Why Two 5 Micron Filters Are Not the Same Filter

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Before the number, the convention. A micron rating on its own is not a specification, because there are two standard ways of arriving at one and they can differ by a factor of four on the same piece of media. A nominal rating names the size at which the filter catches most of what arrives, where most is often defined somewhere between 60 and 85 percent and is frequently not defined at all. An absolute rating names the size at which it catches essentially everything. The same cartridge can therefore be sold as a 5 micron filter or as a 20 micron filter, both honestly, and the label is not obliged to say which convention produced the number.

One note before going further. This page is about ratings, flow and pressure drop. It makes no claim about what any filter removes from water, about any contaminant, or about health, and nothing here should be read as one.

The curve, and the two points people pick off it

A real filter does not have a cutoff. It has a retention curve: it catches almost nothing far below its working size, rises steeply through a transition band, and approaches total capture without ever quite getting there. Picking a single number to print means picking a point on that curve, and the industry has two habits about where to pick.

The formal way to write it is the beta ratio, which is the number of particles of a given size going in divided by the number coming out. Efficiency is one minus one over beta, which makes the arithmetic easy. Beta 2 is 50 percent. Beta 20 is 95 percent. Beta 75 is 98.7 percent, the older absolute convention. Beta 1000 is 99.9 percent, the modern one.

Now put real counts through it. Send ten thousand particles of the rated size at a filter sitting at a nominal 85 percent and about fifteen hundred come out the far side. Send the same ten thousand at the absolute point and about ten do. That is a hundred and fiftyfold difference in what gets through, produced by two numbers that read identically on a box. And it gets worse at the bottom of the nominal range: a filter quoted nominal at 60 percent is letting four thousand of your ten thousand straight past the size it is named after.

Which is the real answer to the original question about an air compressor intake. Ten micron nominal is the usual general purpose answer, and the reason it is sufficient is not that ten micrometres is a magic size. It is that intake air is being protected against the particles that score a cylinder, those are mostly well above ten micrometres, and a nominal rating catching most of them is plenty while costing the compressor very little in breathing. Specify absolute there and you would be paying for retention you do not need in the currency the next section is about.

What a finer filter costs, which is flow

Flow through a porous medium follows Darcy's law: the rate is the permeability times the area times the pressure difference, divided by the fluid viscosity and the thickness. Three of those are things a designer sets. The fourth, permeability, is set by the pore size, and the Kozeny Carman relation says it goes as the square of that size at a given porosity.

The square is the whole problem. Halve the pore size and you quarter the permeability. Go from a 20 micrometre working point to a 5 micrometre one, a factor of four in size, and permeability falls by sixteen. At the same supply pressure, through the same housing, you get a sixteenth of the flow.

There are only three ways out and all are visible on products. Raise the area, which is what pleating does and why a pleated cartridge can carry ten to twenty times the media of a plain wound one in the same tube. Reduce the thickness, which lowers resistance and also lowers the depth available to hold dirt. Or raise the pressure, which a mains supply will not do for you. In a fixed housing with a fixed supply, a finer filter simply passes less water. That is not a defect. It is the arithmetic.

Where the adjective sits instead of the number

Our Pousbo faucet filter cartridge kit, a 50 pack of bag style mesh pouches publishes its filter media as a dense polyester mesh pouch. Dense is doing the work a micron rating would do, and it is not a quantity: it could describe anything from a coarse strainer to something that will slow your tap to a dribble.

The listing also publishes something that is the pressure drop in disguise, under the heading softens the splash: the bag structure breaks up the stream to reduce scattered droplets at the sink. A stream that arrives softer than it did before is a stream that has lost energy on the way through, and the energy it lost is the pressure drop across the mesh. The feature and the cost are the same physical fact described twice, once as a benefit. Our record's drawback line says the mesh needs regular replacing to stay effective, without a replacement interval or a capacity, which brings us to the next section. Record in the Pousbo faucet filter cartridge kit review.

Why capacity belongs in gallons and almost never is

A filter does not wear out. It fills up. Captured particles sit in and across the pores and progressively narrow them, which has two consequences that surprise people in opposite directions. The first is that a loading filter gets finer as it ages, so its retention improves over its life, right up until the end. The second is that permeability is falling the whole time, by the same square law, so the flow collapses long before the retention does.

That makes capacity a cumulative quantity. It is a number of gallons, or a number of hours at a stated flow, because what kills the filter is the total mass of material it has caught. It is not a number of months, because a month is not a quantity of water.

Our AQUA CREST RV inline hose filter publishes both halves of that and lets you see the gap. It states a flow rate of 0.5 gallons per minute and a service life of about three months. Multiply them out at the rated flow and three months is 0.5 times 60 times 24 times 90, which is 64,800 gallons, and nobody means that. So the three months is assuming an unstated duty. At thirty gallons a day it is 2,700 gallons. At a weekend use pattern of thirty gallons a fortnight it is about 190. Those two owners have the same filter, the same label and the same three months on it, and they are a factor of fourteen apart in what they have put through it. One is replacing a filter that still has most of its capacity left and the other is long past it, and the label cannot tell them apart.

