Why Desiccant Packaging Matters: MVTR and Measuring Moisture
Moisture Vapor Transmission Rate MVTR
MVTR is a measure of the passage of gaseous H2O through a barrier. It is also known as W.V.T.R. or "Water Vapor Transmission Rate".
How is moisture resistance measured?
Moisture resistance for a material or a type of sack for packaging is measured in a special chamber, divided vertically by the substrate/barrier material. A dry atmosphere is in one chamber, and a moist atmosphere is in the other. A 24-hour test is run to see how much moisture passes through the substrate/barrier from the "wet" chamber to the "dry" chamber. Standard test procedures (TAPPI T-464, ASTM E96) can specify any one of five combinations of temperature and humidity in the "wet" chamber. Tough test conditions are 100°F / 90%RH (Relative Humidity). MVTR results reported without reference to the test conditions are largely meaningless.
How quickly desiccant bags gain moisture when left unprotected
The graph above was developed for an industry standard 100 grams of Molecular Sieve in an environment of 70% relative humidity and a temperature of 70° F.