What a micron rating claims
A micron (µm) is a thousandth of a millimetre — a human hair is roughly 70 µm, and the eye stops seeing particles at about 40 µm. A cartridge's micron rating states the particle size it removes. The catch is the word removes, because there are two standards of honesty:
Nominal rating means the cartridge removes most of the particles at the stated size — depending on the manufacturer, anywhere from 50% to 90%. A nominal 5 µm cartridge passes a real fraction of 5 µm particles, and plenty of smaller ones.
Absolute rating means a defined, tested cut-off at the stated size. There is no single industry definition of the efficiency behind the word: many makers state 99.9% (beta 1000), others 99.98% (beta 5000), and the figure only means something with the test method named alongside it. A 5 µm absolute cartridge is a genuine barrier at 5 µm to the efficiency and test its own data sheet states — so read the efficiency and the standard, not just the word.
Unless a datasheet explicitly says absolute, assume nominal. Most spun-bonded and string-wound sediment cartridges are nominal; absolute ratings usually come from pleated membrane-media cartridges and cost accordingly.
When nominal is completely fine
Most duty is bulk dirt removal: protecting taps, geysers, irrigation nozzles and appliances, taking the visible sediment out of borehole water, extending the life of a carbon stage. Here the cartridge is a workhorse taking out the mass of solids, a partial pass of fines is harmless, and nominal cartridges hold more dirt per rand of media. Buying absolute for this duty is wasted money.
When you must specify absolute
The difference matters wherever a specific size has a specific consequence:
- Ahead of UV disinfection. UV works by line of sight; particles shadow microorganisms. Cryptosporidium oocysts are 4 – 6 µm, so "5 micron" protection that passes a third of 5 µm particles is not protection. Specify absolute-rated filtration ahead of UV where health is the point.
- Ahead of RO membranes where SDI is marginal — an absolute final cartridge is the last barrier before an expensive element.
- Ahead of dosing equipment and small orifices, where one oversize particle jams a check valve.
- Any process with a validated specification — food, beverage, pharmaceutical — where "removes most of it" fails the audit.
Reading between the lines of a datasheet
- A rating with no efficiency stated is nominal, whatever the marketing says.
- Efficiency at rating (e.g. "90% at 5 µm") tells you how leaky the nominal rating is — useful for comparing brands.
- Dirt-holding capacity matters as much as the rating for service life; two cartridges with the same rating can differ threefold in how much dirt they hold before the pressure drop forces a change.
- Finer is not better. Every step finer raises pressure drop and shortens life; specify the coarsest rating that protects the downstream risk.
Rule of thumb: nominal for dirt, absolute for danger. If the particle size has a health or equipment consequence attached to it, the word you are looking for on the datasheet is absolute — and if it is not there, it is nominal.