Frequently Asked Questions for Customers regarding the new particle count classification

Introduction: BTS is in the process of transitioning customers over the updated particle count contamination codes (ISO 4406:1999) and ISO 11171:1999). Customers familiar with looking at their results data will see some changes, particularly if reviewing trends. This note is designed to answer any questions that may arise.

Q1: Do I have to do anything different in collecting my sample?
A: No. Continue to follow your normal procedure when drawing your samples. Continue to exercise care, and draw off any dead leg fluid, and rinse the bottle with the fluid to be tested prior to filling the bottle and capping it tightly.

Q2: Will my numbers and contamination codes change?
A: The changes implemented are designed so as not to have a change in your results. If there is a change, however, it will be no more than a one-code difference. Any change observed will be most pronounced in the lower size range. Please see the example below:

Result using old ACFTD calibration

Particle count is: 20/17/12
Result using ISO MTD Calibration

Particle count is: 21/17/12

The particle count increased in the lower ranges, but decreased in the higher sizes.

Q3: Can I still receive my results in NAS distribution and class?
A: No. You will however receive your results in an SAE distribution, and have a n SAE class number. This standard supersedes the NAS 1638 standard. Overall, the SAE class number is the equivalent to the NAS Class. For example, a sample with cleanliness NAS 7 will be an SAE 7.

For those who track the actual particle distributions, there are significant changes:

1. The NAS distribution was differential, i.e. the 5 ­15 µm range corresponded to all the particles between 5 and 15 microns exclusively. The SAE distribution is cumulative, i.e. the >6 µm[c] range corresponds to all particles greater than 6 microns[c] collectively. This change means that the actual counts per 100 ml will be much greater than previous results, though the overall class numbers will remain equivalent.
2. The size ranges are completely different. See table:
   
   

   NAS 1638 Distribution	SAE AS4059 Distribution
   2-5 µm	>4 µm(c)
   5-15 µm	>6 µm(c)
   15-25 µm	>14 µm(c)
   25-50 µm	>21 µm(c)
   50-100 µm	>38 µm(c)

   
   Q: Why the change?
   For several years, different test laboratories around the world were noticing fairly significant variations in repeatability and reproducibility when counting a standard calibration fluid used throughout the industry. Batches of calibration fluid could vary widely, and variations were difficult to explain. The increasing accuracy of particle counters and emphasis on tight quality control, combined with the greater awareness and reliance on consistent particle count results by several industries forced the question about the possibility of using NIST (National Institute for Standards and Technology) traceable standard for particle counting.
   
   The standard calibration reference for particle counting in the fluid power industry has been AC fine test dust (ACFTD) in MIL-H-5606 hydraulic fluid. Typically, 5 mg/l of this test dust was added to super clean hydraulic fluid to create a known reference fluid. ACFTD (also known as Arizona Road dust) was selected because it typified real world contamination- it had a relatively consistent distribution of discrete particles. For many years, General Motors AC Spark plug division supplied the test dust to industry. They stopped selling it in 1992, and a few small powder suppliers stepped in to fill the void.
   
   The NIST were formally asked by several working committees and agreed to develop a traceable standard. The existing standard was reviewed by the NIST, and they found that that they were significant differences in what size and number of particles present versus what was reported to be present. They reexamined the standard, and reclassified the size ranges present.
   
   Much of the error has to do with the difference in how particles are measured. Particles may be measured using optical microscopy techniques or electronic measurements (electron microscopy, light blocking diode sensor).
   
   Optical Microscopy
   
   Examining an irregular shaped particle under an optical microscope is a common technique. In characterizing the particle, one must estimate the size of the particle, something that is not straightforward for irregular shaped objects. Measurement is achieved by applying the chord measurement technique, i.e. determining the major diameter of the irregular shaped particle. The major diameter measurement (MD) is what is reported, and is used to discriminate the particle into a size range for contamination code evaluation. This technique is widely used for analytical ferrography and optical examinations.
   
