WVTR Water Vapor Transmission Rate

WVTR (Water Vapor Transmission Rate) measures the transmission of water vapor through a material. WVTR is measured in either grams/100 in²/24 hours or grams/m during 24 hours (according to the standard ASTM - E398).

A critical function of flexible packaging is to keep dry products dry and moist products moist. Without protective packaging, products will quickly gain or lose moisture until they are at equilibrium with the environmental relative humidity.

WVTR is the standard measurement by which films are compared for their ability to resist moisture transmission. Lower values indicate better moisture protection. Only values reported at the same temperature and humidity can be compared, because transmission rates are affected by both of these parameters.

Testing material for WVTR properties requires that the test specimen be held such that it separates two sides of a test chamber. The "wet side" of the specimen is exposed to a high relative humidity atmosphere, while the "dry side" is subjected to a zero relative humidity atmosphere. Infrared sensors on the "dry side" detect the amount of water vapor present. Testing is complete when the concentration of water vapor in the dry side atmosphere is constant.

 

WVTR/OTR Test