Correlating Gas Leakage Rates to Fluid Ingress in Sensor Seals
Correlating gas leakage rates to liquid ingress requires deriving equivalent pore radius to confirm capillary entry pressure exceeds target hydrostatic head.
Leakage Rate Correlation
Metrological Relation
Mathematical conversion factors allow the comparison of fluid transport rates across different media and pressure conditions. Establishing a leakage rate correlation is necessary when a component is tested using a gas but operates with a liquid. This comparison is based on the physical properties of the fluids, such as viscosity and molecular weight.
Understanding these relationships allows quality engineers to set appropriate gas-test rejection limits that guarantee liquid tightness in the field.
Physical Principle
Fluid dynamics through small orifices depends on the regime of the flow, which can range from viscous to molecular. The leakage rate correlation between gas and liquid flow assumes that the geometry of the leak path remains constant. For gas flow, compressibility must be factored into the equations, while liquid flow is treated as incompressible.
Viscous flow occurs in larger leak channels, whereas molecular flow dominates in very small gaps where gas molecules interact with the channel walls.
Calculation Method
Standard formulas are used to calculate the equivalent gas leak rate based on the maximum allowable liquid bypass. To establish the leakage rate correlation, the Knudsen and Poiseuille flow equations are applied to the specific operating pressures. This calculation requires accurate values for the dynamic viscosities of both the test gas and the process fluid.
For example, the viscosity of helium is much lower than that of water, which allows helium to penetrate smaller passages.
Calibration Impact
Verification of the mathematical models is achieved by testing reference components with known capillary dimensions. Technicians confirm the calculated leakage rate correlation by measuring the flow of both gas and liquid through a micro-capillary tube under controlled laboratory conditions. This calibration ensures that the theoretical models match the physical behavior of the seals and fittings.
The results are used to refine the safety factors applied during production testing, which reduces the occurrence of false passes and false failures. By maintaining this metrological traceability, manufacturers can confidently guarantee that components passing the dry helium test will remain completely dry when exposed to high-pressure hydraulic fluids in the field.