Gas leakage between E-11 and E-15 std cc He/s can be measured by a helium accumulation method. The leak detector uses a quadrupole mass spectrometer with a minimum detectable partial pressure of 5E-15 torr. The mass spectrometer is attached to a small ultrahigh vacuum chamber constructed of stainless steel treated for a low hydrogen outgas rate of 2E-15 torr·liter/cm2·s. The chamber has a nozzle to which a test object can be attached. The chamber is also equipped with a non-evaporable getter (NEG) pump that pumps active gases but not noble gases such as helium. The purpose of the NEG and the ultra-low hydrogen outgas feature of the chamber is to maintain low background pressure and high sensitivity of the quadrupole during helium accumulation. Helium tracer gas coming through a defective seal or material is accumulated in this chamber for a few minutes, and the rate of rise of the helium signal is compared with that of a calibrated reference leak. In this manner, leakage down to 1E-15 std cc He/s can be measured with a good confidence factor.
