Water Vapor Transmission Rate (WVTR)
Definition - What does Water Vapor Transmission Rate (WVTR) mean?
The water vapor transmission rate (WVTR) measures the passage of water vapor through a substance of a given unit area and unit time. Controlling the water vapor transmission rate is important because varying working temperatures may lead to condensation and the formation of moisture, which can cause corrosion.
Water vapor transmission rate is also known as moisture vapor transmission rate (MVTR).
Corrosionpedia explains Water Vapor Transmission Rate (WVTR)
Controlling moisture is important in many industries. The food and pharmaceutical industries put moisture control practices into their packaging by reducing the water vapor transmissin rate (WVTR) to achieve the required quality of the product. Similarly, the manufacturers of building materials such as cement, bricks, clay and sand also manage the WVTR of architectural components to ensure the correct moisture levels are maintained.
There are various tools to measure WVTR, such as gravimetric techniques that are used to check for an increase or decrease in moisture content by mass. There are also very sophisticated instruments that in some designs can measure extremely low transmission rates.
Special care must be taken to control the moisture content of porous substances. There are several industry specific methods and ways to measure the moisture content, which are described in ISO, DIN and ASTM industrial standards. Instrument manufacturers frequently provide test methods for their customers.
The moisture vapor transmission rate is a key measurement unit used to determine the degree to which a film layer can resist moisture infiltration. This is of particular importance when selecting a coating or lining for corrosion prevention on a metallic surface. A coating with a high moisture vapor transmission rate will perform poorly as a preventive layer because it allows moisture to seep through and react with the metallic surface, ultimately causing corrosion, degradation and weakening of the surface. Protective substances with a low moisture vapor transmission rate are advisable in preventative applications.
The WVTR can be determined by the formula:
Amount of H2O lost
Water Vapor Transmission Rate = -----------------------------
(Time X Area)