ISO 4793 · Sintered glass filters
Porosity grade is the single most important number on a sintered glass filter — and the one most often chosen wrongly. This guide covers what the grades mean, how to pick one, and how the European and American systems compare.
The basics
A sintered (or fritted) glass filter is not a membrane with drilled holes. It is made by packing graded borosilicate glass particles into a mould and heating them until they fuse at their contact points without fully melting. What is left behind is a rigid disc threaded with an irregular, three-dimensional network of interconnected channels.
Because the channels are irregular, a filter cannot be given a single pore diameter. Instead it is classified by the maximum pore size — the largest channel that survives the manufacturing process, measured by a bubble-point test. That is the number the grade refers to.
The practical consequence matters: a grade P3 filter does not retain everything above 40 µm and pass everything below it. It retains reliably well above the maximum pore size, retains partially inside the stated range, and passes freely below the minimum.
Interactive
Pick a porosity grade and a particle size, and watch what the filter does with it.
0.3 µm500 µm
Result
Not to scale. Pore openings and particles are drawn on a shared logarithmic scale so that grades can be compared side by side in one frame.
Reference
This is the system used throughout Europe and on all Synthware sintered glassware. The grade number runs from coarse to fine — a higher number means a finer filter.
| Grade | ISO designation | Older marking | Max. pore size | What it is for |
|---|---|---|---|---|
| P0 | P250 | Por. 0 | 160–250 µm | Coarse filtration, gas dispersion, and as a support bed for a finer filter medium such as paper or a membrane. |
| P1 | P160 | Por. 1 | 100–160 µm | Coarse precipitates and gas dispersion where a high flow rate matters more than fine retention. |
| P2 | P100 | Por. 2 | 40–100 µm | Medium and crystalline precipitates; washing of gases. The general-purpose workhorse grade. |
| P3 | P40 | Por. 3 | 16–40 µm | Analytical work with medium-sized precipitates. The most commonly stocked grade for Büchner and filter funnels. |
| P4 | P16 | Por. 4 | 10–16 µm | Analytical work with fine precipitates, and mercury filtration. |
| P5 | P1.6 | Por. 5 | 1.0–1.6 µm | Very fine precipitates and bacterial filtration. Slow, and needs vacuum to run at a usable rate. |
A coarser grade than P0 exists. Discs marked P500 (sometimes written P00, 250–500 µm) are made for gas sparging and as supports, where free flow matters far more than retention. They are not part of the common P0–P5 run and are ordered on request.
Note the gap. There is no standard ISO grade between 1.6 and 10 µm. If your separation sits in that window you are choosing between a P4 that will let some material through and a P5 that will run very slowly — or moving to a membrane on a P0 support.
Cross-reference
Glassware bought in North America is usually described in words — coarse, medium, fine — rather than by an ISO grade. The two systems were defined independently and do not line up cleanly.
| ISO 4793 (Europe) | Max. pore |
|---|---|
| P0 · P250 | 160–250 µm |
| P1 · P160 | 100–160 µm |
| P2 · P100 | 40–100 µm |
| P3 · P40 | 16–40 µm |
| P4 · P16 | 10–16 µm |
| P5 · P1.6 | 1.0–1.6 µm |
| ASTM description (US) | Pore size |
|---|---|
| Extra coarse | 170–220 µm |
| Coarse | 40–60 µm |
| Medium | 10–15 µm |
| Fine | 4–5.5 µm |
| Very fine | 2–2.5 µm |
| Ultra fine | 0.9–1.4 µm |
Do not treat these as translations. American medium (10–15 µm) is close to European P4 (10–16 µm), not to P3 — even though P3 sits in the middle of the European range and is often called “medium” in conversation. If a published method specifies a filter in American terms, match it on the micron figure, not on the word.
Practical
Wash a new filter before first use. Fresh sintered glassware carries loose glass dust from manufacturing. Draw dilute hydrochloric acid through it, then rinse thoroughly with distilled water, before anything analytical goes near it.
Always clean by back-flushing. Push solvent through from the stem side, against the direction of filtration. Rinsing from the top drives the retained solids further into the channels, where they set hard and permanently reduce the flow rate.
Match the cleaning agent to the residue. There is no universal solvent for a blocked frit, and the wrong one can set the blockage permanently.
| Residue | Approach |
|---|---|
| Organic residues, greases and oils | A solvent the residue actually dissolves in, followed by acetone, then detergent and water. |
| Inorganic salts and mineral deposits | A warm dilute acid soak, then a thorough rinse with distilled water. |
| Stubborn or unidentified deposits | Time rather than force. A long soak clears far more than a stronger reagent, and costs you nothing if it fails. |
Never use hydrofluoric acid, hot phosphoric acid, or strong hot alkali on a frit. All three attack borosilicate glass itself. All three attack borosilicate glass itself. They widen the pores instead of clearing them, so the disc no longer matches its stated grade.
Shop
Synthware filtration glassware is stocked across the standard porosity grades in borosilicate 3.3.