Viscous Flow
Fluid flow through a tiny orifice or capillary at low velocities where viscous forces dominate over inertial forces defines a predictable leak mode. This regime, characterized as laminar leakage, produces a flow rate that is directly proportional to the pressure differential across the leak path. Metrologists model this behavior to determine leak rates in high-vacuum and pressure containment systems.
The linearity of this flow simplifies the calculation of leak sizes.
Pressure Dependence
Unlike turbulent flows which scale with the square root of pressure, laminar leakage scales linearly. This linear relationship allows extrapolation of leak rates at different pressures from one measurement. It is useful when testing sealed housings.
Sensor Verification
Flow-based leak detectors must be calibrated using standard leaks that generate a known, constant flow rate. When verifying these detectors, the laminar leakage through a certified capillary provides a stable reference. The temperature of the gas must be held constant because gas viscosity rises with temperature, which reduces the leak rate.
This temperature effect is the primary source of uncertainty in leak metrology.
Calibration Standard
Certified leak artifacts with micro-capillary channels are used to calibrate industrial leak testing systems. Since these capillaries are prone to clogging, they must be checked regularly against a primary flow standard. The laminar leakage through the artifact is verified by measuring the pressure drop in a reference volume over time.
Any deviation from the certified flow rate indicates contamination or physical damage to the capillary.
