Meaning
Fluid dynamics equations convert measured gas flow rates into equivalent leak rates under different pressure and temperature conditions. In leak testing, viscous flow conversion calculations determine how a measured helium leak rate translates to the flow of liquid electrolyte or air. This conversion is necessary because different fluids move through micro-cracks according to different physical laws.
Flow Equation
The calculations are based on the Poiseuille equation for laminar flow, which accounts for the viscosity of the fluid and the geometry of the leak path. Because helium has a much lower viscosity than liquid electrolyte, it flows through a leak path much faster. Converting these rates allows engineers to set realistic and achievable leak limits for production-line testing.
Sourcing Integration
Procurement specifications often define the leak limit in terms of equivalent air or helium flow to simplify quality control. Suppliers must use these converted values to configure their leak testing equipment. This ensures that the test results are directly comparable across different manufacturing sites.
Conversion Accuracy
The conversion assumes that the leak path is a clean, uniform capillary and does not change when different fluids flow through it. In reality, a leak path can be irregular, and the presence of liquid electrolyte can block the flow of gas. For this reason, leak testing is best performed on empty cell housings.
