Cable glands have a public size standard to look up. Vent plugs do not — the thread is standard, but the airflow is a property of the membrane inside, and it differs from one product to the next. So this page gives you the threads, the calculation, and an honest note on what to ask for.
A cable gland is easy to tabulate because its critical dimension is a thread, and threads are standardised. M20×1.5 is 20 mm on a 1.5 mm pitch in every factory on earth.
A vent plug is different. Only the thread is standard. How much air it passes depends on the membrane inside — its pore structure, its effective area and how it is supported — and that is a design choice each manufacturer makes for themselves. Two M12 vent plugs that look identical from the outside can differ substantially in airflow, in the pressure they develop, and in how they behave after two years of dust.
Publishing a single airflow number without stating the test pressure would be worse than publishing nothing. So this page is built the other way round: we give you the threads, we give you the physics that decides how much air you need, and we tell you what to demand from any supplier — including us.
Enter the free air volume inside the cabinet and the temperature swing. The calculator works from the ideal gas law on a rigid, sealed box — the worst case — and tells you the pressure that would build up, how much air has to move, and the average flow rate. Everything here is textbook physics; no product data is involved.
Read this before you use the number. The pressure shown assumes a perfectly rigid, perfectly sealed enclosure. A real cabinet relieves part of it through panel flex, door movement and minor leakage, so the measured figure is normally lower. Use the result to judge whether a breather is needed at all and roughly how much air has to move — not as a design pressure for the panel.
The thread part is standard, so this part we can tabulate. Diameters follow the published metric thread standard; the mounting hole is the clearance hole for a nut-and-washer fit.
| Thread | Thread OD | Typical wall | Common use |
|---|---|---|---|
| M5×0.8 | 5.0 mm | 1 – 3 mm | Hand-held instruments, small sensors, sealed luminaires |
| M6×1.0 | 6.0 mm | 1 – 3 mm | Small cameras and sensor housings |
| M8×1.25 | 8.0 mm | 1.5 – 4 mm | Compact enclosures, junction boxes |
| M12×1.5 | 12.0 mm | 2 – 6 mm | The workhorse size on outdoor telecom and control cabinets |
| M16×1.5 | 16.0 mm | 2 – 8 mm | Larger cabinets, inverters, where more airflow is wanted |
| M20×1.5 | 20.0 mm | 2 – 10 mm | Big cabinets, battery racks, high-volume breather assemblies |
Thread outside diameters are standard values. The wall thickness column is a practical range for retention with a nut and washer, not a specification of any particular product — a thick, cast or reinforced panel may need a longer thread or a different fixing, so tell us the wall thickness and we will confirm it rather than leave you to guess.
A bigger plug is not a higher rating. Ingress protection comes from the pore structure and hydrophobic treatment of the membrane, not from its area. A larger plug passes more air at the same pressure difference; it does not pass a harder water test.
A breather does not stop condensation. ePTFE passes water vapour quite happily — it blocks liquid water, not humidity. If a cabinet is collecting water inside, a vent plug will not dry it out. That needs a heater, a desiccant or a thermal design change.
Our own vent plug programme is in development, so we do not yet publish a full airflow table for every size. We would rather tell you that than fill a table with numbers we cannot back with a test.
If you tell us the cabinet volume, the temperature swing and the wall thickness, we will come back with a thread size and an honest statement of what airflow we can and cannot document.
Three numbers and we can size the breather for you — and tell you plainly which figures we can document today and which are still being tested.
