Hehe..... you're softening, but it takes a while.

You make one point that seems contentious:

You think that a depth filter that captures
particles of decreasing size as a function of depth, is a
filter whose pore sizes are actually decreasing
with depth.
It ain't necessarily so. I assert that a depth filter of
constant pore size has just this characteristic also.

At 15µM and larger presses use a resinated technical paper, woven
'technical' fabrics, etc. .... now a world monopoly supply by
Ahlstrom of Finland.
Under 15µM the paper used is a thick 'chinese vacuum process paper'
thats laid down on a fine screen and with vacuum pulling from the
bottom, if this is done correctly the 'pore size' distribution will be
more open on the top, opening a bit through the middle and very tight
near the screen (bottom section) . this is the 'classic' filter paper
and the way its made by the principals of this industry: (EU - Seitz/
Schenk (the 'inventor' of modern depth-filtration - during WWI),
Begerow, Carlson, etc.; USA - ErtelAlsop, 3M-Cuno, Pall (SeitzSchenk),
Cellulo, etc.)

The 15µM paper is typically used with filter aids (perlite/DE/Carbon,
etc.) used as 'pre-coats and body feed' ... the paper used principally
as a 'septum' to hold the filter aid, the filter aid being the 'agent'
of filtration. The 15µM papers are isotropic in (statistical) 'pore'
structure and are all 'nominally' rated to a 'loose' degree. MOST ALL
traditional 'chinese process vacuum screen paper' used in depth
filtration are ALL anisotropic (graded pore) in pore structu 0,2
µM 15 (97-98% eff.).

The same anisotropic media (40-1µM absolute) can be found in 2.5"Ø X
10" long common industrial configurations ... usually of meltblown
polypropylene microfibers .... GE-Osmonics, Pall, 3M-Cuno, Parker (but
not Racor Div.), Filterite (now Pall), and a host of cheap 'imitators'
from Eastern EU and the 'orient'. Polypropylene swells in contact
with fuel oil due to the oleophillic characteristic of polypropylene
(thats what they use in the 'oil-booms'; but, no matter just use the
'next larger' pore size ..... usually available in 1, 2.5, 5, 10, 15,
20, 30, 40, 60µM. But DONT use any 'knife edge seals', use either
flat gasket end caps down to ~10µM and double O-ring adapters 5µM or
less. ************This is probably the cheapest way to go for boat
'recirculation polishing'********** as the (carbon steel) housing
costs are the best value. Typical gravimetric removal will be the
range of 30-50 grams per 10" length. Maximum pressure differential is
~30-40 psid.
There is a lot of CRAP on the filter market of melt-blown technology
but is usually isotropic in pore distribution .... and these are
simply used as 'classifiers' .... with a very sharp particle size 'cut-
off' (but low beta-vale reduction --- log reduction value vs. pore
size) but they dont last long as most of the debris is captured in
first 5% of the 'depth' ... quickly blind off --- this is the crap
usually found in Home Depot's etc. used for 'swimming pool filters',
etc. Typical 'dirt capacity' will be ~ 5 grams per 10" length.
The SAME melt-blown (also 'felting') technology is also found in 'bag
filters' (socks at ~ 8" dia X 12 or 24" long ... looks like a condom)
some very ' highly technical and accurate', some cheap and dirty
(assembled with sewing machines). The 8" dia X 20" length (#2) will
have about 8 pounds of dirt capacity.

Comments?