MACHINERY LUBRICATION- INDIA NOVEMBER-DECEMBER 2019 | Page 29

MLI
ISO
CODE
PARTICLES
NAS 1638
PER MILLILITER > & SAE
10 µm
AS4059F
26/23 140,000 25/23 85,000 23/20 14,000 21/18 4,500 20/18 2,400 20/17 2,300 20/16 1,400 19/16 1,200 10
18/15 580 9
17/14 280 8
16/13 140 7
15/12 70 6
14/12 40 14/11 35 5
13/10 14 4
12/9 9 3
11/8 5 2
10/8 3 10/7 2.3 10/6 1.4 9/6 1.2 0
8/5 0.6 00
7/5 0.3 6/3 0.14 5/2 0.04 2/0.8 0.01
12
11
1
Cleanliness Level Correlation Table
classify the oil’s cleanliness with a range of
00 to 12, based on the number of particles
in each size range. The lower the number,
the cleaner the oil. Prior to this time, a
coding system to quantify oil cleanliness
had not been established. This method
worked well and was largely accepted by
industry through the 1970s and ’80s.
With improved particle filters, the invention
of automatic particle counters and the push
for ISO 9000 during the ’80s and ’90s,
several other attempts were made to create
industry standards for cleanliness levels by
the Society of Automotive Engineers (SAE),
the Aerospace Industries Association of
America (AIA) and others.
The improvements in particle filters tipped
the scales of natural distribution. Filters
became more efficient at removing larger
particles, which made the larger size
ranges less representative of the particle
distribution in the oil. With this change in
the natural distribution of particles, ISO
decided it was no longer necessary to report
the concentration of these larger particle
ranges.
ISO 4406:87
Recognizing the importance of hydraulic
oil cleanliness, the shortcomings of NAS
1638 and the deficiencies of other standards
organizations, ISO aspired to create a
standard that would more accurately
ref lect the concentration of particles.
The organization also sought to make its
classification or code easier to understand
while expanding the standard to all
lubricating fluids so there would be one
accepted standard across all industries.
table was born. The classifications spanned
from 0.9 to 30, with each doubling from
the lowest to the highest acceptable value.
This approach was intended to make each
step more meaningful and impactful. It also
allowed for a simple method of expressing
very small and large particle counts with a
single value.
For example, a cleanliness code might be
something like 18/14. This would indicate
that there were somewhere between
1,301 and 2,500 particles larger than
5 microns and 81 to 160 particles larger
than 15 microns. ISO later dropped the
0.9 code and started the chart at 1 when
it was determined that obtaining this
cleanliness level was highly unlikely and
thus unreasonable to include it in the chart.
ISO 4406:99
The process began by reducing the number
of categories from five to two with an
optional third. It was also decided to use a
scale that would count particles of specific
sizes and larger, moving away from the
range approach. The representative particle
sizes chosen were 2, 5 and 15 microns with
the 2-micron category being optional.
Unlike NAS 1638, which categorized all
particle counts for the various classifications
with a single number, ISO 4406 represented
each size individually. An ISO 4406 code is
always shown with the micron sizes listed
from smallest to largest. In the 1990s, there was a push for
industry to become ISO 9000 compliant.
During this time, it was discovered
that the current method for calibrating
automatic particle counters (APCs) did
not meet the requirements of the ISO
9000 standard. Previously, all APCs were
calibrated according to ISO 4402 using
Air Cleaner Fine Test Dust (ACFTD).
During the process of becoming ISO
9000 compliant, it was determined that
this calibration material was untraceable.
The exact quantity and size of the particles
in the ACFTD were unknown, resulting in
inaccurate calibrations. The company that
produced the ACFTD calibration fluid
also announced that it would no longer
be manufacturing the fluid. This led to
ISO 11171, which utilizes ISO Medium
Test Dust (ISO MTD) from the National
Institute of Standards and Technology
(NIST). NIST employs a scanning electron
microscope (SEM) to accurately measure
the number and size of particles down to
1 micron.
The second major modification involved
changing the scale altogether. What would
later become known as the Renard series In addition to the lack of control over the
calibration material, it was also discovered
that the particle sizes being reported were
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