"Air permeability: 85." Eighty-five what? Measured over which area, at what pressure, on which face? A surprising number of technical data sheets and even test reports give a bare figure, and a surprising number of supplier–buyer disputes on airbag fabric, filter media, sportswear and workwear begin there.
What ISO 9237 defines
ISO 9237 defines air permeability as the velocity of an air flow passing perpendicularly through a specimen under specified conditions of test area, pressure drop and time. The instrument draws air through a clamped circular area of fabric, the flow is adjusted until the chosen pressure drop is reached, and the flow rate is read.
The standard allows four test areas and four pressure drops, and recommends one combination for each end use:
| Options in ISO 9237 | Recommended | |
|---|---|---|
| Test area | 5, 20, 50 or 100 cm² | 20 cm² |
| Pressure drop | 50, 100, 200 or 500 Pa | 100 Pa for apparel fabrics, 200 Pa for industrial fabrics |
The alternatives are used by agreement, when the recommended pressure cannot be reached or is unsuitable. The standard itself notes that results should be compared only when the same test area and pressure drop were used.
The result is calculated in mm/s from the mean flow rate and the test area. For very open fabrics and nonwovens it may be given in m/s. Either way, the report should also carry the coefficient of variation and the 95 % confidence interval.
Mistake 1: changing the pressure and "scaling" the result
A fabric that gives 100 mm/s at 100 Pa does not necessarily give 200 mm/s at 200 Pa. The relationship between pressure and flow through a fabric is not reliably linear, particularly for dense or compressible structures. If the specification says 100 Pa, test at 100 Pa. If a customer wants a figure at another pressure, measure it.
Mistake 2: confusing ISO 9237 with ASTM D737
Both standards measure the same property, but with different default conditions.
| ISO 9237 | ASTM D737 | |
|---|---|---|
| Default pressure difference | 100 Pa (apparel) / 200 Pa (industrial) | 125 Pa |
| Default test area | 20 cm² | 38.3 cm² |
| Usual unit | mm/s | cm³/s/cm² (or ft³/min/ft²) |
Unit conversion is the easy part: 1 cm³/s/cm² equals 10 mm/s. The pressure and area differences are not something a conversion can fix. An ISO 9237 result at 100 Pa and an ASTM D737 result at 125 Pa are two different measurements.
Mistake 3: edge leakage
Air that escapes around the clamped edge of the specimen is counted as air through the fabric. ISO 9237 says precautions should be taken against it, and describes options: a guard ring with its own pressure measurement, rubber facings on the clamps, or measuring the leakage separately (by covering the specimen with a rubber sheet) and subtracting it.
Leakage matters most for low-permeability fabrics — coated fabrics, airbag fabric, dense technical weaves — where a small leak is a large fraction of the reading. For coated specimens, the standard advises mounting the coated side towards the low-pressure side to reduce leakage.
Mistake 4: not recording which face was tested
Permeability can differ between the two faces of a fabric. ISO 9237 asks for the face tested — and, where relevant, the direction of air flow — to be stated. For laminates, brushed fabrics and anything with a finish on one side, this is not a detail.
Mistake 5: too few readings
ISO 9237 calls for at least ten tests at different places on the sample, under the same conditions, with the flow recorded after at least one minute or once it is steady. Woven fabrics are not uniform at the scale of a 20 cm² test head; one or two readings tell you about that spot, not the fabric. Avoid edges, creases and folds — a crease is a local hole.
Mistake 6: testing straight from the store
Like most physical tests, ISO 9237 is carried out on specimens conditioned and tested in the ISO 139 standard atmosphere. Moisture swells many fibres and changes the pore structure. A sample tested in a dry winter office and the same sample tested in a humid summer warehouse are not the same sample.
Keep the instrument honest
The informative annex of ISO 9237 recommends checking the instrument with a calibration check plate of known permeability — weekly when the instrument is in continuous use, before testing when it is used occasionally, and after it has been moved or repaired — and checking the pressure gauge periodically. It is a five-minute routine that catches a drifting flowmeter long before a customer does.
A complete result
Air permeability, ISO 9237: [mean] mm/s, test area 20 cm², pressure drop 100 Pa, face tested [face], 10 tests, CV [%], 95 % confidence interval [±].
Our air permeability tester has a 20 cm² test head as standard, optional 5, 25, 38, 50 and 100 cm² heads, and pressure differences up to 500 Pa, so different test areas and pressure drops can be set up on one instrument. It is made in our own workshop in Istanbul.