ⓘ 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 ..

                                     

ⓘ 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.

                                     

1. 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. 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.

                                     

2. German Institute for Standardization unit

A variant of the metric perm is used 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.

                                     
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