The electrolytic (coulometric) sensor method is a high-precision instrumental technique for determining the water vapor transmission rate (WVTR) of plastic films, composite laminates, and finished packages, standardized in ISO 15106-3 and GB/T 21529. Based on Faraday's law, the method quantitatively electrolyzes the moisture that permeates through a specimen, achieving detection limits as low as 0.001 g/m²·24h and making it ideal for evaluating medium- to ultra-high-barrier packaging materials. This article explains the measurement principle, applicable sample types, test items, operating procedure, typical test conditions, industrial applications, and frequently asked questions.
Technical Overview
Water vapor transmission rate (WVTR), expressed in g/m²·24h, defines the mass of water vapor that permeates through unit area of a packaging material in 24 hours under a specified temperature and humidity gradient. It is one of the most critical barrier performance indexes for packaging, because moisture ingress directly drives product caking, hydrolysis, microbial growth, loss of potency, and shelf-life reduction.
The electrolytic method, also known as the coulometric detection sensor method, is an instrumental technique in which a specimen is mounted as a barrier between a humid chamber and a dry chamber swept by a carrier gas (high-purity nitrogen). The permeated water vapor is carried to an electrolytic sensor cell where it is completely absorbed by a phosphorus pentoxide (P2O5) layer and electrolyzed. Because the electrolysis charge is rigorously related to the amount of water through Faraday's law, the method is effectively an absolute quantitative measurement with excellent sensitivity and repeatability.
Compared with the traditional gravimetric dish method, the electrolytic method offers a much lower detection limit, full automation, faster equilibration, and objective electrical signal acquisition. It is particularly suitable for high-barrier structures such as metallized films, aluminum foil laminates, and oxide-coated (AlOx/SiOx) films, where gravimetric methods lack sufficient resolution.
Relevant Standards
- ISO 15106-3: Plastics — Film and sheeting — Determination of water vapor transmission rate — Part 3: Electrolytic detection sensor method
- GB/T 21529: Determination of water vapor transmission rate of plastic films and sheeting — Electrolytic sensor method
- YBB 00092003: Water vapor transmission rate of pharmaceutical packaging materials and containers — Electrolytic method
- ASTM E96/E96M and ISO 2528: Gravimetric (dish) methods, commonly referenced for comparison
- ASTM F1249 and ISO 15106-2: Infrared sensor methods, cited for cross-method correlation
- JIS K 7129: Instrumental determination of water vapor transmission rate of plastic films
Testing Samples
Monolayer Plastic Films and Sheets
Typical samples include LDPE, LLDPE, CPP, BOPP, BOPET, BOPA, PVC, and CPE films in the thickness range of approximately 20–200 µm. WVTR values for these materials typically span from below 1 g/m²·24h (e.g., high-density PE, BOPET) to more than 100 g/m²·24h (e.g., thin CPP or PE films), and the instrument range must be verified before testing.
High-Barrier Films
This category covers PVDC-coated (K-coated) films, EVOH coextruded structures, vacuum-metallized films (VMPET, VMCPP), transparent oxide-coated films (AlOx- or SiOx-coated PET), and aluminum foil laminates. These materials are the primary application field of the electrolytic method because their transmission rates often fall to the 0.001–0.5 g/m²·24h level, below the practical detection capability of gravimetric testing.
Multilayer Composite Laminates
Flexible packaging laminates such as PET/AL/PE, PET/VMPET/CPP, PA/PE, paper/AL/PE, and similar structures can be tested as complete laminates. The result characterizes the laminate as a functional whole, including the contribution of adhesives and coating layers. Specimen edges must be sealed with wax or masking material to prevent lateral penetration and edge leakage.
Pharmaceutical Packaging Materials
Composite hard films for blister packaging (e.g., PVC/PVDC, PA/AL/PVC cold-formable foil, tropical blister laminates), aluminum-plastic composite sheets, and infusion or diagnostic pouches are routinely tested under pharmaceutical packaging standards using the electrolytic method, often at 40 °C/90% RH or other accelerated conditions.
Finished Packages and Containers
With dedicated package fixtures, complete pouches, sachets, bottles with closures, tubes, and thermoformed cups can be evaluated in finished form. This verifies not only the material barrier but also the integrity of seals, seams, and closures, and the result is usually expressed as mg of water vapor per package per day.
Testing Items
The core deliverable of the test is the water vapor transmission rate (WVTR) of the specimen under specified conditions, reported in g/m²·24h. Depending on the purpose of the evaluation, the following items can be derived and reported:
- Water vapor transmission rate (WVTR), g/m²·24h, under the selected temperature and relative humidity conditions
- Water vapor permeance, obtained by normalizing WVTR to the water vapor partial pressure difference across the specimen
- Water vapor permeability coefficient (g·cm/cm²·s·Pa), calculable for homogeneous monolayer materials of known thickness
- Whole-package moisture permeation (mg/day/package) for finished packages and containers
- Condition-specific and comparative testing, for example 23 °C/85% RH versus 38 °C/90% RH, for shelf-life modeling
A typical report includes the individual and mean values of all specimens, standard deviation, test area, specimen thickness, test temperature and humidity, conditioning state, and the applicable standard. Testing should be performed by accredited third-party laboratories or by in-house laboratories with calibrated instruments and traceable reference materials.
