Silicon Piezoresistive Pressure Diaphragm Encapsulation Selection Criteria
Encapsulation selection balances gel potting low cost against oil-filled cell thermal stability to control long term zero drift and field recalibration expenses.
Glass-to-Metal Header Seal
Hermetic Isolation
Electrical signals must pass through structural walls of high-pressure sensors without compromising the isolation between internal electronics and harsh external fluids. The assembly known as a glass-to-metal header seal achieves this isolation by melting a specialized glass bead around a conductive pin within a metal outer ring. This structure maintains a tight physical barrier that prevents gas or fluid migration across the sensor boundary.
Thermal Matching
Manufacturing this component requires precise matching of the thermal expansion coefficients of the glass, the pin and outer housing. In a compression seal, the metal ring has a higher thermal expansion coefficient than the glass, which causes the metal to shrink more during cooling and place the glass under continuous compressive stress. Because glass is stronger in compression than in tension, this stress profile prevents microcracks from propagating through the seal when the assembly undergoes high mechanical pressure.
A mismatch in these expansion values leads to radial cracks that destroy the hermetic integrity of the feedthrough.
Leakage Verification
Verification of the seal involves helium mass spectrometry testing to check the leakage rate under high vacuum. The leakage limit is often specified to be less than one times ten to the minus nine standard cubic centimetres of helium per second. This testing occurs after thermal cycling and mechanical shock exposure to confirm that the seal can survive field stresses.
The calibration of the helium leak detector is verified using an internal reference leak standard with a traceably calibrated flow rate.
Dielectric Constraint
Electrical insulation resistance represents the primary functional constraint of this glass seal. Under high-humidity conditions, surface condensation can degrade the insulation performance. This degradation leads to low shunt resistance and signal drift.