Choosing a micron rating for a sintered metal filter starts with one question: what size particles are you actually trying to remove, and how many of them? The rating on the specification is only useful once you know your particle-size distribution and your duty. A rating that is too coarse lets contaminants through; one that is too fine drives up pressure drop, shortens service life, and wastes money. This guide walks through absolute vs nominal ratings, how to read a beta ratio, and how to map pore size to real applications.
Absolute vs nominal rating
Two rating systems appear on filter specs, and confusing them is the most common sourcing mistake.
- Nominal rating is a loose, often unverified claim — typically the size at which a filter removes “most” particles (often quoted around 80–98%). It is not standardized, so a 10 micron nominal from one supplier is not comparable to a 10 micron nominal from another.
- Absolute rating is the smallest particle size a filter is proven to retain with a defined efficiency (usually 99.9% or better, verified by bubble-point or challenge testing). For engineered systems this is the number you specify.
For sintered metal media, always specify an absolute rating. The rigid, fully bonded pore structure makes absolute ratings meaningful and reproducible batch to batch — which is exactly why sintered media is trusted in hydraulic, food and pharmaceutical duty.
Reading a beta ratio (β)
Beta ratio (βx) is the most honest way to express filtration performance. It is the ratio of particles of size x upstream to particles of size x downstream.
| β value | Removal efficiency |
|---|---|
| β = 10 (at x µm) | 90% of x µm particles removed |
| β = 100 | 99% removed |
| β = 1000 | 99.9% removed (this x is the absolute rating) |
If a supplier quotes “10 µm” without a beta value, ask for β1000 (or the equivalent verified absolute). That single number tells you more than the nominal label ever will.
Matching pore size to your particles
The right rating follows the particle distribution, not a round number. A practical approach:
- Protective / pre-filtration (remove bulk debris, protect downstream elements): 25–100 µm absolute.
- Process clarification (steady removal of fines in liquids or gases): 5–25 µm absolute.
- Critical / final protection (valves, sprays, instruments, sterile air): 0.2–5 µm absolute.
Sintered metal lets you build a graded structure — coarse pores on the upstream face stepping down to fine pores at the downstream face. That gives high dirt-holding capacity at low pressure drop, which a single-layer rating cannot match.
Sintered-specific considerations
Two properties make sintered stainless steel and bronze different from disposable media when you pick a rating:
- Reusability. A sintered element is cleanable and re-rated in service, so you can start conservative and confirm the rating against real operating data.
- Rigid pores. Pores do not migrate or shed, so the absolute rating holds for the life of the element — important where downstream damage from a broken fibre would be costly.
Our sintered bronze and sintered stainless steel elements are supplied against a written absolute rating with a bubble-point verification record on request.
Common mistakes to avoid
- Specifying by nominal rating alone and assuming comparability across suppliers.
- Matching the rating to the largest particle only, ignoring the bulk distribution.
- Over-filtering — picking 1 µm when 10 µm would do, then fighting pressure drop and change-out cost.
- Forgetting that gas and liquid duty differ: the same µm rating behaves differently across viscosity and flow.
FAQ
What micron rating do I need for hydraulic oil?
For most hydraulic circuits a final rating of 3–10 µm absolute (β1000) protects pumps and servo valves; pre-filters upstream run 25–50 µm. Base the exact number on your ISO 4406 target cleanliness code.
Is nominal or absolute better for sintered filters?
Absolute. The bonded pore structure supports a verified, reproducible absolute rating, which is what engineered systems should be specified against.
Can I change the rating after installation?
With cleanable sintered elements you can validate performance in service and re-rate conservatively; for disposable media you are locked into the original spec.
Conclusion
The right micron rating is derived from your particle distribution and duty, expressed as a verified absolute rating (with a beta value), and supported by a graded pore structure where flow and dirt-holding matter. Send us your particle data or the component you need to protect, and we will specify the element — and back it with a sample.