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.
Reference Cavity
Baseline Volume
A structural component provides a stable environment containing a known substance or vacuum against which an external measurement is compared. The reference cavity ensures that the sensor output remains proportional to the target variable by isolating the internal sensing element from ambient pressure or temperature fluctuations. This design is fundamental to absolute pressure transducers and infrared gas analyzers.
The stability of the measurement depends entirely on the integrity of this sealed volume over the life of the instrument.
Environmental Isolation
Manufacturers use laser welding or glass to metal seals to hermetically close the chamber and prevent any exchange with the outside atmosphere. Within a reference cavity, the internal gas or vacuum must remain constant to avoid drifting the zero point of the sensor. High grade materials like sapphire or stainless steel are chosen to minimize outgassing and thermal expansion.
If the seal fails, the resulting change in internal pressure will manifest as an uncorrectable error in the sensor output.
Thermal Regulation
Integrated heaters or passive insulation systems maintain the volume at a constant temperature to eliminate thermal noise.
Verification Process
Checking the health of the internal volume involves monitoring the sensor output at a known zero point or under a specific load. A reference cavity that has leaked will show a characteristic shift in response that deviates from the factory calibration. Specialized test chambers simulate the extreme conditions of the field to verify that the structure maintains its seal under mechanical and thermal stress.