Testing Methods
Test Principle
The specimen is clamped in a two-chamber diffusion cell. The upper (wet) chamber is maintained at a constant relative humidity by distilled water or a humidity generator, while the lower (dry) chamber is continuously swept by high-purity dry nitrogen. Water vapor permeating through the specimen is entrained by the carrier gas and transported to the electrolytic sensor cell, where a phosphorus pentoxide coating absorbs the moisture quantitatively. Under an applied DC voltage, the absorbed water is electrolyzed into hydrogen and oxygen, regenerating P2O5, and the electrolysis current is measured. According to Faraday's law, two moles of electrons correspond to one mole of water, so:
WVTR = I × M × 86400 / (2 × F × A)
where I is the steady-state electrolysis current, M is the molar mass of water (18.02 g/mol), F is the Faraday constant (96,485 C/mol), and A is the effective test area. Once a steady state is reached, the electrolysis current is directly proportional to the permeation rate, yielding WVTR without empirical sensor calibration.
Apparatus and Materials
- Electrolytic WVTR analyzer with temperature-controlled test chamber, humidity control, mass flow controllers, P2O5 electrolytic sensor cells, and data acquisition software
- High-purity nitrogen (typically ≥99.999%), dried to a dew point low enough to establish a stable baseline
- Sealing wax, masking film, or high-vacuum grease for edge sealing of specimens
- Certified reference films traceable to a national metrology institute, for periodic verification and calibration
Specimen Preparation
Specimens are sampled free of wrinkles, creases, pinholes, and visible defects, and are normally conditioned at 23 °C/50% RH for at least 48 hours (or as specified by the material standard). At least three specimens are tested for each material. Thickness is measured at several points and the effective test area (commonly 5–50 cm² depending on the cell) is recorded. For laminates with exposed cut edges, the perimeter is sealed to suppress lateral diffusion.
Test Procedure
- Set the chamber to the target temperature and allow it to stabilize within the required tolerance (typically ±0.5 °C or better)
- Establish the required relative humidity on the wet side (commonly 90% ± 2% RH) and purge the dry side with nitrogen at the flow rate specified by the standard or instrument
- Verify the zero baseline and system tightness, then mount the specimen and start the test
- Monitor the electrolytic signal until a steady state is reached, defined as a variation of the computed transmission rate within a small percentage (for example 2–5%) over a defined time window
- Record the steady-state WVTR, repeat for all specimens, calculate the mean and deviation, and report results with full test conditions
Typical Test Conditions
- 38 °C / 90% RH — the most widely used condition for flexible packaging, equivalent to a tropical climate
- 23 °C / 85% RH or 23 °C / 90% RH — ambient reference conditions
- 40 °C / 90% RH — accelerated condition frequently specified for pharmaceutical packaging
- 25 °C / 90% RH, 37 °C / 100% RH, or customized conditions as required by product specifications
Because WVTR is strongly temperature- and humidity-dependent, the test conditions must always be stated together with the result, and comparisons between materials are valid only at identical conditions.
Applications
The electrolytic method is applied throughout the packaging value chain, from film manufacturing to finished-product quality control:
- Food packaging: shelf-life evaluation and material selection for biscuits, snacks, milk powder, coffee, dried foods, condiment sachets, and retort pouches; incoming inspection of barrier films
- Pharmaceutical packaging: qualification of blister foils, aluminum-plastic laminates, and pouches for moisture-sensitive tablets, effervescent dosage forms, powders, and traditional medicine products, supporting pharmaceutical packaging standards and ICH stability programs
- Electronics packaging: barrier performance verification of moisture barrier bags for moisture-sensitive devices, in line with handling requirements such as IPC/JEDEC J-STD-033
- New energy and photovoltaics: water vapor barrier evaluation of photovoltaic backsheets and aluminum-plastic laminated films for lithium battery cells
- Medical devices and diagnostics: moisture protection of sterile barrier systems, test strips, and diagnostic reagent packaging
- Research and development: comparison of coating processes (AlOx, SiOx), metallization quality, adhesive systems, film structures, and humidity-aging behavior of materials
FAQ
What is the difference between the electrolytic method and the gravimetric (dish) method?
The gravimetric method per ASTM E96 or ISO 2528 determines WVTR by weighing a desiccant dish over long exposure periods; its practical detection limit is roughly 0.1–1 g/m²·24h and tests may take days to weeks. The electrolytic method measures permeated moisture electrically via Faraday's law, reaches detection limits around 0.001 g/m²·24h, runs unattended, and is far more suitable for high-barrier materials.
How does the electrolytic method compare with the infrared sensor method?
Both are high-sensitivity instrumental techniques. The infrared method (ASTM F1249, ISO 15106-2) detects water vapor by infrared absorption, while the electrolytic method (ISO 15106-3, GB/T 21529) provides an absolute coulometric measurement in which the electrolysis charge is directly proportional to the permeated water mass. The choice usually depends on the governing specification; results from the two methods correlate well when tests are run under identical conditions.
What is the measuring range of the method?
Depending on the instrument configuration, the typical range extends from approximately 0.001 g/m²·24h up to several hundred g/m²·24h. Very high-transmission samples may require area or flow adjustment to remain within range, while ultra-high-barrier samples mainly require sufficient equilibration time and a well-controlled baseline.
How long does a test take?
Moderate-barrier films such as BOPET typically reach steady state within a few hours. High-barrier and foil laminates exhibit long lag times and may require 24 hours to several days. Modern automated instruments run the measurement unattended and terminate automatically once the steady-state criterion is satisfied.
How many specimens should be tested?
At least three specimens per material are recommended, and more for heterogeneous or anisotropic structures. The mean, individual values, and deviation are reported so that localized defects such as pinholes can be identified.
Why is edge sealing important?
Water vapor can migrate laterally through the cut edges of laminates and through leaks at the clamping interface, which inflates the apparent transmission rate. Sealing the perimeter with wax or masking material ensures that only transmission through the flat test area is measured.
How is the instrument verified?
Although the electrolytic principle is absolute, instruments are periodically verified with certified reference films traceable to a national metrology institute, along with zero checks using an impermeable specimen, temperature and humidity verification, and carrier-gas purity control. This guarantees the traceability and reliability of reported WVTR values.
