The differential pressure method gas permeability test is a foundational laboratory technique for measuring how quickly gases such as oxygen, carbon dioxide, and nitrogen permeate through plastic films, laminates, sheets, and elastomeric materials. Performed in accordance with ASTM D1434, ISO 15105-1, and GB/T 1038, this manometric approach delivers accurate gas transmission rate (GTR), permeance, and permeability coefficient data that support packaging design, barrier material development, and quality control.
Technical Overview
The differential pressure method, also known as the manometric or pressure-sensor method, is one of the two principal techniques for determining the gas barrier properties of flexible materials, the other being the equal pressure (coulometric sensor) method. In the differential pressure configuration, the specimen is clamped as a hermetic barrier between an upper high-pressure chamber and a lower low-pressure chamber. Dry test gas is introduced into the upper chamber at approximately atmospheric pressure while the lower chamber is evacuated, creating a controlled pressure difference across the specimen.
Gas molecules dissolve into the upstream surface, diffuse through the polymer matrix, and desorb on the downstream side, causing a gradual pressure rise in the evacuated chamber. A high-resolution pressure transducer records this pressure increase over time, and the steady-state slope of the pressure-time curve is converted into quantitative permeation parameters. Because the driving force is a total-pressure gradient rather than a concentration gradient maintained by a carrier gas, the method is not restricted to a specific electrochemical sensor or to oxygen alone.
- No gas-specific sensor is required: virtually any dry, pure test gas (oxygen, nitrogen, carbon dioxide, air, inert gases) can be measured on a single instrument
- Wide measurement range covering ultra-high-barrier laminates through to highly permeable porous substrates
- Direct determination of permeability, diffusion, and solubility coefficients through time-lag analysis
- Excellent repeatability when temperature, leak rate, and sealing quality are tightly controlled
- Considerations: the specimen is exposed to vacuum, which precludes liquids and may dry hydrophilic films, and the pressure differential can slightly deform very soft or extremely thin unsupported materials
Testing Samples
Monolayer Plastic Films and Sheets
Common materials include polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), polyamide (nylon, PA), polyvinyl chloride (PVC), polystyrene (PS), polyvinylidene chloride (PVDC), ethylene-vinyl alcohol copolymer (EVOH), and polylactic acid (PLA), typically ranging from about 10 µm to 3 mm in thickness. Specimens are die-cut into circular discs free of pinholes, wrinkles, and surface contamination.
Multilayer, Laminated, and Metallized Structures
Barrier laminates for flexible packaging, aluminum foil laminates, metallized films such as VMPET, and inorganic oxide coated films (SiOx, AlOx) can all be evaluated. Because the specimen is tested as a complete system, the method is well suited to verifying the overall barrier performance of a composite structure rather than that of individual layers.
Rubber and Elastomeric Sheets
Butyl rubber innerliners, gaskets, seals, gloves, and tubing are tested per ISO 2782-1. Thicker or mechanically weak elastomeric specimens are often supported by perforated grids to prevent deformation under the pressure differential.
Coated Papers, Foils, and Specialty Substrates
Typical substrates include coated papers and paperboards, aluminum-plastic laminated pouch film for lithium-ion cells, photovoltaic backsheets, and transparent high-barrier encapsulation films used in flexible electronics.
Testing Items
A differential pressure permeability experiment typically reports the following parameters, each characterizing a different aspect of gas transport through the material.
- Gas Transmission Rate (GTR): the volume of gas permeating through unit area per unit time under a stated partial pressure difference, commonly expressed in cm³/(m²·24h·0.1MPa)
- Gas Permeance: the transmission rate normalized to the applied pressure difference, expressed in cm³/(m²·24h·MPa), useful for characterizing a structure of defined thickness
- Gas Permeability Coefficient (P): an intrinsic material property independent of thickness, expressed in cm³·cm/(cm²·s·Pa)
- Diffusion Coefficient (D): the rate at which gas molecules move through the polymer matrix, in cm²/s, derived from time-lag analysis
- Solubility Coefficient (S): the amount of gas dissolved per unit volume of material at equilibrium, in cm³/(cm³·Pa), calculated as S = P/D
- Comparative barrier rankings for oxygen, carbon dioxide, nitrogen, and air at one or more test temperatures
Testing Methods
Specimen Preparation and Conditioning
Specimens are conditioned in a controlled environment, typically 23 ± 2 °C and 50% relative humidity, or under dry conditions where the standard requires it. Thickness is measured at multiple points across the test area, and a thin, even layer of vacuum grease is applied to the sealing surfaces to guarantee leak-tight clamping between the two polished metal chambers.
