Very low gas flow measurements for UHV/XHV and leak calibration

Abstract

The main factor that affects the extension of the lower limit of a constant conductance gas flowmeter is the outgassing of the inner surface of the ballast chamber and pipes. A novel method is demonstrated to extend the lower limit using a non-evaporable getter (NEG) pump to eliminate the outgassing influence. The NEG pump has two remarkable characteristics: one is the large pumping speed for active gases especially for hydrogen at ambient temperature, and the other is no pumping speed for inert gases. It is known from numerous experimental investigations on outgassing that the residual gas inside an ultrahigh vacuum and extremely high vacuum (UHV/XHV) facility made of stainless steel is virtually pure hydrogen. So it can reduce the outgassing influence and maintain a low base pressure without changing the test gas quantity in the ballast chamber when inert gas is used as the test gas. By this method, the lower limit of the constant conductance gas flowmeter is successfully extended to 10−12 Pa m3 s−1 (at 23 °C) with a combined relative standard uncertainty of 0.94% (k = 1). A vacuum leak with a leak rate of 10−10 Pa m3 s−1 (at 23 °C) is calibrated, and the uncertainty of the non-linearity of a quadrupole mass spectrometer is avoided. An ionization gauge is calibrated in the pressure range from 10−10 Pa to 10−6 Pa with the dynamic expansion method, and the uncertainty of conductance ratio is avoided compared with the separated flow method.