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 US perm is defined as 1 grain of water vapor per hour, per square foot, per inch of Mercury.
A different unit, the metric perm is defined as 1 gram of water vapor per day per square meter per millimeter of Mercury (mmHG or Torr). (The equivalent of 1.51735 US perms)
Because it depends on non-metric units (days and mmHg), the "metric perm" is not part of the International System of Units. The equivalent SI measure is the nanogram per second per square meter pascal. (The equivalent of 0.0174784, or 1/57.2135 US perms)
Another unit, used in DIN Standards is the gram per square meter per day given a "standard" vapor pressure of 17.918 mmHg. (The equivalent of 0.084683 of a US perm)
In each case, lower values represent less permeance of a membrane.
References
- Michon, Gérard P. (April 29, 2003) "Permeability and permeance". Final Answers: Physics of Gases and Fluids. - accessed August 13, 2007
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