What a micron is, and what 5 of them stop
A micron is a thousandth of a millimetre. A human hair is somewhere around fifty to seventy microns across; the smallest thing an eye can pick out unaided is around forty. Fine sand is in the hundreds of microns, silt in the tens, and clay below about two. A 5 micron sediment cartridge, then, stops sand, rust flakes and most silt, and lets the finest clay and everything dissolved straight through.
That is by design. The sediment stage is not there to make the water clean. It is there to keep grit off the carbon block and the membrane, which are the stages that do the cleaning and are the ones that cost money. Five microns is fine enough to protect them and coarse enough to pass water freely for months. Every LUXGEN sediment cartridge is rated at 5 microns: the Branded-Fit and Universal-Fit wound cartridges, the spun pack of two and the 9-inch yarn-wound candle.
Nominal against absolute
A micron rating comes in two kinds, and the number on the label means different things in each. A nominal rating means the cartridge stops most particles of that size and above: a large share of them, not all. An absolute rating means it stops nearly every particle of that size, and it is tested to prove it. Absolute-rated cartridges are laboratory and pharmaceutical products. Domestic sediment cartridges are, as a rule, nominal ratings, and that is the right way to read the 5 on any of them, including LUXGEN's.
The difference matters for one reason. A nominal 5 micron cartridge will let some 5 micron particles through, and it will catch a good many smaller ones by chance as the media loads up. What it will not do is guarantee anything. So the sediment stage is never the reason your water is safe. That job belongs to the membrane and the UV lamp, and the guide to what TDS cannot see explains what those have to deal with.
Why finer is not better
Swap a 5 micron cartridge for a 1 micron one and three things change, none of them in your favour.
- It clogs faster. The finer the pores, the sooner the same water blocks them. A cartridge rated for months on your water can be down to weeks.
- Pressure drops sooner. A loading cartridge is a restriction. The booster pump works harder against it, flow at the membrane falls, and rejection falls with it, because a membrane needs pressure to reject salt. The booster pump guide covers what a starved pump does next.
- It catches nothing the next stage needed catching. Whatever passes a 5 micron cartridge and would have stopped at 1 micron is still far larger than anything the membrane lets through. The membrane is stopping dissolved ions. The finer sediment stage is protecting it from particles it was never troubled by.
The same logic runs the other way. A 10 or 20 micron cartridge lasts longer but passes silt that the carbon block then has to hold, which shortens the carbon stage instead. Five is where the two costs balance for a domestic purifier, which is why the industry settled on it.
What each stage is for
| Stage | Stops | Scale | LUXGEN part |
|---|---|---|---|
| Sediment | Sand, rust, silt | 5 micron, nominal | LUX-SED-10-UNI · LUX-SPN-10-2 · LUX-YW-9 |
| Carbon block | Chlorine, odour, taste; and fine particles, since it is also a 5 micron block | 5 micron, plus adsorption | LUX-CF-10-AGLP · LUX-CF-10-UNI |
| UF membrane, where fitted | Bacteria, cysts, colloids | Hundredths of a micron | Not sold here |
| RO membrane | Dissolved ions: salts, hardness, fluoride, nitrate, heavy metals | Molecular; a micron rating does not apply | LUX-RO-80 · LUX-RO-100 · LUX-RO-100M |
| UV lamp | Kills what is in the storage tank | Not a filter | Not sold here |
Read the table top to bottom and the pattern is plain: each stage removes what the next one cannot tolerate. Sediment keeps grit off the carbon. Carbon keeps chlorine off the membrane, which chlorine destroys. The membrane removes what no filter can, and the UV keeps the tank sterile. The carbon cartridges, Branded-Fit at ₹250 and Universal-Fit at ₹220, carry the same 5 micron figure as the sediment cartridges, but their job is chemical, and their exhaustion shows as a chlorine taste rather than as slow flow.
Choosing on micron, or not
Between the LUXGEN sediment cartridges there is no micron decision to make, because they are all 5 micron. The decisions are the fit, which the fit guide settles, and the media, wound or spun, which the spun vs wound guide settles. The one thing to avoid is a cartridge bought on its micron number from a seller who does not say whether the rating is nominal, whether it is a gradient or a single density, or what housing it fits. A 5 micron cartridge of the right fit, changed on the schedule, protects a membrane better than a 1 micron cartridge of the wrong one that clogged in six weeks.
Questions
What does 5 micron mean on an RO filter?
The cartridge is rated to stop particles five thousandths of a millimetre across and larger: sand, rust flakes and most silt. On a domestic cartridge the rating is nominal, meaning it stops most such particles rather than every one. Its job is to protect the carbon block and the membrane, not to make the water safe.
Is a 1 micron filter better than 5 micron for RO?
Not in a domestic RO purifier. A 1 micron cartridge clogs sooner, drops the pressure the membrane needs, and stops nothing the membrane was troubled by, since the membrane removes dissolved ions far smaller than any sediment cartridge reaches. Five micron is the balance the industry settled on.
What is the difference between nominal and absolute micron ratings?
A nominal rating stops most particles of that size; an absolute rating stops nearly all of them and is tested to prove it. Absolute-rated cartridges are laboratory products. Domestic sediment cartridges, including LUXGEN's, carry nominal ratings, and the 5 should be read that way.