A filter that is marked “10 microns” has some capability
in capturing particles as small as 10 microns. However,
there is no one accepted method to measure and
describe the size of particles that a filter can capture
or the total amount of particles that the filter can
hold. When you see the filter marked “10 microns”,
you will not know exactly what this means unless you
also have a description of the test and standards used
to determine the filter rating. Filter micron ratings are
often based on one of these methods, but with many
possible variations:
A. Nominal Micron Rating (NMR)
NMR usually means the filter can capture a
given percentage of particles of the stated size.
For example, a filter might be said to have a
nominal rating of 90% at 10 micron.
B. Absolute Micron Rating (AMR)
AMR is a single pass test and is obtained by
passing fluid containing glass beads through
a flat sheet of filter material. Any beads that
pass through are captured and measured.
C. Multi-Pass Beta Rating (MPBR)
The MPBR has been accepted by many
machinery manufacturers, as well as filter
manufacturers (but not used in a public way by
most of them to identify or specify their filters),
especially for filters used in fluid power
applications; hydraulics, controls, transmissions,
power steering and so forth.
Single/Multi-Pass tests use contaminant specially
graded by particle sizes added regularly in measured
quantities to the fluid which is pumped continuously
through the filter. Measured samples of fluid are taken
at timed intervals upstream and downstream of the
filter. The contaminant in these samples is measured
for particle sizes and the quantity of each size or
ranges of sizes.
Filtration Efficiency
Fuel filters are supplied for various applications and,
therefore, there is a need for different levels of filtration
efficiency in the removal or retention of particles. The
hydraulic industry uses a rating method that uses the
term “Beta Ratio” to describe a filtration efficiency
level. The diesel fuel filtration industry generally uses
simple filtration efficiency as the method of rating a
fuel filter. Since there is no such thing as a fuel that
provides absolute filtration of the particle sizes that
are cause for concern, the industry uses terms like 96%
at 5 microns. This term means, that when tested to
SAE or ISO test methods the filter will retain 96%
of all 5 micron size and larger particles.
Some suggestions:
1. Use filters of high quality.
2. Obtain filters by catalog listing, not just by
“micron rating”. Other important qualities
should also be considered.
3. Pay close attention to service intervals and good
service practices for best economy of operation.
Attention: TDR Forum Junkies 
