Mesh is a wire count, not a hole size
The defaults describe a fairly common screen: 150 mesh woven from 0.0026 in wire. A hundred and fifty wires to the inch puts the pitch at 169.3 microns, the wire takes 66.0 of that, and what is left is a 103.3 micron opening — 0.0041 in.
Change nothing but the wire. At 0.0035 in wire the same 150 mesh gives an 80.4 micron opening, 22 percent smaller. The mesh number on the label is identical in both cases. This is why two filters carrying the same mesh number can behave differently, and why a mesh-to-micron conversion table is only ever right for the wire it was written for.
Most of a screen is wire
The opening on the defaults is 61 percent of the pitch in one direction. Square that, because the weave is a grid and the wire blocks in both directions, and only 37.2 percent of the screen surface is actually hole.
Run 12 GPM through a 60 sq in screen and you are pushing it through 22.33 sq in of opening at 0.172 ft per second. That is a comfortable figure, and it stays comfortable only while the open area holds. Blind half the screen and the velocity doubles while the loss across it follows roughly the square, which is why a screen filter gives very little warning: the pressure drop sits almost flat for weeks and then climbs quickly over a few days. It is also the reason screen filters are built with a surface area many times the bore of the pipe feeding them.
What the ratio to the emitter means, and does not
With a 700 micron emitter passage entered, the page reports that the passage is 6.78 times the screen opening. That is arithmetic on two numbers, and it is useful for seeing where you stand rather than as a pass mark.
The figure to work to is the filtration requirement the emitter manufacturer publishes for that emitter, which is their number for their product. And a screen does nothing at all about some of what blocks emitters. Fine silt passes straight through. Iron and calcium precipitate downstream of any filter. Algae deforms and extrudes through openings it should not fit through. None of that is visible in a geometry calculation, and this page does not pretend otherwise.
Flushing costs almost nothing
Four 200 ft laterals of 0.57 in tubing hold 2.651 gallons each. Pushing two line volumes through each at 1 ft per second takes 0.795 GPM per lateral and 6.7 minutes, and one flush of all four comes to 21.2 gallons. Six flushes a season is 127 gallons, which at four dollars fifty a thousand is 57 cents.
Set that against a zone running at 12 GPM, which puts 720 gallons out in an hour. A whole season of flushing is about eleven minutes of watering. The reason emitters block is almost never one large particle arriving; it is fines settling at the ends of the laterals where the velocity is lowest, month after month, until the passage narrows. Opening the ends and moving water fast enough to carry that out is the cheapest maintenance on the whole system, and it is the one that gets skipped because nothing appears to be wrong until several emitters are already gone.
Questions people ask
How do I convert mesh to microns?
You cannot, from the mesh number alone. Mesh counts wires per inch, so the pitch is 25,400 divided by the mesh number, and the opening is that pitch less the wire diameter. On the defaults here, 150 mesh with 0.0026 in wire gives a 103.3 micron opening; the same 150 mesh with 0.0035 in wire gives 80.4. Any conversion table is quietly assuming one particular wire, so use your own screen specification.
How much of a screen is actually open?
The open share in one direction is the opening divided by the pitch, and the area share is that squared, because wire blocks in both directions. On the defaults that is 61 percent in one direction and 37.2 percent by area — so a 60 sq in screen presents 22.33 sq in of hole. That is why screen filters carry far more surface area than the bore of the pipe they sit in.
What mesh should I use for my drip system?
Work to the filtration requirement the emitter manufacturer publishes for the emitter you are using. That is their figure for their product and it is the one that matters. This page compares the opening you have to the emitter passage you enter, which shows you where you stand, but it makes no judgement about whether that is right for your water — and some things that block emitters are not screen problems at all.
How much water does flushing a drip system use?
On the defaults — four 200 ft laterals of 0.57 in tubing, two line volumes at 1 ft per second, six times a season — 127 gallons, or about eleven minutes of the zone running. Flushing costs almost nothing in water. What it saves is the emitters at the ends of the laterals, where the velocity is lowest and the fines settle out.
Why does my filter pressure drop suddenly get worse?
Because the loss across a screen follows roughly the square of the velocity through the remaining open area, and the open area falls as the screen loads. That makes the pressure drop sit almost flat for a long time and then climb steeply once a real fraction of the screen is blinded. The page shows what the velocity does at half the screen blinded, which is the shape of the problem rather than a prediction of when it happens.