Moisture vapor transmission rate
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance.
There are many industries where moisture control is critical. Moisture sensitive foods and pharmaceuticals are put in packaging with controlled MVTR to achieve the required quality, safety, and shelf life. In clothing, MVTR as a measure of breathability has contributed to greater comfort for wearers of clothing for outdoor activity. The building materials industry also manages the moisture barrier properties in architectural components to ensure the correct moisture levels in the internal spaces of buildings. Optoelectronic devices based on organic material, generally named OLEDs, need an encapsulation with low values of WVTR to guarantee same performances over the device lifetime.
There are various techniques to measure MVTR, ranging from gravimetric techniques that measure the gain or loss of moisture by mass, to highly sophisticated instrumental techniques that in some designs can measure extremely low transmission rates. Note that special care has to be taken in measuring porous substances such as fabrics as some techniques are not appropriate. Likewise for very low levels, many techniques would not have the resolution to provide a reliable result. Numerous standard methods are described in ISO, ASTM, BS, DIN etc., -- these are quite often industry-specific. Instrument manufacturers will often be able to provide test methods developed to fully exploit the specific design which they are selling. The search for the most appropriate instrument is a zealous task which is in itself part of the measurement.
The conditions under which the measurement is made has a considerable influence on the result. Both the temperature of and humidity gradient across the sample need to be measured, controlled and recorded with the result. An MVTR result without specifying these conditions is almost meaningless. Certainly no two results should be compared unless the conditions are known. The most common international unit for the MVTR is g/m²/day. In the USA, g/100in²/day is also in use, which is approximately 1/15 of the value of g/m²/day units. (More precisely, the ratio is 1/15.500031, or very close to 2/31.) Typical rates in aluminium foil laminates may be as low as 0.001 g/m²/day, whereas the rate in fabrics can measure up to several thousand g/m²/day.
Often, barrier testing is conducted on a sheet of material. Calculations based on that can be useful when designing completed structures, clothing, and packages. Seams, creases, access points, and heat seals are critical to end-use performance. For example, the glass of a glass bottle may have an effective total barrier but the screw cap closure and the closure liner might not. Performance verification and validation of complete containers, structures, or irregular objects is often recommended.
For the special case of OLEDs, where the levels of allowed permeation are in the 10e-6 g/m²/day level  , the methods preferred exploit an oxidation of a metal upon the exposure to water  .
- Carbon dioxide transmission rate
- Moisture sorption isotherm
- Oxygen transmission rate
- Shelf life
- Vapor barrier
- Bell, L.N., and Labuza, T.P. 2000. "Practical Aspects of Moisture Sorption Isotherm Measurement and Use". 2nd Edition AACC Egan Press, Egan, MN
- Brody, A. L., and Marsh, K, S., "Encyclopedia of Packaging Technology", John Wiley & Sons, 1997, ISBN 0-471-06397-5
- Massey, L K, "Permeability Properties of Plastics and Elastomers", 2003, Andrew Publishing, ISBN 978-1-884207-97-6
- ASTM F1249-06 Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor
- ASTM E398-03 Standard Test Method for Water Vapor Transmission Rate of Sheet Materials Using Dynamic Relative Humidity Measurement
- ASTM D1434 - Standard Test Method for Determining Gas Permeability Characteristics of Plastic Film and Sheeting
- ASTM D3079 - Standard Test Method for Water Vapor Transmission of Flexible Heat-Sealed Packages for Dry Products
- ASTM D4279 - Standard Test Methods for Water Vapor Transmission of Shipping Containers-Constant and Cycle Methods
- ASTM E96 - Standard Test Methods for Water Vapor Transmission of Materials
- ASTM E398 - Standard Test Method for Water Vapor Transmission Rate of Sheet Materials Using Dynamic Relative Humidity Measurement
- ASTM F1249 - Standard Test Method for Water Vapor Transmission Rate Through Plastic Film and Sheeting Using a Modulated Infrared Sensor
- ASTM F2298- Standard Test Methods for Water Vapor Diffusion Resistance and Air Flow Resistance of Clothing Materials Using the Dynamic Moisture Permeation Cell
- ASTM F2622 - Standard Test Method for Oxygen Gas Transmission Rate Through Plastic Film and Sheeting Using Various Sensors
- , Water Vapor Permeation Testing of Ultra-Barriers: Limitations of Current Methods and Advancements Resulting in Increased Sensitivity, M. Stevens et Al.
- , "Institute of Materials Research and Engineering (IMRE) MEDIA RELEASE, Embargoed Release for 29 April 2008, Singapore research produces world’s best protection from moisture and oxygen"
- , Water Vapour Transmission Rate Measurements
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- , Electrical Calcium Test for Measuring Barrier Permeability, National Renewable Energy Laboratory