Water Vapor Transmission Testing of Packaging Materials by the Gravimetric (Cup) Method

The water vapor transmission rate (WVTR) of packaging materials determines how effectively they protect foods, pharmaceuticals, and electronics from moisture during storage and distribution. The gravimetric method—also known as the cup method or dish method—is the internationally recognized reference technique for quantifying moisture barrier performance in films, sheets, laminates, and finished packages. This article explains the test principle, applicable standards, sample requirements, procedures, calculations, and typical applications of gravimetric WVTR testing.

Technical Overview

Moisture is one of the most destructive agents affecting packaged goods. It softens crispy snacks, causes powdered products to clump, degrades active pharmaceutical ingredients, and corrodes electronic components. The ability of a packaging material to resist the passage of water vapor is therefore a fundamental quality characteristic, quantified as the water vapor transmission rate (WVTR), typically expressed in grams per square meter per 24 hours (g/m²·24h).

The gravimetric method is the oldest and most widely accepted approach for measuring WVTR. In this technique, a test specimen is sealed across the mouth of a shallow dish or cup containing either a desiccant (dry cup method) or distilled water (wet cup method). The assembled test cup is placed in a controlled temperature and humidity environment. Water vapor migrates through the specimen driven by the partial pressure difference across it, causing the cup to gain mass (dry cup) or lose mass (wet cup). By periodically weighing the cup and plotting mass change against time, the steady-state transmission rate is calculated from the slope of the linear portion of the curve.

Because the measurement is based directly on mass change, the gravimetric method serves as the primary reference method against which instrumental techniques such as infrared sensor, electrolytic sensor, and dynamic relative humidity methods are calibrated. The method is governed by several international and national standards, including ASTM E96/E96M, ISO 2528, JIS Z 0208, DIN 53122-1, and GB/T 1037, ensuring consistency of results across accredited third-party laboratories worldwide.

Testing Samples

Plastic Films and Sheets

Monolayer and multilayer flexible films are the most frequently tested sample category. Common materials include polyethylene (PE), polypropylene (PP), polyester (PET), polyamide (PA), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC)-coated films, and coextruded structures. Film thickness typically ranges from 20 µm to 200 µm, and specimens must be flat, crease-free, and free from pinholes.

Laminated and Metallized Structures

High-barrier laminates such as PET/AL/PE, BOPP/VMPET/PE, paper/AL/PE, and foil-containing cold-form laminates for pharmaceutical blisters are routinely evaluated. For laminates incorporating aluminum foil, gravimetric testing verifies foil integrity and detects micro-leaks at fold lines or seal areas, where transmission may be dominated by defects rather than permeation through the bulk material.

Paper, Paperboard, and Coated Substrates

Packaging papers, greaseproof paper, wax-coated paper, cellophane, and coated or printed papers used for pouches, sachets, and carton liners can be assessed using the gravimetric approach. Because paper is hygroscopic, specimens require careful conditioning before testing to stabilize the moisture content of the substrate itself.

Rubber, Textile, and Other Sheet Materials

Rubber sheeting, coated fabrics, barrier membranes, and other sheet materials specified in ISO 2528 and DIN 53122-1 may also be tested, provided the material can be securely sealed to the test dish without damage.

Packaging Containers

Complete packages such as bottles, cans, jars, pouches, and sealed barrier bags can be evaluated by gravimetric container methods, in which desiccant or water is sealed inside the package and the entire package is weighed periodically. ASTM D3079 and related container-level standards apply to this configuration.

Testing Items

The core deliverable of the gravimetric test is the water vapor transmission rate, but the test program may include the following related items:

• Water vapor transmission rate (WVTR), expressed in g/m²·24h or g/m²·h
• Water vapor permeance, expressed in g/m²·24h under specified conditions, normalized for the driving vapor pressure difference when required by the client or standard
• Water vapor permeability coefficient, derived for homogeneous materials when thickness is taken into account
• Moisture barrier classification of the material under specified climatic conditions
• Comparison of barrier performance before and after conditioning, printing, lamination, or simulated transport stress
• Verification of seal and fold integrity in foil-based laminates and finished packages

Testing Methods

Test Principle

A circular specimen is sealed over the mouth of a test dish with molten sealing wax or a mechanical clamping ring, defining a precise transmission area. Inside the dish, either anhydrous desiccant maintains approximately 0% relative humidity (dry cup) or distilled water maintains 100% relative humidity (wet cup). The assembly is exposed to a controlled atmosphere in an environmental chamber. Water vapor flows through the specimen until a steady state is reached, and the resulting mass change of the assembly is recorded by periodic weighing on an analytical balance.

Dry Cup Method (Desiccant Method)

In the dry cup configuration, the dish contains a conditioned desiccant such as anhydrous calcium chloride or molecular sieve. Water vapor from the chamber atmosphere permeates the specimen and is absorbed by the desiccant, so the cup mass increases. This configuration simulates a moisture-sensitive product protected by the package in a humid environment and is preferred for moderate and high barrier materials. Care must be taken to ensure the desiccant does not become saturated during the test, which would invalidate the constant driving force.

Wet Cup Method (Water Method)

In the wet cup configuration, the dish contains distilled water, leaving a defined air gap below the specimen. Water vapor passes from inside the cup outward into the chamber atmosphere, and the cup mass decreases. The wet cup method is suitable for materials with higher transmission rates and for conditions simulating water-rich contents such as liquids or freshly baked goods. An inverted wet cup variant may be used for certain specifications where the water contacts the specimen directly.

