What are the differences between the ASTM and ISO test methods?

The ASTM and ISO test methods for measuring fabric air permeability are similar, but there are some differences between them. These differences include:

1. Test apparatus: The ASTM and ISO test methods use different test apparatus to measure fabric air permeability. The ASTM test method uses a test instrument with a circular orifice, while the ISO test method uses a test instrument with a square orifice.

2. Test pressure: The ASTM test method measures air permeability at a constant airflow rate, while the ISO test method measures air permeability at a constant pressure drop. The pressure drop used in the ISO test method is typically lower than the pressure drop used in the ASTM test method.

3. Units of measurement: The ASTM test method reports air permeability in cubic feet per minute per square foot of fabric (CFM/ft2), while the ISO test method reports air permeability in liters per second per square meter of fabric (L/s/m2). 

4. Test sample size: The ASTM and ISO test methods specify different sample sizes for testing fabric air permeability. The ASTM test method requires a sample size of at least 40 square inches, while the ISO test method requires a sample size of at least 100 square centimeters.

5. Test conditions: The ASTM and ISO test methods specify slightly different test conditions, such as the temperature and humidity of the test environment, the conditioning of the fabric sample, and the number of readings to be taken.

Despite these differences, both the ASTM and ISO test methods are widely recognized and accepted in the textile industry, and they provide accurate and reliable measurements of fabric air permeability. The choice between the two methods often depends on regional or industry preferences, as well as the specific requirements of the application or regulation. It is important to select a test method that is recognized by the relevant industry standards organizations and that meets the specific needs of the application.