How to Measure Water Vapor Transmission Rate of Packaging Film

How to measure water vapor transmission rate comes down to three choices: the method, the test conditions and how you report the result. Each method seals a film specimen between a humid side and a dry side and measures the water that passes through.

What WVTR tells you about a film

Water vapor transmission rate (WVTR) is the mass of water vapor that passes through a unit area of material in a unit time, under stated temperature and humidity. ISO 2528 expresses it in grams per square metre per day, g/(m²·d).

WVTR is often loosely called permeability, but the two are not the same thing. ASTM F1249 reports WVTR, permeance and, for homogeneous materials, a permeability coefficient as separate results.

For packaging, WVTR can often be related directly to shelf life and product stability.

Three ways to measure WVTR

The three method families differ in how they detect the water that passes through the film. Pick the one that matches your film's barrier level and the method your customer names.

Gravimetric cup method

ASTM E96 measures WVTR by a gravimetric procedure and gives two basic methods, the Desiccant Method and the Water Method. ISO 2528 describes the desiccant dish version for sheet materials such as paper, board, plastic films and laminates.

In ISO 2528, dishes holding desiccant are sealed with the test material and placed in a controlled atmosphere. They are weighed at intervals, and WVTR comes from the mass gain once that gain has become proportional to time.

ISO 2528 does not generally recommend its dish method for rates below 1 g/m² per day or for materials thicker than 3 mm. For those, the standard points to ISO 9932 instead.

Infrared sensor method

ASTM F1249 measures WVTR through plastic film and sheeting with a modulated infrared sensor. It applies to sheets and films up to 3 mm thick, including multilayer films, foils and coated materials.

The F1249 interlaboratory study demonstrated the method for materials from 0.052 g/(m²·day) to 26 g/(m²·day). ISO 15106-2 is the ISO method that uses an infrared detection sensor.

Electrolytic sensor method

ISO 15106-3 specifies an instrumental method for plastic film, sheeting and multilayer structures using an electrolytic detection sensor. Like the infrared part, it notes that the method gives rapid measurement over a wide range of transmission rates.

Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].

Test conditions that change the result

WVTR is not a linear function of temperature, and generally not of the humidity difference either. ISO 2528 therefore warns that a result from one set of conditions is not necessarily comparable with one from another.

ASTM E96 makes the same point. A result from one method and one set of conditions cannot be used to predict the result under other conditions or with the other method.

E96 lists commonly used conditions as Procedures A to E in an appendix, but does not make them mandatory. It advises choosing conditions close to the material's real exposure when possible.

When a test is run for classification or compliance, the conditions usually come from the code, specification or supplier literature. Agree them with your customer before the first test.

Calculating WVTR from cup weight gain

The arithmetic follows the definition: mass gain per unit area per unit time. The 12.0 mg mass gain in the example is a hypothetical value. The 79.8 mm template and the 0.1 mg balance reading come from ISO 2528.

1. Test area: a 79.8 mm template gives 50 cm², which is 0.0050 m².
2. Weigh each dish at regular intervals on a balance that reads to 0.1 mg.
3. Wait until the mass gain per interval is steady, for example a hypothetical 12.0 mg per 24 h.
4. WVTR = 0.0120 g ÷ 0.0050 m² ÷ 1 day = 2.4 g/(m²·d).

Use only the steady part of the weight gain. Early readings, before the gain becomes proportional to time, do not represent the film's transmission rate.

Our WTC-303H water vapor transmission rate tester is one of the water vapor transmission rate testers we supply.

For the other tests a flexible packaging lab usually runs, see our guide to flexible packaging testing equipment.

Reporting WVTR so results can be compared

A WVTR number on its own says little. E96 requires the test conditions to be reported with the results in all cases.

• The method used, for example ASTM E96, ASTM F1249, ISO 2528 or ISO 15106-3.
• Test temperature and the humidity on each side of the specimen.
• The result in g/(m²·d), or in the unit system the method uses.
• The test area, if it differs from the standard template size.
• Specimen thickness and whether the film was flat or creased.

ISO 2528 asks for a non-standard template diameter to be stated in the report. It also gives a separate procedure in Annex A for creased material.

F1249 data can serve as a referee method, provided buyer and seller have agreed on sampling, standardization, test conditions and acceptance criteria. Settle those points in the specification before results are compared.

Frequently asked questions

Can I convert WVTR to a different film thickness?

Only with caution. ASTM F1249 notes that the inverse relationship of WVTR to thickness, and its direct relationship to the water vapor pressure difference, may not always apply.

Is ASTM E96 limited to plastic film?

No. ASTM E96 is written for a wide range of materials, including paper, plastic films, coatings, foams, fiberboards, gypsum and plaster products, wood products and plastics. Check the scope of the method you plan to use against your own material.

Talk to our factory team

We sell direct from our factory, so you work directly with our factory team. Questions about a model or a test? Email Amy Zhao, Export Manager, at [email protected].