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{{Short description|Non-SI unit of gas permeability}}
The '''barrer''' is a non-[[SI]] unit of [[permeation|permeability]] of [[gas]]es used in the membrane technology and [[contact lens]] industry. It is named after the New Zealand-born chemist [[Richard Barrer]].

==Definition==
The barrer is defined as follows:

:<math>\rm {1} \ barrer = {10^{-10}} \frac{ cm^3_{STP} \cdot cm}{cm^2 \cdot s \cdot cmHg} </math>

Confusingly, the centimetre notation is used in four different ways. 

* To denote an amount of substance, the 'cm<sup>3</sup><sub>STP</sub>' is [[standard cubic foot|standard cubic centimeter]], which is a unit of [[amount of substance]] rather than a unit of [[volume]]. It represents the number of gas molecules or moles that ''would'' occupy one cubic centimeter at [[standard temperature and pressure]], as calculated via the [[ideal gas law]].
* To denote a pressure differential, the notation 'cmHg' is used; a 'centimetre of mercury', which is ten times the more familiar '[[millimetre of mercury]]'.
* And finally, the centimetre and square centimetre are used in the normal way to measure thickness and area.

The [[centimeter|cm]] corresponds in the permeability equations to the thickness of the material whose permeability is being evaluated, the cm<sup>3</sup><sub>STP</sub>cm<sup>−2</sup>s<sup>−1</sup> to the [[flux]] of gas through the material, and the cmHg to the pressure drop across the material. That is, it measures the rate of fluid flow passing through an area of material with a thickness driven by a given pressure. See [[Darcy's law|Darcy's Law]].

In [[SI]] units, the barrer can be expressed as: 
:<math>\rm {1} \ barrer = {3.35 \times 10^{-16}} \frac{ mol \cdot m}{m^2 \cdot s \cdot Pa} </math>

To convert to CGS permeability unit, one must use the following:<ref>{{Cite journal|last=Stern|first=S. A.|date=1968-11-01|title=The "barrer" permeability unit|journal=Journal of Polymer Science Part A-2: Polymer Physics|language=en|volume=6|issue=11|pages=1933–1934|doi=10.1002/pol.1968.160061108|bibcode=1968JPoSB...6.1933S |issn=1542-9377|doi-access=}}</ref>

:<math>\rm {1} \ barrer = {M \ \times} \ {3.35 \times 10^{-13}} \ \frac{ g \cdot cm}{s \cdot cm^2 \cdot bar}</math>

Where M is the molecular weight of the penetrant gas (g/mol).

Another commonly expressed unit is '''Gas Permeance Unit''' ('''GPU'''). It is used in the measurement of gas [[permeance]]. Permeance can be expressed as the ratio of the permeability with the thickness of membrane.<ref>{{Cite journal|date=2014-05-01|title=CO2-selective membranes for hydrogen production and CO2 capture – Part I: Membrane development|journal=Journal of Membrane Science|language=en|volume=457|pages=149–161|doi=10.1016/j.memsci.2014.01.020|issn=0376-7388|last1=Lin|first1=Haiqing|last2=He|first2=Zhenjie|last3=Sun|first3=Zhen|last4=Vu|first4=Jimmy|last5=Ng|first5=Alvin|last6=Mohammed|first6=Moyeen|last7=Kniep|first7=Jay|last8=Merkel|first8=Timothy C.|last9=Wu|first9=Tony|last10=Lambrecht|first10=Robert C.}}</ref>

:<math>\rm {1} \ GPU = 10^{-6} \ \frac{cm^3_{STP}}{cm^2 \cdot s \cdot cmHg}
         =7.501\ \times \ 10^{-12} \ \frac{m^3_{STP}}{m^2 \cdot s \cdot Pa}</math>

Or in SI units:
:<math>\rm {1} \ GPU = 3.35\ \times \ 10^{-10} \ \frac{mol}{m^2 \cdot s \cdot Pa}</math>

== References ==
<references />

[[Category:Units of measurement]]


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