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.
1 U.S. perm | = 0.659045 metric perms |
≈ 57.2135 ng·s−1·m−2·Pa−1 |
1 metric perm | = 1.51735 US perms |
≈ 86.8127 ng·s−1·m−2·Pa−1 |
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 |
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.