   Electronic Measurement/Surface area measurement
   
   Laser light particle counters are used to evaluate the particle sizes and quantities when measuring contamination levels. These instruments use various principles, (light blocking, light scattering etc.); all aimed at defining the particle size by measuring the extinction pattern (shadow of the particle) or redirected light (reflection of particle) in order to quantify the particle surface area. The particles are thus discriminated by this measurement into various size ranges for contamination classification. This technique is used heavily nowadays for contamination verification.
   
   When the ACFTD was first used in the early fifties, particle sizes were defined by the optical microscopy technique. Most automatic measurements, combined with electron microscopy, measure the surface are of the particle, and from that derive a particle with equivalent diameter size. This variation in measurement approach is what the NIST focused on.
   
   Example:
   

   Take a long fiber particle

   
   
   Optical Microscopy analysis: This is a 10 µm MD (major diameter) particle by optical measurement. For particle counting consistency, particles are defined by their equivalent spherical diameter. Therefore, this particle would be reported as a 10µm diameter particle.
   
   A particle counting sensor would measure the surface area of this particle, in this case a long fiber. As this long fiber passes between the light source and sensor, the sensor will detect a shadow of this particle. The shadow area for this particle is:
   
   Surface area: r².h
   Where r: radius of particle - h: height of particle
   
   So for the above particle, the surface area is: X1 X10 = 31.4159 µm²
   
   However, particle counters are designed to report particles as equivalent size spheres for distribution and comparison purposes. A sphere has a surface area of 4 r².
   
   Using this surface area, and solving for r = (31.4159/4? ) = 1.58 µm
   
   A sphere with an equivalent surface area of 31.4159 µm2 would have a radius of 1.58 µm. The particle counter would report this particle as having a diameter of 3.16 µm.
   
   This clash of different approaches is often noticed by end users; particularly when fibers and irregular shaped particles are observed by wear particle analysis techniques, yet particle count measurements do not indicate significant levels of particulate contamination.
   
   In summary, the calibration material was characterized by optical microscopy, whereas most techniques rely on surface area measurement approaches. The NIST simply uses surface area measurement to categorize the calibration material with electron microscopy; thereby eliminating the error developed using different measurement approaches.

Q4: I receive my data electronically, and input into my own software program for trending. Will there be any changes?
A: It depends. We have been aware of this change for some time, so most ASCII test upload files contain either the additional fields, or the fields with particle count data will have a different label. For most trend programs, the new data will go right in where the old data resided, for consistency. Calculation of ISO or SAE codes will not change.

Q5: So what you are telling me is that the particles I have been monitoring all this time are not what I thought they were. I went out and bought expensive filters to remove contamination based on these results. Was I wasting my money and time?
A: No you were not. Any time you invested in filters to remove particles from your system, you made a smart decision. The cleaner the system, the better the health of the machine. You ended up removing particles, just not the size you thought, i.e. if you installed a 2µm absolute filter, yo u were not filtering out particles to that size, more like 4.6 µm. The filter industry was using the same standards as everyone else to qualify how efficient their filters were, and so were subject to the same error.

Q6: I monitor several hydraulic systems in my facility. I was told that particles in the 2 to 5 µm range needed to be monitored and filtered whenever possible, as these particles contributed to silt locking of my servo valves. You are saying now I wasn’t removing them. What do I do?
A: Particles ranging from sub micron to 5µm, deposited on servo valve guides, can cause silt locking, a condition where the material promotes “stiction”. The answer to this question is based on the answer to this question: did you ever have a situation where the particle count results and actual operations/observations differed? Did operations report sticking servos, or symptoms such as sluggish controls, pulsations, and/or control errors/timeouts relating to sloppy hydraulics, and your particle count trend analysis was trending low? If the answer is yes, you were not seeing/removing the particles of concern. You will see particle counts more representative of your system because of this change. If the answer is no, your system was able to operate despite some of this material present.

Filter companies who specialize in removing very fine particulate are reviewing the implications of this standard change.

Q7: Does the lab do anything different with this new standard?
A: All new particle counters are supplied calibrated to this newer standard. Older machines are calibrated with the new standards in order to comply. For the lab technicians, the test procedure remains the same.

This graph summarizes the change in result between the old and new standard