The permeability test is used to determine the amount of air that can flow through a given
cloth area. Permeability is defined in ASTM Standard D-737-69 as the volume of air that
can flow through one square foot of cloth at a pressure drop of no more than 0.5 in. w.g.
(125 Pa). Because air permeability is not a linear function of the pressure difference measured
across fabric surfaces, the ASTM method prescribes that permeability tests be made
at a pressure drop of 0.5 in. w.g. (125 Pa). Certain fabrics may be too dense or too open to
maintain this pressure drop. In these cases, the ASTM method states that measured pressure
drop be given in the test report.
The permeability of clean felts usually ranges between 15-35 ft/min (8-18 cm/s), while
lighter-weight woven materials have permeability values greater than 50 ft/min (25 cm/s).
Permeability can be measured on clean or dirty bags. Dirty bags are usually tested in the
"as received" state. They are then cleaned by vacuuming or washing and retested. These
measured values can be compared to the original clean permeability of the fabric to determine
if bags that have been in service have become blinded. It is also possible that the
pores in the fabric will open wider after extended use, which is shown by permeability values
higher than the original values. This condition, however, does not occur as frequently as blinding.
cloth area. Permeability is defined in ASTM Standard D-737-69 as the volume of air that
can flow through one square foot of cloth at a pressure drop of no more than 0.5 in. w.g.
(125 Pa). Because air permeability is not a linear function of the pressure difference measured
across fabric surfaces, the ASTM method prescribes that permeability tests be made
at a pressure drop of 0.5 in. w.g. (125 Pa). Certain fabrics may be too dense or too open to
maintain this pressure drop. In these cases, the ASTM method states that measured pressure
drop be given in the test report.
The permeability of clean felts usually ranges between 15-35 ft/min (8-18 cm/s), while
lighter-weight woven materials have permeability values greater than 50 ft/min (25 cm/s).
Permeability can be measured on clean or dirty bags. Dirty bags are usually tested in the
"as received" state. They are then cleaned by vacuuming or washing and retested. These
measured values can be compared to the original clean permeability of the fabric to determine
if bags that have been in service have become blinded. It is also possible that the
pores in the fabric will open wider after extended use, which is shown by permeability values
higher than the original values. This condition, however, does not occur as frequently as blinding.
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