Water Vapor Permeability Coefficient Testing by the Gravimetric (Dish) Method

Water vapor permeability coefficient testing by the gravimetric method is the industry-standard technique for quantifying the moisture barrier performance of packaging films, laminates, papers, and membranes. This article explains the gravimetric (dish) method principle, applicable standards such as ASTM E96/E96M, ISO 2528, and GB/T 1037, typical sample types, test conditions, calculation formulas, and applications across the packaging, pharmaceutical, and construction industries.

Technical Overview

The gravimetric method, also known as the dish method or cup method, is one of the most widely used and long-established techniques for measuring the water vapor transmission properties of flexible and semi-rigid barrier materials. The method is standardized internationally in ASTM E96/E96M, ISO 2528, GB/T 1037, DIN 53122-1, and JIS Z 0208, making it a reference technique for quality control, material qualification, and research.

The principle of the test is straightforward: a flat specimen is sealed across the mouth of a shallow, impermeable test dish containing either a desiccant (dry cup) or distilled water (wet cup). The sealed assembly is placed in a chamber maintained at constant temperature and relative humidity, which establishes a stable water vapor partial pressure difference across the specimen. Water vapor migrates through the specimen under this gradient, causing the dish assembly to gain mass (dry cup) or lose mass (wet cup). The dish is weighed periodically, and once the mass change becomes linear with time, the slope of the curve is used to calculate the water vapor transmission rate (WVTR).

For homogeneous materials, the water vapor permeability coefficient is derived by normalizing the WVTR with the specimen thickness and the water vapor partial pressure difference across the specimen. Common reporting units include g/m²·24h for transmission rate, g/m²·s·Pa for permeance, and g·cm/(cm²·s·Pa) for the permeability coefficient.

Testing Samples

Plastic Films and Sheets

Monolayer and coextruded films such as PE, PP, BOPP, PET, PVC, PA (nylon), CPP, and CPE are routinely tested. These materials form the basis of flexible packaging and are frequently evaluated at multiple thicknesses to establish structure-property relationships.

Composite and Multilayer Structures

Laminated structures such as PET/AL/PE, BOPP/VMPET/PE, PET/PA/AL/CPP, and paper/aluminum/plastic laminates are common test articles. These structures combine multiple functional layers, and the gravimetric method measures the combined barrier performance of the finished laminate rather than individual layers.

Paper, Paperboard, and Coated Barrier Papers

Waxed or polymer-coated papers, greaseproof papers, and barrier-coated paperboards used for food contact, dry goods packaging, and medical packaging can be characterized using the dish method, with TAPPI T464 and ISO 2528 providing paper-specific procedures.

Rubber, Elastomers, and Coatings

Free films and sheets of rubber, thermoplastic elastomers, paints, and functional coatings can be tested as flat specimens. For coatings, a free-standing film is typically prepared before testing so that the coating itself is characterized independent of the substrate.

Textiles, Nonwovens, and Building Membranes

Waterproof-breathable membranes, geotextiles, house wraps, roofing underlays, and vapor retarder membranes used in construction are evaluated to balance moisture management with vapor tightness. Highly permeable materials are well suited to the gravimetric approach because of its wide measurement range.

Testing Items

The gravimetric method can deliver the following results, reported under clearly defined temperature and humidity conditions:

• Water Vapor Transmission Rate (WVTR), typically in g/m²·24h or g/m²·day
• Water Vapor Permeance, in g/m²·s·Pa, used for multilayer or heterogeneous structures where thickness normalization is not meaningful
• Water Vapor Permeability Coefficient (P), in g·cm/(cm²·s·Pa), applicable to homogeneous materials and normalized by thickness and vapor pressure differential
• Comparative barrier ranking between dry cup and wet cup conditions, which reveals humidity-dependent permeability behavior
• Effect of temperature, relative humidity, and specimen thickness on moisture barrier performance

Testing Methods

Dry Cup (Desiccant) Method

In the dry cup configuration, the dish is filled with anhydrous calcium chloride, silica gel, or molecular sieve desiccant, and the specimen is sealed over the opening. The assembly is placed in a controlled-humidity chamber, typically at 38°C and 90% RH or 23°C and 50% RH. Water vapor from the chamber atmosphere diffuses through the specimen and is absorbed by the desiccant, so the dish gains mass over time. This configuration is preferred for materials that must limit moisture ingress from a humid external environment into a dry interior, such as dry food and pharmaceutical packaging.

