Quantifying Water Vapor Diffusion across Elastomer Pressure Sensor Seals
Water vapor diffusion across elastomer seals causes reference cavity pressurization and dielectric drift, requiring coulometric ISO 15105-2 verification.
Coulometric Electrolytic Sensor
Electrochemical Analysis
An electrochemical transducer determines moisture content by measuring the current required to electrolyze water absorbed from a gas stream. This coulometric electrolytic sensor relies on Faraday’s laws of electrolysis to relate charge to mass. The process is absolute because it calculates moisture mass directly from electrical charge rather than relying on a comparative calibration curve.
Accuracy remains high across the parts per million range until the phosphorus pentoxide film becomes saturated or contaminated.
Current Conversion
Electrical current flows between two platinum electrodes coated with a thin film of hygroscopic material. As the gas flows over the film, the coulometric electrolytic sensor converts every water molecule into hydrogen and oxygen through electrolysis. One ampere of current for one second equals a fixed mass of water based on the chemical equivalent weight.
This relationship provides a fundamental measurement that requires no periodic field adjustment against a reference gas. The electronics measure the resulting current and display it as a moisture concentration. Because the sensor consumes the water it measures, it provides a continuous update on the gas purity without the lag associated with passive adsorption.
Operational Constraint
Performance depends on maintaining a constant flow rate and preventing electrode fouling by oils or corrosive vapours. If the gas flow deviates from the calibrated set point, the resulting moisture reading will be proportionally incorrect despite a stable current.
Verification Standard
Precision is verified by introducing a known moisture standard generated by a permeation tube or a pressure swing saturation system. These checks confirm that the coulometric electrolytic sensor maintains a linear response across its entire operational span. The drift is typically minimal unless the electrolyte film undergoes physical degradation or chemical poisoning.