Nominal vs Absolute Micron Rating: What 5 µm Really Means

Short answer: A nominal micron rating is a claim that a filter removes some percentage of particles at the stated size, and there is no single standard for what that percentage is. Published definitions run from 60% to 98%. An absolute rating is usually taken to mean 99.9% (3-log, beta 1000), but even that varies: the ultrapure-water industry uses 99.99%, and some industrial suppliers call 98–99% “absolute.” So “5 micron” on a cartridge is not a specification. It becomes one only when the supplier states three more things: the efficiency at that size, the test method, and the flow rate. This guide explains what each rating system actually certifies (beta ratio, NSF/ANSI 42 particulate classes, NSF/ANSI 53 cyst reduction) and how to write a micron spec into a purchase order so a supplier cannot quietly substitute a looser cartridge.

We manufacture PP sediment, carbon block and UF cartridges in Ningbo, so we are the people who print that micron number on the label. This article is written from the factory side of it.

Why “5 Micron” on Its Own Is Not a Specification

A micron number describes a particle size. It does not describe how many of those particles the filter stops. That second number is the one that matters, and it is the one most labels leave out.

Here is how different bodies and markets have defined the two terms:

Source	”Nominal” means	”Absolute” means
Common industry practice (WC&P survey, 2000)	60%, 70%, 80% or more, depending on the maker	Not defined; third-party validation advised
Water Quality Association (as cited in WC&P)	85% rejection at the stated size	99.9% (3-log) rejection
Ultrapure water industry	—	99.99% (4-log) reduction
Typical cartridge-maker usage	”Usually understood as below 90%“	Often 98–99.9%, defined by the maker
NSF/ANSI 42 particulate classes	≥85% reduction in the class size range	Not used. All classes are nominal

Two consequences follow for a buyer.

First, two “5 µm nominal” cartridges are not interchangeable. At 60% efficiency, 40 of every 100 particles at 5 µm get through. At 95%, five get through. That is an 8× difference behind an identical label.

Second, “absolute” without a number is no better. If a supplier calls a depth cartridge “5 µm absolute,” ask “at what efficiency, measured how?” If they cannot answer in one line, the word was chosen by the marketing department.

Beta Ratio: The Number That Makes Suppliers Comparable

The beta ratio removes the vocabulary problem. It is a count:

β(x) = particles ≥ x µm upstream ÷ particles ≥ x µm downstream

Efficiency follows directly: efficiency = (1 − 1/β) × 100%.

Beta ratio at x µm	Removal efficiency	Particles passed per 1,000	What suppliers tend to call it
β = 2	50%	500	Coarse “nominal”
β = 10	90%	100	Good nominal
β = 100	99%	10	”Near-absolute”
β = 1,000	99.9%	1	Absolute (WQA / 3-log)
β = 5,000	99.98%	0.2	Absolute, critical service
β = 10,000	99.99%	0.1	Absolute, ultrapure (4-log)

The jump from β10 to β1000 looks like a few decimal places. It is a 100× difference in particles passed. That is why the beta ratio should be on any spec where the downstream stage is expensive (an RO membrane, a coffee boiler, a dental unit) or where a health claim depends on it.

The multi-pass beta test was standardized for hydraulic oil (ISO 16889). Water cartridges are usually tested single-pass with a standard test dust, most often ISO 12103-1 A2 Fine or A4 Coarse. The method changes the absolute number, which is one more reason to write the method into the order.

What NSF/ANSI 42 Particulate Classes Actually Certify

Many buyers read an NSF/ANSI 42 particulate listing as proof of an absolute rating. It is not. Every class is a nominal, ≥85% claim:

NSF/ANSI 42 class	Particle size range	Required reduction	Absolute?
Class I	0.5 to <1 µm	≥85%	No
Class II	1 to <5 µm	≥85%	No
Class III	5 to <15 µm	≥85%	No
Class IV	15 to <30 µm	≥85%	No
Class V	30 to <50 µm	≥85%	No
Class VI	≥50 µm	≥85%	No
For comparison: NSF/ANSI 53 cyst reduction	3 µm microspheres or live Cryptosporidium	≥99.95%	Effectively yes, for that claim

