Media Isolation Unit
Hermetically sealed subassemblies consisting of a micro-machined sensing chip submerged in an incompressible liquid core isolate fragile semiconductor dies from destructive chemical or industrial process environments. An oil-filled capsule transfers process pressure from an external flexible metal diaphragm through its internal liquid fill directly to a silicon piezoresistive or capacitive sensor die. The encapsulation concept ceases to apply in dry gas applications where bare silicon dies interface directly with clean, non-conductive media without protective barrier fluids.
Hydraulic Transfer Dynamic
Incompressibility of the internal liquid medium ensures immediate, proportional hydraulic transfer of external diaphragm displacements onto the sensing die surface. Highly refined silicon oils, mineral oils, or fluorinated hydrocarbon liquids are degassed under vacuum down to minute fractional air contents before hermetic sealing to prevent compressible gas bubble formation. The viscosity of the chosen liquid introduces high-frequency mechanical dampening, which filters out hydraulic pressure spikes but simultaneously attenuates sensor dynamic response above designated frequency cutoffs.
Operating temperature fluctuations cause the oil volume to expand or contract against the compliance of the isolation diaphragm. This expansion causes zero-shift drift that requires factory compensation across the operational envelope of an oil-filled capsule.
Hermetic Boundary Integrity
Fabrication requires microscopic electron-beam or laser welding of a thin metallic diaphragm onto a rigid cylindrical housing containing the sensor die and electrical feedthroughs. The electrical connections exit the sealed chamber through ceramic or glass-to-metal seals designed to maintain insulation resistance above one hundred megaohms under elevated operating pressures. Helium mass spectrometer leak tests verify that the housing maintains an absolute hermetic boundary, as any loss of internal oil results in total instrument failure.
When internal fluid leaks into the process stream, the diaphragm collapses onto the internal backing plate, causing signal clipping at baseline pressures.
Metrological Verification Procedure
Factory qualification requires measuring span error, zero offset drift, and hysteresis across multiple thermal and pressure cycles. Metrologists characterize the assembly inside temperature-controlled liquid baths, mapping the repeatable temperature-dependent offset curve into local non-volatile memory or dedicated digital signal processing chips. Mechanical overpressure stops are machined directly into the internal capsule walls behind the isolation diaphragm, allowing the thin metal foil to bottom out harmlessly when process overpressures occur.
An oil-filled capsule maintains structural stability and calibration fidelity only as long as process pressures remain within the structural yield limit of the welded isolation diaphragm.
