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
- U.S. perm
- The U.S. perm is defined as 1 grain of water vapor per hour, per square foot, per inch of mercury.
| 1 U.S. perm
|
= 0.659045 metric perms
|
|
≈ 57.2135 ng·s−1·m−2·Pa−1
|
- 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.
| 1 metric perm
|
= 1.51735 US perms
|
|
≈ 86.8127 ng·s−1·m−2·Pa−1
|
- Equivalent SI unit
- The equivalent SI measure is the nanogram per second per square meter per pascal.
| 1 ng·s−1·m−2·Pa−1
|
≈ 0.0174784 US perms
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≈ 0.0115191 metric perms
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The base normal SI unit for permeance is the kilogram per second per square meter per pascal.
| 1 kg·s−1·m−2·Pa−1
|
≈ 1.74784x1010 US perms
|
|
≈ 1.15191x1010 metric perms
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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 U.S. perm.