Two details from the test method matter in practice. Both are described by NSF’s Rick Andrew in Water Conditioning & Purification:

1. Shedding is subtracted. Background particle counts from the flushing samples are subtracted from the effluent counts. A cartridge that sheds fibers can still pass Class I. If your end user cares about fines downstream (espresso machines, solenoid valves, RO membranes), ask for flush-water turbidity separately.
2. The tests do not predict each other. A system can pass particulate Class I and fail cyst reduction, or pass cyst reduction and fail turbidity. They measure different characteristics. A Class I listing is not a cyst claim, and a micron number is never a cyst claim on its own.

For how these listings fit together across NSF/ANSI 42, 53, 58 and 401, see our NSF 42 vs 53 vs 401 comparison.

Why Depth Cartridges Are Hard to Make “Absolute”

Most sediment cartridges sold worldwide are depth filters: melt-blown or string-wound polypropylene. They trap particles throughout a thick wall, not at a single surface. That gives them high dirt-holding capacity and a low price. It also makes their efficiency move with operating conditions:

• Flow rate. Higher face velocity pushes particles deeper and through. A cartridge rated at 1 gpm per 10-inch length will pass more fine particles at 4 gpm. That is why the WC&P survey says a micron rating “must be accompanied with a flow rate.”
• Loading and unloading. As the cartridge loads, pressure drop rises. Pressure surges and water hammer can push trapped particles out the downstream side. The same survey notes that string-wound cartridges “sometimes deform and dump sediment” under high pressure or water hammer.
• Clean vs loaded efficiency. Some depth cartridges filter better as they load (the cake becomes part of the media). A rating measured on a part-loaded cartridge can flatter a new one.

Media type	Filtration mechanism	Rating basis you can realistically get	Unloading risk	Typical 10” FOB (1,000 pcs)
String-wound PP	Depth, uniform density	Nominal; widest spread between makers	High (yarn shifts under surge)	~10–20% below melt-blown
Melt-blown PP, gradient density	Depth, graded outer-to-inner	Nominal; β10–β100 achievable at stated flow	Moderate	$0.35–$0.55 (5 µm)
Pleated PP / polyester	Surface	Near-absolute to absolute at higher micron sizes	Low	Several times melt-blown
Carbon block (CTO)	Depth + adsorption	Nominal; cyst claim only via NSF/ANSI 53 test	Low (rigid)	Higher; media-dependent
UF hollow fiber membrane	Surface, pore-size controlled	Absolute-type, verified by integrity test	Very low	Highest of these

The practical rule we give distributors: if you need an absolute claim, change the media type, not the label. A melt-blown cartridge can be made tighter. But a “1 µm absolute” melt-blown cartridge usually loads in a fraction of the time and has a pressure drop your customer will notice. For a deeper look at how melt-blown, string-wound and pleated PP compare in production, see our PP sediment filter cartridge guide.

Five Ways a Micron Rating Gets Gamed or Misread

These come up repeatedly when we review competitor data sheets that importers send us:

Red flag on the data sheet	What it hides	What to ask for
Micron number with no efficiency	Could be 60% or 95%	Efficiency % or beta ratio at that size
No flow rate	Rating may have been measured at a trickle	Flow per 10-inch length (e.g. 2 gpm)
Cites ASTM F795	Standard withdrawn in 2002	The method actually run, with test dust
Cites EN 1822	Air-filter (HEPA/ULPA) standard, not water	A liquid test method
”Bubble point verified” on a melt-blown cartridge	Bubble point qualifies membranes, not depth media	Particle-count challenge data
”Efficiency by weight” (gravimetric)	Big particles dominate mass; fines can pass	Particle-count efficiency at the rated size

If a supplier’s quality section for a melt-blown or string-wound cartridge leans on bubble point, treat it as a sign they are copying a membrane template. Ask for the particle-count challenge data instead.