Test Conditions

Standard climatic conditions are selected to represent the intended distribution environment:

• 38 °C and 90% RH — tropical conditions, widely used for high-barrier food and pharmaceutical laminates
• 23 °C and 85% or 90% RH — ambient high-humidity conditions
• 23 °C and 50% RH — standard laboratory atmosphere
• 40 °C and 90% RH — accelerated conditions for high-barrier structures

Temperature and humidity stability within the chamber are critical, because the driving force for vapor transfer depends directly on the water vapor partial pressure difference across the specimen.

Specimen Preparation and Sealing

Specimens are conditioned at the test environment or at 23 °C/50% RH prior to assembly, unless otherwise specified. Each specimen must be free of wrinkles, pinholes, and surface contamination. Sealing is achieved either with a molten wax blend applied to a masking template to ensure a reproducible exposed area, or with gasketed clamping flanges. Leak-tightness of the seal is essential, since leakage at the edge will overstate the transmission rate. Multiple specimens, typically three to five per material, are tested to characterize variability.

Weighing and Data Collection

After assembly, cups are placed in the environmental chamber and weighed at regular intervals, for example every 24 hours, on a balance with a resolution of 0.1 mg to 1 mg depending on the expected transmission level. The first data points are discarded if necessary to exclude the transient period before steady state. The mass change is plotted against elapsed time, and the slope of the straight-line portion is determined by linear regression.

Calculation

The water vapor transmission rate is calculated from the steady-state slope of the mass-versus-time curve:

WVTR = (Δm / Δt) / A, converted to g/m²·24h, where Δm/Δt is the slope of mass change in grams per hour and A is the exposed test area in square meters. For container tests, the result may be expressed as grams per package per 24 hours. Test reports state the method used (dry or wet cup), temperature, humidity, specimen thickness, mean value, and standard deviation of replicate specimens.

Reference Standards

• ASTM E96/E96M — Standard Test Methods for Water Vapor Transmission of Materials
• ISO 2528 — Sheet materials: Determination of water vapour transmission rate, gravimetric (dish) method
• GB/T 1037 — Determination of water vapour transmission rate of plastic film and sheeting by the cup method
• JIS Z 0208 — Testing methods for determining water vapour transmission rate of moisture-proof packaging materials (dish method)
• DIN 53122-1 — Determination of water vapour transmission rate of plastic film, rubber sheeting, paper, board and other sheet materials by gravimetry
• ASTM D3079 — Water vapor transmission of flexible packaging by the double-chamber gravimetric method for complete packages

Applications

Gravimetric WVTR testing supports decision-making across the entire packaging value chain:

• Food packaging: qualification of barrier films for snacks, bakery products, powdered beverages, dried foods, coffee, and condiments, where crispness and caking are moisture-sensitive quality attributes
• Pharmaceutical packaging: verification of high-barrier laminates for tablets, capsules, effervescent products, and effervescent or hygroscopic dosage forms, supporting stability studies and shelf-life justification
• Electronics packaging: evaluation of moisture barrier bags and static-shielding laminates used to protect semiconductors and PCB assemblies from humidity-induced damage
• Materials development: comparison of resin grades, coating formulations, metallization quality, and lamination structures during research and development
• Incoming inspection and quality control: routine verification of film lots against purchase specifications and barrier class requirements
• Compliance and certification: generation of test data for regulatory submissions, customer audits, and accreditation-based reporting from accredited third-party laboratories
• Shelf-life modeling: use of measured WVTR values together with product sorption isotherms to predict moisture uptake over the intended distribution life of the packaged good

FAQ

What is the difference between the dry cup and wet cup methods?

The dry cup method uses desiccant inside the dish and measures mass gain as chamber humidity permeates the specimen, while the wet cup method uses water inside the dish and measures mass loss. The two configurations produce different relative humidity gradients across the specimen, which can yield different results for moisture-sensitive polymers. Standard practice is to select the configuration that best simulates the actual package contents and use environment, and to report the configuration with the result.

How does the gravimetric method compare with instrumental sensor methods?

The gravimetric method is a direct, primary measurement based on mass change and requires no sensor calibration, making it the reference method for most standards. Instrumental methods using infrared or electrolytic sensors offer faster results and better sensitivity for very high-barrier materials but rely on calibration against known references. Agreement between the two approaches is generally good when test conditions are matched, and gravimetric data are frequently used to verify instrumental readings.

How long does a gravimetric WVTR test take?

A typical test runs from several days to about two weeks. The duration depends on the barrier level of the material: high-transmission films reach steady state within one to three days, while high-barrier foil laminates may require extended weighing schedules to accumulate a measurable mass change. Minimum test duration requirements in the standards ensure that at least the specified number of data points falls within the steady-state linear region.

What sample size and quantity are required?

A circular specimen of approximately 60 to 100 mm in diameter is usually sufficient to seal over standard dish apertures, with the exposed test area defined by the dish opening, commonly around 25 to 50 cm². Standards typically require a minimum of three replicate specimens per material to characterize variability, and additional material should be supplied to allow resealing or retesting if a seal leak or specimen defect is detected.

What factors most commonly cause inaccurate results?

The dominant error sources are leaks at the specimen-to-dish seal, desiccant saturation, unstable chamber temperature and humidity, inadequate specimen conditioning, insufficient balance resolution for high-barrier materials, and edge effects around the sealed perimeter. Meticulous sealing technique, verified chamber control, and compliance with weighing interval requirements are essential for reliable, reproducible results.

Can finished packages be tested gravimetrically?

Yes. Container-level gravimetric methods seal desiccant or water inside the actual package, weigh the whole package periodically, and report moisture gain or loss per package per 24 hours. This approach captures the combined effect of material permeation, seal quality, and closure performance, making it valuable for validating the total moisture protection delivered by the finished pack.