perm

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. Definitions U.S. perm The U.S. perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury. {| |- |1 U.S. perm  |= 0.659045 metric perms |- |||≈ 57.2135 ng·s−1·m−2·Pa−1 |} Metric perm The metric perm (not an SI unit) is defined as 1 gram of water vapor per day, per square meter, per millimeter of mercury.