Let me please repeat .... 30µM (optional)--> 10µM -->
15-18µM engine guard filter ---> engine.
Filters for oil, fuel and other automotive fluids are NOT 'screen doors' that remove
ALL the particles at that arbitrary µM rating. All filters, including those discussed here, have efficiencies .... the 'good' automotive filters are typically between 95-97% efficient at their arbitrary 'µM rating'. This implies that a LOT of 'crap' is NOT captured and therefore the downstream filters must be selected on a 'statistical basis' to 'narrow down' that removal to 'acceptable'.
In the case of the 10µM Racor-type filter (@ ~95-97% efficiency.), for every 100,000 10µM particles filtered, 3000 10µM particles
will pass through !!!!!!!!! and probably in the order of 500 to 1000 20µM and LARGER µM particles will go
through that 10µM 'filter'.
To redfine the NEED of that 15-18µM downstream filter: The reason for that 'small surface area' LARGER µM filter (... besides being there in case one of the upstream filters 'unloads' / breaks, etc.) is that it too will on a
statistical basis remove 'most' of those remaining 10µM and a high degree of those 20µM particles that didnt become captured by the upstream 10µM.
Sorry folks, filtration is
not selected based on µM ratings but the statistical removal efficiencies (over the entire range of possible particle sizes), the viscosity of the fluid, the amount of fluid flow per surface area, the 'dwell time' that the fluid remains inside that filter material, etc. etc. etc. ..... STATISTICS!!!!! not only 'micron' / micrometer ratings.
The current spec. for most small marine diesel engines still remains
20µM and such a fuel filtration sequence is OPTIMIZED for the reduction/removal of that 20µM 'most damaging' particle and that recommended sequence still remains: 30µM (optional) --> 10µM -->
15-18µM engine mounted 'guard' filter.
Yes indeed many of the newer modern engines are trending towards a 10µM spec. because of the tighter 'clearances' inside these newer engines .... yet, very few of these are on recreational boats at this time. If your engine was
designed around the 20µM spec., using filtration based on the newer 10µM spec. can result in LESS total efficiency and LESS removal ..... all based on the 'statistics' involved with filtration.
Here's an elemental layman's (with working math background) write-up on whats known as filtration "Beta Ratio" - one of the
first things a filtration engineer looks at when specifying and applying
any filter. Its the 'key' data plot that opens that statistical door of filtration efficiency in order to apply the 'correct' filter
and filter 'µM rating':
http://machinerylubrication.com/Read/564/filter-beta-ratios
Rx: 30µM (optional) --> 10µM -->
15-18µM engine mounted 'guard' filter .... for probably 98+% of the small diesel engines found in recreational boats.
I keep referring to Wix Filters ... the company that 'invented' spin-on automotive filters and are probably the main automotive filter manufacturer including the private label filters used by engine car truck manufacturers, automotive parts distributors, etc. They have a 'wonderful database' that includes the µM (~97% eff) rating on virtually ALL automotive engines.
wixfilters.com --> filter lookup --> vehicle lookup ---> Off Highway ---> marine engines .... etc. Use this data as a cross-reference for ANY other filter manufacturer, including OEM private label / OEM branded filters.
Hope this helps.