Wet Cup (Water) Method

In the wet cup configuration, the dish contains distilled water covered by the sealed specimen. The chamber is maintained at a lower relative humidity than the saturated condition inside the dish, driving water vapor outward. The dish loses mass as vapor permeates through the specimen. This configuration simulates packaging that contains moisture-rich products or experiences direct liquid contact. An inverted wet cup variant, in which the water layer directly contacts the specimen, is used to assess performance under hydrostatic or condensed-water conditions.

Test Conditions

Condition selection should reflect the intended service environment of the material. Commonly specified combinations include:

• 38°C / 90% RH — the classic tropical condition widely used in GB/T 1037 and ASTM E96 evaluations
• 23°C / 50% RH — ambient temperate condition for general comparison
• 40°C / 75% RH or 40°C / 90% RH — accelerated conditions for shelf-life studies
• 37°C / 90% RH — conditions relevant to medical and healthcare product packaging

Procedure and Calculation

Specimens are first conditioned to equilibrium, typically at 23°C and 50% RH, cut to the required diameter, and sealed to the dish with molten wax, a gasket, or a mechanical clamp to eliminate edge leakage. The assembly is weighed at regular intervals, commonly every 24 hours, using a balance with suitable resolution. Weighing continues until at least three consecutive mass changes agree within the reproducibility limit defined by the standard, confirming that steady-state transmission has been reached.

The transmission rate is calculated from the steady-state slope:

WVTR = Δm / (A × t), where Δm is the mass change in grams, A is the exposed test area in square meters, and t is the elapsed time in days.

The permeability coefficient is then obtained by normalizing for thickness and driving force:

P = (WVTR × d) / Δp, where d is the specimen thickness and Δp is the water vapor partial pressure difference across the specimen, equal to the saturation vapor pressure at the test temperature multiplied by the relative humidity difference on the two sides.

Applications

Gravimetric water vapor transmission data support critical decisions across many industries:

• Food packaging: shelf-life modeling for snacks, cereals, powdered beverages, and dry goods, and verification of barrier performance after printing, lamination, or converting
• Pharmaceutical packaging: qualification of blister films, foil laminates, and desiccant-containing systems for tablets, capsules, and moisture-sensitive formulations
• Medical devices: assessment of sterile barrier systems and humidity protection for diagnostics and electronic medical products
• Electronics: moisture barrier evaluation of protective films, encapsulation materials, and component packaging
• Construction: selection of vapor retarders, breather membranes, and roofing underlays with the appropriate vapor resistance class
• Textiles and consumer goods: verification of waterproof-breathable membranes for apparel, footwear, and outdoor equipment
• Photovoltaics: characterization of backsheet and encapsulant films for solar modules exposed to long-term humidity and heat

FAQ

What is the difference between the dry cup and wet cup methods, and which should I choose?

The dry cup method places desiccant inside the dish and measures mass gain, simulating moisture ingress into a dry package from a humid environment. The wet cup method places water inside the dish and measures mass loss, simulating moisture escaping from a wet product. Choose the configuration that best matches the real-world humidity gradient the material will experience; for materials whose barrier properties are humidity dependent, both configurations provide complementary information.

What are the typical units for reporting results?

The water vapor transmission rate is most often reported in g/m²·24h. Permeance is reported in g/m²·s·Pa, and the permeability coefficient for homogeneous materials is reported in g·cm/(cm²·s·Pa). Always state the test temperature, relative humidity, and specimen thickness alongside the result, since transmission behavior is strongly condition dependent.

How long does a gravimetric test take?

Most tests run from three days to two weeks. Highly permeable papers or films may reach steady state within a few days, whereas high-barrier laminates may require longer weighing schedules. Steady state must be confirmed by consistent linear mass change across successive weighings before the final calculation is made.

What is the practical measurement range of the gravimetric method?

The dish method performs best for transmission rates from roughly 0.5 to several thousand g/m²·24h. For ultra-high barrier materials below this range, longer test durations or sensor-based techniques such as the modulated infrared sensor method described in ASTM F1249 may offer better sensitivity and shorter cycle times.

How is the permeability coefficient different from the transmission rate?

The transmission rate describes the mass of vapor passing through a unit area per unit time under specific test conditions. The permeability coefficient additionally normalizes for specimen thickness and the vapor pressure difference, allowing intrinsic material comparison between specimens of different thicknesses, but it is only valid for homogeneous single materials, not for multilayer composites.

What precautions ensure accurate results?

Specimens must be defect-free and free of pinholes, wrinkles, or contamination. Sealing integrity is critical, since any leakage at the dish rim will inflate the result. Consistent chamber temperature and humidity control, careful handling with gloves, a calibrated balance, and replicate specimens all contribute to reliable, reproducible data. Accredited third-party laboratories routinely verify these controls as part of their quality management systems.