Perm (unit)

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

;US perm:The US perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.

:

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 US perm.

References

* Michon, Gérard P. (April 29, 2003) " [http://home.att.net/~numericana/answer/gas.htm#perm Permeability and permeance] ". "Final Answers: Physics of Gases and Fluids". - accessed August 13, 2007


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