Vapor permeability test
Introduction
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance. It is a measure of the permeability of vapor barriers.
There are many industries where humidity control is critical. Moisture sensitive foods and pharmaceutical products are placed in MVTR controlled packaging to achieve the required quality, safety and shelf life. In clothing, MVTR as a measure of breathability has contributed to greater comfort for outdoor clothing wearers. The building materials industry also manages moisture barrier properties in architectural components to ensure correct humidity levels in the internal spaces of buildings. Optoelectronic devices based on organic material, generally called OLED, require encapsulation with low WVTR values to guarantee the same performance throughout the useful life of the device.
Measurement
There are several techniques for measuring MVTR, ranging from gravimetric techniques "Gravimetry (Geophysics)") that measure moisture gain or loss by mass, to highly sophisticated instrumental techniques that in some designs can measure extremely low transmission rates. Special care must be taken when measuring porous substances such as fabrics, as some techniques are not appropriate. For very low levels, many techniques do not have adequate resolution. Numerous standard methods are described in ISO, ASTM, BS"), DIN, etc., often industry-specific. Instrument manufacturers are often able to provide test methods developed to fully exploit the specific design they are selling. The search for the most suitable instrument is a painstaking task that is itself part of measurement.[1].
The conditions under which the measurement is carried out have a considerable influence on the result. Both temperature and humidity gradients across the sample must be measured, monitored and recorded with the result. An MVTR result without specifying these conditions is almost meaningless. Certainly two results should not be compared unless the conditions are known. The most common international unit for MVTR is g/m²/day. In the US, g/100in²/day is also in use, which is 0.064516 (approximately 1/15) of the value of g/m/day units. Typical rates in aluminum laminates can be as low as 0.001 g/m²/day, while the rate in fabrics can measure up to several thousand g/m²/day.