Barrer

Barrer (after Richard Maling Barrer ) is a unit in the CGS system of units ( SI unit no ) for the gas permeability of fabrics. The unit is, inter alia, used in the description of the properties of membranes and sealing materials.

A similar unit which describes the permeability of porous materials for liquids, is the Darcy.

Definition

Notwithstanding the permeability ( SI unit m ) the permeability is defined in terms of Barrer as:

With

• The dynamic viscosity ( SI unit )
• The flow rate ( rate of permeation ) through the material relative to the volume at standard conditions, and, therefore, given in cm3 (STP) / s
• The thickness of the material in cm
• Of perfused area in cm ²
• The pressure difference in cmHg.

The Barrer is defined as:

Conversion to SI units:

Auxiliary calculation: the flow rate can be represented via the ideal gas law in mol / s (see molar volume ):

With

• P - pressure
• V - volume
• N - amount of substance
• Rm - or universal molar gas constant
• T - absolute temperature
• T - time.

This results in:

Permeation rate

The rate of gas permeation follows the direction of the partial pressure:

It increases linearly with the pressure and with the Durchdringungsquerschnitt, it decreases linearly with the length of the Permeationsweges and behaves like a molecular flow.

Permeation coefficient

The leak detection you are instead of the permeation rate of their product with the pressure difference, so the power loss

The permeation coefficient defines the permeation of a combination gas to Material:

With

• P - power loss in (W = Watt)
• X - length of Permeationspfades in cm
• A - permeation cross in cm2
• - Partial pressure in bar.

The permeation coefficient is, for example for

• Helium through Teflon:
• Hydrogen by Teflon:
• Helium through Pyrex glass.

Solving for the power dissipation results in:

Thus, for example, the power loss of helium through a Teflon membrane having a thickness and an area at a pressure differential: