From: Tom Jaszewski (tomj@lvcm.com)
Date: Fri Dec 20 2002 - 18:33:29 EST
I find this micron and mesh issue confusing..I found this today..
Don't confuse MESH with MICRON
* Contrary to the accepted norm; mesh size, by itself, means absolutely
nothing!
* Remove the concept of trying to equate an arbitrary number (mesh) to a
fixed number (micron)
* Stick with a micron retention in particle removal
What is mesh?
It refers to the number of openings in a lineal inch measured from the
center of one wire to a point 1" distant. It has nothing to do with the size
of the hole and this is where confusion reigns. Someone gives you a mesh
size (and this happens all the time) and through some hocus-pocus, a size in
microns is given. How it ever evolved is beyond me and has caused more
problems than people realize.
A simple example:
A 40 mesh filter screen has 40 holes per lineal inch - so what? Depending on
which manufacturers catalog you use, you can end up with, and I hate doing
this as a comparison, a micron retention ranging from 307 microns to 435
microns. For some applications, the difference is minimal, but for most
companies, it can mean rejection of product or contamination.
What determines the disparity when you convert to microns?
That's a good question, and I will answer it as simply as I can. The reason
is this: manufacturers use different wire diameters. Why? My guess is due to
the structural integrity of the screen, and a need to please everyone (you
know what happens when you try that!).
Using our 40-mesh example, a manufacturer will use a wire diameter of
0.013", which results in smaller holes per linear inch. This calculates to a
width opening of 0.012". The same manufacturer will then use a 0.008" wire
diameter, which produces bigger holes and corresponds to a width opening of
0.017". Remember, we still have 40 holes per linear inch but our micron size
has changed from 307microns to 435 microns.
What can a company do?
Well, the first thing you need to do is think in terms of microns. I know,
your thinking what is a micron? For those who are not sure or forgot, a
micron is 0.000039". How is it applied? In an ideal world, a particle
distribution count would be taken - this breaks down the size and quantity
of suspended particles in a sample liquid. Unfortunately, most companies do
not have the budget or time to do this (the cost can run anywhere from $300
to over a $1000). So the next best thing is to evaluate (guess) or contact a
filter specialist. To help, use the mesh /micron chart provided - the most
common are listed. It isn't complete by any means, but it will get you
pointed in the right direction.
As a general rule of thumb, anything below 40 microns cannot be seen with
the eye. Keep in mind that a mesh screen you're presently using, may not
correspond to our chart. If this occurs, you will need to look at the
nearest mesh/micron size and evaluate which is better (above or below).
Better yet, determine which micron size is needed for particle removal.
Conversion Chart
Mesh
Nominal Particle Retention
Micron Rating
10
.065"
1666
20
.035"
897
30
.023"
590
40
.015"
384
50
.011"
282
60
.009"
230
70
.008"
205
80
.007"
180
90
.0051"
130
100
.0055"
141
150
.0046"
118
200
.0033"
84
250
.0024"
60
400
.0018"
45
700
.0012"
30
Tom Jaszewski
www.livesoil.com
702-595-7012
-----Original Message-----
From: Jeff Lowenfels [mailto:jeff@gardener.com]
Sent: Friday, December 20, 2002 12:17 PM
To: compost_tea@yahoogroups.com
Subject: Re: [compost_tea] mist blowers for CT
100 Microns sounds too small.....400 , I think, is the order....
Cheers,
Jeff
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