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.
Time Lag Method
Permeation Measurement
An analytical technique determines the diffusion and solubility coefficients of a gas through a solid membrane by measuring the delay in gas transport. The time lag method relies on the period between the initial exposure of the membrane to a gas and the point where a steady state flow is established on the other side. This interval provides a direct measurement of how quickly molecules migrate through the polymer structure.
It is a standard procedure for characterizing the barrier performance of packaging materials and sensor seals.
Mathematical Derivation
The steady state permeation rate is extrapolated back to the time axis to identify the specific moment the gas began to emerge. Using the time lag method allows researchers to separate the effects of solubility from the speed of diffusion within the material. This distinction is necessary for predicting how a seal will behave when exposed to different types of gases or vapours.
The resulting data is used to select materials that provide the best protection for sensitive electronic components.
Experimental Setup
Constant high vacuum is maintained on the downstream side of the sample to ensure that the accumulation of gas does not interfere with the measurement. Precise pressure sensors record the rise in pressure over time as the gas permeates through the membrane.
Material Selection
Results from these tests guide the development of new polymers with improved resistance to gas migration. Standardized by organizations like the International Organization for Standardization, the procedure ensures that data from different laboratories can be compared directly. Each test run must account for the thickness of the sample and the temperature of the environment to maintain accuracy.