PPM Distribution, Is it Better than ISO Code for Interpreting Particle Count Data?

by Ray Garvey[i]

Emerson Process Management

Email:  ray.garvey@compsys.com

 Originally published in Lubrication & Fluid Power Magazine  

ABSTRACT:

A brilliant new method called “PPM distribution” calculates precisely how much particulate debris is in the lubrication system.   The systems debris values are calculated by multiplying the parts per million of debris by the lubricant volume to determine milliliters (ml or cc) of particulate debris in three adjacent size ranges:  4- to 6-microns, 6- to 14-microns, and >14-microns.  Peaks in the distribution plot indicate the nominal size for each different source of contamination or wear.  The proportional area under each peak reveals how much contamination or how much wear debris is in the lubrication system.  This quantitative new method presents particle count information in an intuitive format.  The result is easy interpretation of system contamination and realistic insight about the size of wearing defects.  Examples are given showing how the PPM distribution can provide insights that might be missed by using the ISO 11171 code values for contamination control and wear indications.

ARTICLE DETAILS:

Wouldn’t you really like to know how much debris is in your lubrication systems and how much of it is larger and smaller than the filter cutoff size? Wouldn’t you like to know the size of the damage you expect to find on bearings and gears simply as a result of non-intrusive oil analysis?  Both of these volume and size measurements can now be done using the data from a good laser particle counter[1].  The new patent pending method is called, “PPM distribution.”

The article, “Is Your Particle Counter Giving you Particle Count and Size Distribution,” gave an example of how PPM distribution can be used to reveal two different problems that are taking place at one time.  In that case dust contamination came into a roots blower and caused abrasive wear.  The distribution showed a peak at 24 microns corresponding to the dust contamination and a second peak at 40 microns corresponding to the metallic wear debris.

This article provides additional examples and explanations of how the PPM distribution works and how it adds real value to traditional ISO code representation of particle count information.

Figure 1 below shows four examples that have all been normalized so that each has exactly one PPM of total particulate debris.  It is interesting how the four examples have significantly different ISO codes although all four have exactly the same volume of debris.  This is easy to comprehend considering the fact that ISO codes represent the cumulative numbers of particles greater than 4-, 6-, and 14-microns, without allowing for how much greater in size the particles actually are.

Just to report that particles are larger than 6-microns is not sufficient.  It makes a BIG difference depending on much greater than 6-six microns the particle s actually are.  For example in the 6- to 14-micron category, a single 13-micron particle is ten times more massive than a 6-micron particle. 

You will see on Figure 1 that the target cleanliness level of 18/16/13 is achieved in example “A” and close to being achieved in example “B” while the other two examples (“B” and “C”) appear to be worse.

Figure 2, on the other hand shows that samples “A” and “D” are actually the ones with problems evidenced by the peaks in the > 14-micron size ranges.  Sample “A” has 700 particles greater than 4-microns with 15 of these particles in the vicinity of 40 micron size.  You can see that these larger particles occupy approximately 90% of the volume of debris in the system!  Example “D” has a similar circumstance with double peaks in the larger size ranges.
Figure 1.  Four examples with particle count reported in ISO format, each with precisely 1-PPM

Figure 2.  Same four examples with PPM distribution graphs.

What is PPM by weight?

Normally when someone says, “PPM”, they mean parts per million by weight.  PPM w/w means that the weight of one part divided by weight of whole (x10^6). 

Chemists use w/w because it works well with single-phase solutions.  However, nobody really knows the density of particles – specific gravity varies from 1 to 5 depends on material & morphology; while oil specific gravity is about 0.9.  Spectrometers report PPM w/w. 

Spectrometers are calibrated with single-phase solutions, not with particulate mixtures.  The reasons for this are obvious.  A spectrometer only measures the ultra-fine particulate debris. Most everything bigger than 5 microns is missed altogether.  These are serious limitations regarding Fe, Cu, Pb, Sn, and Si.  With all of these we care most about particles > 5 microns.

What is PPM by volume?

PPM v/v means that the volume of one part (e.g. particles) Is divided by volume of whole (e.g. oil) (x10^6).  This is meaningful for mixtures of solids with solids or solids with liquids.  You don’t need to know the density of the constituents. 

