Micron Ratings: Absolute Microfiltration vs. Nominal
One of the most important conventions
in the filtration industry: NOT ALL FILTERS ARE CREATED EQUAL.
An end-user might think that the filter they just bought designates
a definitive, empirical rating. Actually, it probably does not.
There is a significant difference in the way filters are rated,
whether it is a "nominal" rating or an "absolute" rating? Most importantly,
the filtration efficiency and performance for a given micron rating
can significantly differ between different manufacturers.
What is the difference? There are no
industry standards when it comes to micron rating claims on filters.
Each industry has its own differing requirements, making it difficult
for the manufacturers to adopt a uniform standard. A more useful
rating can be the efficiency rating. That is the percent
rejection at the recommended flow rate for the designated micron
rating.
Definitions
- Nominal Rating - Expressed as a percentage
of retention by micron size (For example, 90% of X Microns)
- Absolute Rating - Expressed as the
maximum sized particle which the filter will pass (All particles
of X Microns size). A general rule of thumb is to multiply the
nominal rating by 5 to obtain the more reliable absolute rating.
- Beta Ratio - A mathematical expression
defining the number of particles of a given size upstream of a
filter relative to the number of particles of the same size downstream
of the filter. The Beta ratio (ßx) is an indicator of how well
a filter controls particulate. It is the ratio of the number of
particles (>xµm) entering the filter to the number (>xµm)
that pass through. If one out of every two of the particles (>xµm)
in the fluid pass through the filter, the filter's Beta ratio
at xµm is "2." If only one out of every 200 of the particles (>xµm)
pass through the filter, the Beta ratio at xµm is "200." Therefore,
filters with a higher Beta ratio provide better particulate control
and hence better system protection.
It is important to note that most of
the time these criteria apply only to a new, undamaged, unused filter.
Once the filter has been put into service, it is impossible to know
how it is performing without constant monitoring.
A leading water industry association
defines nominal to mean 85 percent rejection at the stated micron
rating and at the recommended flow rate. Absolute provides a much
stricter efficiency standard for the filter media, typically 98-99%
percent rejection rate. The ultra-pure water industry even defines
absolute as a 99.99 percent (4-log) reduction, or greater. However,
independent validation might be in order for filter manufacturers
making this claim.
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