherical water droplets. Due to the density of metallic wear particles, the largest that should get passed to the plasma is 1-3 microns, depending on the specific metal.

The first 5 pages of the attached article is a good explanation of aerodynamic diameter.

Most ICP operators don't have a clue has to how the instrument works or what they are actually measuring. Some of Blackstone's comments are a prime example of this. The older technology DCP(Direct Current Plasma) spectrometry that was commonly used for oil analysis before ICP was common had a more robust plasma and was more tolerant of larger particles. Their sample induction systems would pass particles in the 10 micron range. I'm guessing that there was some small carryover of particles in the 20 micron range. This may be where the idea that current oil analysis sees 10-20 micron particles comes from.

Ed
 

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ICP (Inductively Coupled Plasma) spectrometry is designed to measure elements in solution, not particles in suspension. Droplets larger than about 4.5 microns destabilize the plasma. The proper operation of the instrument depends on the spray chamber removing "particles" larger than the 4.5-5 micron target range. The finer the droplets the nebulizer produces and the higher the efficiency of the spray chamber system at removing the larger droplets, the better the detection limit and relative standard deviation of the instrument. Manufacturers go to great lengths to send those 4.5+ micron drops down the drain tube instead of into the plasma.

The size of the particle is it's aerodynamic diameter, which is affected by density and shape. The 4.5 micron size is based on spherical water droplets. Due to the density of metallic wear particles, the largest that should get passed to the plasma is 1-3 microns, depending on the specific metal.

The first 5 pages of the attached article is a good explanation of aerodynamic diameter.

Most ICP operators don't have a clue has to how the instrument works or what they are actually measuring. Some of Blackstone's comments are a prime example of this. The older technology DCP(Direct Current Plasma) spectrometry that was commonly used for oil analysis before ICP was common had a more robust plasma and was more tolerant of larger particles. Their sample induction systems would pass particles in the 10 micron range. I'm guessing that there was some small carryover of particles in the 20 micron range. This may be where the idea that current oil analysis sees 10-20 micron particles comes from.

Ed
The most important distinction in this whole thread.

Thank you for your detailed explanation.
 
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