Perm (unit)

Definitions

US perm

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

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.

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.

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.