Perm (unit)
Encyclopedia
A perm is a unit of permeance or "water vapor
transmission" given a certain differential in partial pressure
s on either side of a material or membrane.
Metric perm
Equivalent SI unit
The base normal SI unit for permeance is the kilogram
per second
per square meter per pascal
.
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.
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
transmission" given a certain differential in partial pressure
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
s on either side of a material or membrane.
Definitions
US perm- The US perm is defined as 1 grain of water vapor per hourHourThe hour is a unit of measurement of time. In modern usage, an hour comprises 60 minutes, or 3,600 seconds...
, per square footSquare footThe square foot is an imperial unit and U.S. customary unit of area, used mainly in the United States, Canada, United Kingdom, Hong Kong, Bangladesh, India, Pakistan and Afghanistan. It is defined as the area of a square with sides of 1 foot in length...
, per inch of mercuryInch of mercuryInches of mercury, ' is a unit of measurement for pressure. It is still widely used for barometric pressure in weather reports, refrigeration and aviation in the United States, but is seldom used elsewhere....
.
| 1 US perm | = 0.659045 metric perms |
| |≈ 57.2135 ng·s−1·m−2·Pa−1 |
Metric perm
- The metric perm (not an SIInternational System of UnitsThe International System of Units is the modern form of the metric system and is generally a system of units of measurement devised around seven base units and the convenience of the number ten. The older metric system included several groups of units...
unit) is defined as 1 gramGramThe gram is a metric system unit of mass....
of water vapor per dayDayA day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
, per square meter, per millimeter of mercuryTorrThe torr is a non-SI unit of pressure with the ratio of 760 to 1 standard atmosphere, chosen to be roughly equal to the fluid pressure exerted by a millimetre of mercury, i.e., a pressure of 1 torr is approximately equal to 1 mmHg...
.
| 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 secondSecondThe second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
per square meter per pascalPascal (unit)The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
.
| 1 ng·s−1·m−2·Pa−1 | ≈ 0.0174784 US perms |
| |≈ 0.0115191 metric perms |
The base normal SI unit for permeance is the kilogram
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
per second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
per square meter per pascal
Pascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
.
| 1 kg·s−1·m−2·Pa−1 | ≈ 1.74784x10+10 US perms |
| |≈ 1.15191x10+10 metric perms |
German Institute for Standardization unit
A variant of the metric perm is used in DINDeutsches Institut für Normung
is the German national organization for standardization and is that country's ISO member body. DIN is a Registered German Association headquartered in Berlin...
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.