How to Write the Micron Spec Into Your Purchase Order

The fix is a few lines of text in the PO or the approved-sample sheet. Here is the wording we recommend to importers, filled in for a typical 5 µm RO pre-filter:

Sediment cartridge, 10” × 2.5”, melt-blown PP, 5 µm nominal. Particle-count removal efficiency ≥90% at ≥5 µm (β5 ≥ 10), single-pass, ISO 12103-1 A2 Fine test dust, at 2 gpm (7.6 L/min) per 10-inch length, measured on a clean cartridge. Initial pressure drop ≤ 2 psi (0.14 bar) at 2 gpm. Report flush volume to <1 NTU. Test report per production batch; golden sample retained by both parties.

Each line closes a loophole. The efficiency stops a looser cartridge being substituted. The test dust and flow make results reproducible. The pressure-drop cap stops a supplier meeting the efficiency by simply over-densifying the cartridge. That would pass the particle test but cut cartridge life in half. For how to enforce the line at incoming inspection, see our water filter cartridge QC and AQL inspection guide.

Where Nominal Is Enough, and Where It Isn’t

You do not need an absolute rating everywhere. Paying for one where nominal works just shortens cartridge life.

Application	Nominal acceptable?	What to specify instead of “absolute”
RO pre-filter, municipal water	Yes	5 µm, ≥90% at stated flow; add 1 µm if SDI > 3–5
Carbon block protection	Yes	5–20 µm nominal upstream, sized to sediment load
Whole-house / point-of-entry sediment	Yes	20 µm then 5 µm staging; flow per length
Coffee and espresso machines	Mostly	5 µm with low-shedding flush spec
Cyst (Giardia, Cryptosporidium) claim	No	NSF/ANSI 53 cyst reduction listing, not a micron number
Bacteria claim	No	0.2 µm membrane with ASTM F838 bacterial retention data
Healthcare ice machines	No for health claims	See our ice machine filter and Legionella guide

For the size-by-size breakdown of what each rating removes (1, 5, 10, 20, 50 µm) and the matching pressure-drop trade-off, see the water filter micron rating chart. If you are deciding between sediment, carbon and membrane stages in the first place, start with sediment vs carbon vs RO membrane.

What We Will and Won’t Put on Our Labels

Being specific about our own limits is the point of this article:

• We label melt-blown and string-wound PP as nominal. We do not print “absolute” on depth cartridges, even when a buyer asks. We can build a tighter cartridge and state its efficiency at your flow rate instead.
• We state the efficiency basis (particle count, clean cartridge, test dust and flow) when a buyer specifies one, and we hold a retained sample per batch.
• Our UF cartridges carry a pore-size rating backed by membrane integrity testing. We still do not call 99.99% bacteria reduction a sterilizing-grade claim, because that requires ASTM F838 validation.
• Cyst and health claims depend on the finished system’s NSF/ANSI 53 listing, not on any micron number we print.

Standard PP cartridges ship in 15–20 working days FOB Ningbo, with trial orders from a few hundred pieces and private label typically from 2,000 pieces per SKU. Browse our standard and commercial cartridges or our OEM manufacturing service.

Related Guides

• Water filter micron rating chart: 0.5 to 50 µm explained
• PP sediment filter cartridges: specifications and OEM sourcing
• NSF 42 vs 53 vs 401 certifications explained
• Water filter cartridge QC and AQL inspection
• Turning a water test report into a filter spec
• UF ultrafiltration membrane cartridge guide

Request a Quote for a Spec-Backed Sediment Cartridge

Send us the micron size, efficiency, flow rate and downstream equipment, plus your target market’s certification needs. Our sales team (Cici and Nancy) replies within 2 working hours with an FOB Ningbo quote, a sample plan and the test basis we can commit to in writing.

Request a Quote for This Product

Sources: J. Hunt, “A Survey of Filter Cartridges,” Water Conditioning & Purification, Jan 2000; R. Andrew, “Cyst Reduction, Turbidity Reduction, Nominal Particulate Reduction Class,” WC&P, Jan 2005; NSF/ANSI 42 and 53; ASTM F795-88 (withdrawn 2002); ISO 16889; ISO 12103-1.

Last Updated: 2026-10-12