The 0.5 gallons per minute is worth sitting with on its own, because it is the pressure drop argument made concrete and our own record flags it as a drawback. A standard three quarter inch garden hose at household pressure delivers something in the region of eight to seventeen gallons a minute. Fitting an inline filter rated at half a gallon a minute therefore cuts the flow by roughly twenty times. A five gallon bucket that fills in half a minute from the bare hose takes ten minutes through the filter. That is the entire trade in one line: filtration is bought with flow, and the listing publishes what it took. Record in the AQUA CREST RV inline filter review, and the category is in best whole house water filters. What that filter does or does not remove is outside the scope of this page and we make no claim about it.

A screen is a different kind of filter, and the geometry is exact

Everything above is about depth media, where particles are caught throughout a thickness. A woven screen is the other family: a single plane with holes in it, and because the geometry is simple the arithmetic is exact rather than approximate.

For a square weave with aperture a and wire diameter w, the open area fraction is a squared over the quantity a plus w, squared. A 200 micrometre aperture woven with 100 micrometre wire is 200 squared over 300 squared, which is 44 percent open. Take the aperture down to 50 micrometres on 50 micrometre wire and it is 50 squared over 100 squared, which is 25 percent. So the finer screen has barely half the open area before you have even considered that each individual hole is more restrictive, and for a thin plate the flow through each hole scales as the cube of its radius. Put the two together and flow goes as aperture times open fraction: 200 times 0.44 is 88.9 against 50 times 0.25 is 12.5, so a screen four times finer passes about seven times less at the same pressure.

Which is exactly the trade in our MAVO pour over coffee set with a reusable stainless steel mesh filter. Our own record names the consequence without the mechanism, listing as a drawback that the metal filter passes more oils and sediment than paper. That is the aperture fact seen from the other side: a woven or etched steel mesh has openings far larger than the pore structure of a paper filter, so it passes more and it drains faster.

And draining faster is the part that catches people out, because it changes the brew without changing anything you set. Swap paper for steel and keep the same grind and the same pour, and the water spends less time in contact with the coffee, so you will under extract and get something thin and sour while also getting more sediment in the cup. Both effects are the same hole size. The two levers that fix it cost nothing: grind finer, which slows the bed, or pour more slowly, which extends the contact time directly. Record in the MAVO pour over coffee set review, and there is more on mesh and separation in best yogurt strainers.

One product called a filter that is not filtering at all

Our washing machine hair and lint filter mesh bags, a three pack are worth including precisely because no micron rating could describe them. The listing is clear about the mechanism: a sticky, reusable surface with raised nubs that loose hair clings to. That is adhesion, not sieving. Nothing is being passed through anything, so there is no pore, no retention curve and no pressure drop.

It also means the failure mode is completely different. A sieve fails by clogging, progressively, as the pores narrow. An adhesive collector fails by saturating: once the nubs are covered, capture stops more or less abruptly, and rinsing restores it fully rather than partially. Our record says it needs rinsing between loads to keep working, and that is why. The word filter in the product name is doing marketing work, and the specification you would actually want is a surface area, which is not published either. Record in the washing machine lint filter bags review.

A word that means two unrelated things on this site

Worth flagging, since it comes up every time someone searches for it. In filtration a micron is a micrometre, a unit of length. In vacuum and refrigeration work a micron is a unit of pressure: one micrometre of mercury, which is a thousandth of a torr, or about 0.133 pascals. Our vacuum gauge kits comparison uses it in that second sense, where a digital micron reading describes how deep a vacuum a system has pulled, and a 500 micron target there is a pressure, not a particle. The two usages share nothing but the word, and a search for micron rating will cheerfully mix them.

The honest limit

Not one filter product on this site publishes a micron rating at all. Not the faucet cartridges, not the inline hose filter, not the coffee mesh. So the practical problem is not choosing between a nominal number and an absolute one, it is that the number is usually missing entirely and replaced with an adjective: dense, fine, high quality. When a number is present, assume it is nominal unless the word absolute or a beta ratio appears next to it, because absolute is the expensive claim and makers who have earned it say so.

Which leaves the free moves, and for filtration they are unusually effective.

Judge by flow, not by the label, because flow is the one thing you can measure without any equipment. Time a container filling, write the number down the day you fit the filter, and time it again later. Flow falling is the filter telling you its remaining capacity directly, which is more honest than any interval on a box and accounts for your water and your usage rather than an average household's. A filter that has lost half its flow has lost far more than half its remaining life, because the loading accelerates as the pores narrow.

Fit the coarsest thing that does the job, and stage it if you need fine. Two filters in series, a coarse one first and a fine one after, is not an extravagance: the coarse one takes the bulk of the loading, which is most of the mass, and the fine one then lasts far longer than it would on its own. Cost aside, this is the only arrangement that does not force you to choose between flow and life.

And the free one that beats the purchase: give it more area. Most consumer filtration problems are solved not by a finer medium but by a bigger one, because area is linear in Darcy's law while pore size is squared. A larger housing, a pleated element instead of a wound one, or simply two in parallel will give you the retention you wanted at the flow you had. Buying finer and then complaining about the trickle is the common mistake, and the arithmetic in this post says it was always going to happen.