Optical particle counters directly measure the shadow area of individual particles.  The “size” reported is the diameter of a circle with same area.   It is reasonable to use this “size” to estimate the volume of the particles in ranges measured by the particle counter. 

To get PPM v/v, determine the volume of debris (micro-liter) in a liter of oil.  You can use PPM v/v for wear and contamination analysis.

For wear analysis, if you know how much metal is in the lube system, you know how much material has been removed from the machine.

For contamination analysis, if you know how much particulate is in the oil, you know how much filtration will be needed to remove it.

PPM Distribution is  more than just ISO Code

This measurement provides valuable new information.  The ISO cleanliness code is a convenient way to report the number of particles per milliliter in these size ranges:

> 4 microns

> 6 microns

>14 microns

The PPM distribution includes additional insights because it provides the total amount of wear and contamination 2 ways.  First it reports PPM v/v, and second it determines the system debris in ml.  Peaks and inflections in the PPM distribution plot correspond to specific sources of contamination and wear.

What is “System Debris”?

The “system debris” is the total amount of particulate debris in the lubrication system.  Multiply “PPM v/v” by lubrication system volume (liters x 1000) to get system debris (milliliters).  You can easily visualize how much debris or wear metal this represents using small syringe (or maybe a big one!).  Use this information to calculate the dirt holding capacity for filters to estimate how much it costs to remove “system debris.”  Keep in mind that filter companies report filter capacity in grams assuming a value for specific gravity of dust.

How to test wet/dirty oils?

Yes, you can test wet oils.  Immiscible water droplets in oil will produce counts which are falsely interpreted as solid particles.  This is done using methods for water masking (US Patent 6,064,480).  In this case the cloudiness disappears – oil and water emulsion becomes transparent.  This works with extremely high water contamination, and with water-glycols.  Yes, you can test very dirty oils to get size PPM distribution. 

Also, you will occasionally want to test extremely high particle concentrations.  Recall that above 20,000 particles/ ml the particle counter may be saturated.  Dilution with clean solvent allows testing of all industrial lubricants, even 1,000,000 particles/ml.  Software accounts for dilution ratio and counts contributed by dilution.

How to keep from loosing big particles?

It is very important that the larger particles not be allowed to be lost.  Proper mixing entrains air in the sample.  Degassing takes valuable time while larger particles fall to bottom.  It is a good idea to degas upside down, then dispense the sample from top-to-bottom through laser sensor as shown in Figure 3.

Figure 3.  US Patent 6,418,799 method for degassing samples for laser particle counting.

Key points about PPM Distribution

It is important to measure PPM distribution on all types of machinery.  This includes very low to very high viscosity grades (e.g., VG 32 to VG 680).   It also includes very clean to very dirty samples (ISO 10 to ISO 26.  You will want to trend and alarm on PPM measured in all three ranges.  And you want be sure to sample from the active zone where the oil is hot and circulating.

Be sure to mask water when needed and validate the results using Wear Debris Analysis.  WDA is able to find root cause and severity.  It is perfect for determining what the root causes are that produced the peaks in the PPM distribution plot.  See Figure 4.

Figure 4.  Wear debris analysis

A single sample will often have more than one contaminant.  In this case you characterize each group of particles in the distribution using the WDA template.

Finally, you will want to take advantage of the Systems Debris computation.  Once you know the total systems debris and you have identified which particle group is associated with each peak in the distribution using WDA, then you can proportion the total systems debris into categories corresponding to the area under each peak.  For instance in Example “D” shown in Figure 2 you can estimate that 40% of the debris volume is associated whit the 13-micron nominal size distribution while the remaining 60% is associated with the 26-micron nominal size distribution.

CONCLUSION

The new PPM distribution method adds valuable new insight to industrial oil analysis.  By quantifying the total systems debris, combining this with wear debris analysis to identify which source is responsible for each characteristic peak in the distribution, the user can determine how much filtration is needed or how much wear has taken place recently.  Keep in mind that this measure is only the freshly generated debris that is actively circulating in the oil.  As such the PPM distribution method provides critical insight needed to screen for contamination and wear problems.
 

REFERENCES:

R. E. Garvey, “Is Your Particle Counter Giving You PPM and Size Distribution?” Practicing Oil Analysis Magazine