A perm is a unit of
permeance or "
water vapor transmission" given a certain differential in
partial pressures on either side of a material or membrane.
The equivalent SI measure is the
nanogram per
second per square meter per
pascal.
1 ng·s−1·m−2·Pa−1
≈ 0.0174784 US perms
≈ 0.0115191 metric perms
The base normal SI unit for permeance is the
kilogram per
second per square meter per
pascal.
1 kg·s−1·m−2·Pa−1
≈ 1.74784x1010 US perms
≈ 1.15191x1010 metric perms
German Institute for Standardization unit
A variant of the metric perm is used in
DIN Standard 53122, where permeance is also expressed in grams per square meter per day, but at a fixed, "standard" vapor-pressure difference of 17.918 mmHg. This unit is thus 17.918 times smaller than a metric perm, corresponding to about 0.084683 of a U.S. perm.
References
Michon, Gérard P. (April 29, 2003) "
Permeability and permeance". Final Answers: Physics of Gases and Fluids. - accessed August 13, 2007