Blister packaging · Industry glossary
Water vapour transmission rate (WVTR)
The mass of water vapour transmitted through a unit area of material per unit time under specified conditions.
Also called: MVTR; moisture vapor transmission rate; water vapor transmission rate.
WVTR units and the term MVTR
It expresses water-vapour mass passing through each unit of specimen area per unit time under the reported conditions. WVTR and MVTR are commonly used for this moisture-transmission quantity; neither is the percentage of moisture already inside the product.
Retain the area and time basis. A value per square metre is not numerically interchangeable with one per 100 square inches or per package. Changing units is arithmetic; changing the test environment or specimen is not just a unit conversion.
Conditions for comparing WVTR results
Temperature and the humidity conditions across the specimen belong beside the result. A wet/dry chamber arrangement creates the moisture-driving difference, so a lone humidity value may not fully explain the test boundary.
A lower reported number is meaningful as a comparison only on a compatible measurement basis. Identify the method, construction, thickness and whether the figure is typical or a guaranteed specification; do not rank films measured under materially different conditions as if the numbers were equivalent.
| Record item | What to clarify |
|---|---|
| Conditions | Temperature and humidity boundary on the relevant sides. |
| Specimen | Flat film, laminate or finished package, with its construction. |
| Reporting basis | Mass, area or package basis, and time unit. |
| Claim type | Method and whether the result is typical, specified or bounded by measurement capability. |
Can WVTR be multiplied by area?
For the same flat specimen and unchanged test conditions, multiplying an area-normalised rate by exposed area gives the corresponding mass per time. That arithmetic does not include additional materials, seals, formed regions or defects in a completed blister.
WVTR, leak integrity and shelf life
No. Flat-film transmission, completed-package performance and leakage are different questions. An area-based film projection does not measure the effects of forming, seals or other parts of the finished blister.
A WVTR figure alone does not establish product stability or account for the complete package and use environment. Use the integrity page for the separate question of defects and leak paths; oxygen transfer has its own metric.
IN THE PRODUCTION LINE
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