Standard Test Areas in Textile and Membrane Testing
By Scientific Converters · Updated · Factors from NIST SP 811 and the BIPM SI Brochure
Short answer: ISO 9237 uses a 20 cm² test area (with 5, 50 and 100 cm² alternatives) and ASTM D737 uses 38.3 cm² (about 5.94 in²); permeability is the measured flow divided by that area.
The common test areas
Air permeability and filter-media testers clamp the sample over a circular opening, the test head. The area of that opening turns the measured flow into a permeability. These areas cover nearly all commercial testers:
| Area | in² | ft² | Diameter (mm) | Where used |
|---|---|---|---|---|
| 5 cm² | 0.775 in² | 0.005382 ft² | 25.23 | ISO 9237 (alternative test area) |
| 10 cm² | 1.55 in² | 0.01076 ft² | 35.68 | Instrument option on many permeability testers |
| 20 cm² | 3.1 in² | 0.02153 ft² | 50.46 | ISO 9237 / EN ISO 9237 (standard test area) |
| 25 cm² | 3.875 in² | 0.02691 ft² | 56.42 | Instrument option; used for nonwovens and small samples |
| 38.3 cm² | 5.937 in² | 0.04123 ft² | 69.83 | ASTM D737 (standard test area, ≈ 5.94 in²) |
| 50 cm² | 7.75 in² | 0.05382 ft² | 79.79 | ISO 9237 (alternative test area) |
| 100 cm² | 15.5 in² | 0.1076 ft² | 112.8 | ISO 9237 (alternative); filter media holders (e.g. 32 L/min → 5.33 cm/s) |
Area and diameter of a round test head
For a circular opening, A = π d² / 4, so d = √(4A/π). A 20 cm² head is 50.46 mm across and a 38.3 cm² head 69.83 mm. Measure the inner diameter of the clamping ring when checking a tester; a 1 % error in diameter is a 2 % error in area and therefore in permeability.
Why the area changes the flow you read
At the same permeability, flow scales with area. A fabric of 100 mm/s passes 12 L/min through 20 cm² but about 22.98 L/min through 38.3 cm². Choose a head that keeps the flow in the accurate range of the tester's flow meter.
Choosing an area
- Use the area the specification names; results on different areas should agree, but specifications are written for one.
- Use larger areas (50–100 cm²) for open or variable materials such as nonwovens, so a single thin spot does not dominate.
- Use small areas (5–10 cm²) for small samples or very open materials that would exceed the flow range.