Perm (unit)

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
 * }
 * ≈ 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.




 * 1 metric perm
 * = 1.51735 US perms
 * ≈ 86.8127 ng·s−1·m−2·Pa−1
 * }
 * ≈ 86.8127 ng·s−1·m−2·Pa−1
 * }


 * Equivalent SI unit
 * The equivalent SI measure is the nanogram per second per square meter per pascal.



The base normal SI unit for permeance is the kilogram per second per square meter per pascal.
 * 1 ng·s−1·m−2·Pa−1
 * ≈ 0.0174784 US perms
 * ≈ 0.0115191 metric perms
 * }
 * ≈ 0.0115191 metric perms
 * }


 * 1 kg·s−1·m−2·Pa−1
 * ≈ 1.74784x1010  US perms
 * ≈ 1.15191x1010 metric 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.