App Note: Testing a Tube Bore for Imperfections

Fast and accurate leak checking can be a complicated process requiring a rather complex setup, so when Alicat received an inquiry from a company looking to test bore imperfections in 0.001 bore tubes, Alicat engineers stepped up to the challenge. For years, Alicat has recommended a two-unit leak checking system – an Alicat PC3-Series Remote Sensing Pressure Controller to maintain the required test pressure at the DUT and an Alicat M-Series Mass Flow Meter to measure the leak rate.

The test was simple.  A ported-hollow rod would be passed between two O-rings using a highly accurate, precision motor. This rod would then pressurize the annulus in the bore. Leaks would occur if the annulus passes by minute imperfections in the bore walls. Now all they needed was a device to check for very small leaks (@ 30 PSIG, flow volume (leak) is <1mL per minute) in the system. Alicat engineers determined this system would require two Alicat devices to check for these very small leaks – one PC3-100PSIG-D to maintain constant pressure and an M-1 SCCM-D to detect the small leaks.

If plumbed as shown in the diagram below, at a constant temperature, a constant bleed though the needle valve allows the pressure controller to precisely hold the pressure at the Device under test (DUT), ensuring that any flow through the meter after the bypass valve is closed is due to leakage. Alicat M-Series Mass Flow Meters are available with full scale ranges as small 0.5 SCCM with a flow measurement range of 0.01-100% of full scale, meaning that flows as low as .05 microliters (0.00005 SCCM) can be resolved.

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