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.

The base normal SI unit for permeance is the kilogram per second per square meter per pascal.

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.

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