Water vapour transmission rate is the number behind every "breathable" claim. It is measured gravimetrically: seal a specimen over a cup, put a known driving force across it, and weigh the assembly over time. The slope of mass against time is the transmission rate.
Simple in principle. The two places it goes wrong are the choice of method and the seal.
Dry cup or wet cup
The cup contains either a desiccant or water. That single choice sets the direction of vapour flow and the humidity the specimen actually sees.
| Dry cup (desiccant) | Wet cup (water) | |
|---|---|---|
| In the cup | Desiccant, near 0 % RH | Distilled water, near 100 % RH |
| Vapour flows | From chamber into cup | From cup into chamber |
| Mass change | Cup gains | Cup loses |
| Specimen sees | Low humidity on one face | High humidity on one face |
For hydrophilic membranes and coatings this matters a great deal. Materials that transport moisture by absorption and diffusion — rather than through open pores — become considerably more permeable at high humidity. A hydrophilic film can look mediocre in a dry cup test and excellent in a wet cup test.
Neither result is wrong. They describe behaviour at different humidity levels. Report which one you ran, and never compare across the two.
The seal is the whole test
Any vapour that escapes around the specimen edge, rather than through it, is counted as if it passed through the material. There is no way to detect this from the data — the line still looks straight.
Three practices control it:
- A defined test area. The cup has a specified clear opening, and the reported rate is per unit of that area. If a gasket overlaps into the opening, the true area is smaller than the assumed one and the result is understated.
- A sealing method appropriate to the specimen. Coated and film materials seal well against a gasket. Open fabrics may need a mounting method that does not let vapour track along yarns.
- Blank cups. Run sealed cups with no specimen, or with an impermeable foil, alongside the real ones. Their mass should not change. If it does, the seal — or the balance environment — is the problem, not the material.
Blank cups are the cheapest quality control in this test and the most frequently skipped. On a multi-position instrument, dedicate two positions to blanks permanently.
Weighing is where precision is won or lost
The mass changes involved are small, and the balance resolution sets the shortest usable test.
Two things degrade weighings in practice. First, handling — carrying a cup from a conditioned chamber to a balance on another bench exposes it to a different atmosphere and to the operator's hands. Second, timing — an irregular weighing schedule produces a scattered slope even when the material is perfectly uniform.
This is why automated systems weigh in place, inside the controlled environment, at programmed intervals. It removes both variables at once. Where an external balance is used, it belongs on a vibration-damping table immediately beside the chamber, and the transfer routine must be identical every time.
Reading the curve
WVTR is the slope of mass against time, not a difference between two weighings.
Plot every point and fit a straight line. The first hours are usually curved while the assembly reaches steady state — discard that region and fit only the linear portion. Report the coefficient of determination alongside the rate; a poor fit is a signal to investigate before the number leaves the lab.
A curve that bends late in the test usually means the desiccant is saturating or the water level has dropped enough to change the geometry. Both mean the test ran too long.
Conditions belong on the report
A WVTR figure with no conditions attached is not usable by anyone. The minimum is:
Method (dry cup / wet cup), temperature, chamber relative humidity, test area, duration, air velocity
Air velocity is the one most often omitted. Still air above the specimen builds a boundary layer that resists vapour transfer, and the measured rate drops. Controlled, repeatable airflow across the specimen face is part of the method, not a refinement.
Related but different
- ISO 11092 measures water vapour resistance on a sweating guarded hotplate. It answers a comfort question, not a barrier question, and produces a resistance rather than a rate.
- EN ISO 12572 applies the cup principle to building materials, with its own cup geometry and conditions.
They are all cup or plate methods with the same physics, and none of their numbers convert into each other.