ISO 9932 - Paper and board - Determination of water vapour transmission rate of sheet materials - Dynamic sweep and static gas methods

The ISO9932 standard covers two general test methods for measuring the water vapour transmission.

By using either a dynamic gas or static gas technique.  It is designed for materials up to 38 mm thick and water vapour transmission (Vapour permeability) rates between 0.05 g m-2 d-1 to 65 g m-2 d-1
Water Vapour Transmission Rate (WVTR), which is also known as moisture vapour transmission rate (MVTR), is the rate of a steady-state water vapour flow per unit time through a unit area of a body under specified temperature and partial pressure conditions.  It is a measure of the permeability of a material and is widely expressed in many types of units (see Versaperm Conversion Units).

As with all permeability measurement systems temperature and the relative humidity (or partial pressure) are critical and should be recorded.  Permeability can vary greatly with respect to these two properties.

Method 1 - Dynamic sweep

The first measurement method is the Dynamic sweep gas method where the test piece is mounted between two chambers.  The first chamber is kept at a constant relative humidity while the other is swept through to a sensor by a dry carrier gas (such as desiccated air or dry nitrogen). The water vapour that permeates through the barrier is measured by a sensor and the Water Vapour Permeability is calculated from the result.

Method 2 - Static gas method

In the second method the barrier material is mounted in a cell that also contains an electrolytic element and the entire cell is then placed into an humidity cabinet. The water vapour that permeates through to the cell is electrolyzed and the relative humidity in the cell will stay very low (< 1 %).  Once a steady state (equilibrium) is reached the water vapour transmission rate can be simply calculated from by measuring the electric current.