Vacuum Differential Pressure Procedure
- Clamp the conditioned specimen between the high-pressure and low-pressure chambers and verify a valid seal
- Evacuate both chambers to a low residual pressure to remove ambient air and adsorbed gases, and confirm that the system leak rate is within the acceptance limit
- Charge the upper chamber with the dry test gas at approximately atmospheric pressure while keeping the lower chamber isolated under vacuum
- Monitor the pressure rise in the low-pressure chamber with the manometric sensor, recording data through the transient region until a steady-state linear slope is established
- Apply blank and leak corrections before calculating the final results
Data Processing and Calculation
In the steady-state region, the slope of the pressure-time curve (dp/dt) is used to compute the gas transmission rate as GTR = (dp/dt) × V / (A × p0), where V is the calibrated volume of the downstream chamber, A is the exposed test area, and p0 is the upstream gas pressure, with temperature correction to standard conditions. The permeability coefficient is then obtained from P = GTR × d / Δp, where d is the mean specimen thickness and Δp is the pressure difference across the specimen. The intercept of the extrapolated steady-state line on the time axis yields the time lag θ, from which the diffusion coefficient D = d²/(6θ) and the solubility coefficient S = P/D are calculated.
Reference Standards
- ASTM D1434 — Determining gas permeability characteristics of plastic film and sheet to a gas
- ISO 15105-1 — Plastics, film and sheeting, determination of gas-transmission rate, Part 1: differential-pressure method
- GB/T 1038 — Determination of gas permeability of plastic films and sheeting by the differential pressure method
- JIS K 7126-1 — Plastics, film and sheeting, gas transmission rate, Part 1: differential pressure method
- ISO 2782-1 — Rubber, vulcanized or thermoplastic, determination of permeability to gases by differential pressure methods
- DIN 53380 — Testing of plastic films, determination of gas permeability
Applications
Differential pressure gas permeability data are used across the packaging, electronics, energy, and materials industries wherever gas barrier performance controls product quality, safety, or service life.
- Food packaging: oxygen and carbon dioxide transmission data for modified atmosphere packaging, shelf-life modeling, and barrier film qualification for coffee, dairy, meat, and fresh produce applications
- Pharmaceutical and medical packaging: blister lidding foils, infusion bags, and sterile barrier systems where oxygen ingress affects drug stability
- Energy and electronics: aluminum-plastic pouch film for lithium-ion batteries, photovoltaic backsheets, and high-barrier encapsulation films for organic electronic devices
- Material research and development: comparing polymer candidates, evaluating coating and metallization quality, and studying the temperature dependence of permeation for shelf-life extrapolation
- Quality control and compliance: incoming material inspection, batch release testing, supplier qualification, and independent verification by accredited third-party laboratories
FAQ
What is the difference between the differential pressure method and the equal pressure method?
The differential pressure method establishes a pressure gradient across the specimen and detects permeation manometrically, so it works with any dry test gas and directly yields the permeability coefficient. The equal pressure method maintains approximately one atmosphere on both sides, sweeps the downstream face with carrier gas, and quantifies oxygen with a coulometric sensor; it simulates package conditions more closely but is generally limited to oxygen transmission rate measurement.
Which gases can be tested by the differential pressure method?
Oxygen, nitrogen, carbon dioxide, dry air, and inert gases are routinely measured. Flammable or toxic gases may be evaluated only with appropriate instrument modifications and safety controls. The test gas should always be dry, since moisture in the gas stream affects both the measurement and hydrophilic specimens.
What specimen size and thickness are required?
Circular discs are used, with diameters and exposed test areas defined by the instrument design; exposed areas of roughly 5 to 50 cm² are common. Film and sheet specimens are typically up to about 3 mm thick, while thicker rubber sheets may be tested with mechanical support to prevent deflection.
How long does a typical test take?
High-permeation films may reach steady state within two to three hours, whereas ultra-high-barrier laminates can require twenty-four to forty-eight hours or longer. Evacuation, outgassing, conditioning, and leak verification add additional time to the overall cycle.
How does gas permeability testing differ from water vapor transmission testing?
Gas permeability testing uses dry gases driven by a pressure difference, while water vapor transmission testing establishes a humidity gradient and typically employs gravimetric cups or infrared sensors in accordance with standards such as ASTM E96 or ASTM F1249. Both measurements are usually required for a complete barrier characterization of a packaging material.
Which factors most strongly influence the results?
Temperature is the dominant factor, since permeability of polymers increases steeply with temperature; humidity also strongly affects polar polymers such as EVOH and polyamide. Careful control of sealing integrity, leak rate, thickness uniformity, and specimen conditioning is essential for accurate, reproducible results.