Here's what Raycor has to say about filter micron ratings:
makes filters with various filtration
efficiencies, but its standards for non-OEM (Original
Manufacturer) are 2, 10, and 30 micron
filter elements. The actual efficiency ratings for these
are 98%, 95%, and 90% respectively. Racor
use of a 7 and 20 micron filter medium which are used
to meet certain engine
for a final filter and a primary filter.
Racor’s 2 micron filter medium should only be used
in final or secondary filters where the fuel
filtered by a primary filter. The primary filter for a 2
micron final filter should use a 10 micron medium.
The exception in using a 2 micron filter in place of
a primary filter is to obtain high-efficiency water
separation, and is usually used in marine applications
where the fuel supply may be cleaner but also may
contain water more often.
If the installation
allow the use of a filter large enough, then a 2 micron
filter can serve in a system as the only filter in that
New high pressure common rail fuel injection
systems require high efficiency in removal
particles. The requirement is 95% for 3 micron
particles. Racor fuel filters have a medium designed
for these applications. Replacement elements should
state, “For Use With Common Rail Fuel Injection
Systems.” Dirt levels in fuel also direct the level of
efficiency required. Since the filters removes a
percentage of dirt particles, it follows that when a
much greater amount of dirt is present in the fuel, a
greater number of particles will pass through the filter.
engines used in earth moving or agriculture
should use fuel filters that have higher efficiency than
those for over-the-road or marine
The planning must also take into consideration
whether the filter is to be installed on the engine
the chassis and whether on the vacuum or pressure side
of the system. Filter installations on the engine make
the filter subject to high frequency vibrations which
reduces the efficiency level, (as do spill port metering
See Racor brochure #7550 for additional information. Unquote
SEEMS TO ME, the usual recommendation is for a 10 micron primary filter followed by a 2 micron secondary filter in two-filter setups.
Note that if the filter is mounted on the engine, the filter efficiency is diminished through vibration.
Note also the "pass through" or "blocking" efficiency of the various Raycor filters. The 30 micron filter traps 90% of sediment/particulate matter at least 30 microns in size....that means that 10% pass right on through the filter and reach the secondary filter.
applications may not be the most appropriate model for marine recreational use, due to a number of factors including economy. I'm perfectly willing to buy cases of filters (and do) and change them out at the first sign of a significant increase in vacuum....LONG before they block enough diesel
fuel to starve the itsy-bitsy Perkins
4-108 which only needs 2-3 ounces per minute at cruising speed.
I've never had a problem with the 2 micron filters in a primary application. However, I recognize that someday, somewhere, some nasty stuff might shake loose from my tanks
and be a potential problem. So, I think I'll start loading my filter manifold (two 500G Raycors, either one selectable on the fly) with one 2 micron and one 10 micron filter :-))