Leak detection with tracer gases; sensitivity and relevant limiting factors

Abstract

After briefly dealing with some basic questions concerning vacuum leak detection based on tracer gases, the factors which determine the smallest detectable leak rate are discussed. It is shown that this frequently overrated specification not only depends on the lowest measurable partial tracer gas pressure, but also on the lowest detectable tracer gas concentration, the effective volume flow rate at the inlet of the tracer gas detector and above all other factors on the total gas flow from the test object to the pump. It is demonstrated that high sensitivity leak testing can only commence provided the gas flow which is mainly due to outgassing, is sufficiently low after a suitable pumpdown period. The task of the LN 2-trap of conventional leak detectors is that of pumping out condensible gases to increase the helium concentration, thus reducing the time required for the commencement of the actual leak detection process. In the counterflow-type helium leak detector the same task is performed by a high vacuum pump, whereby it relies on the much lower compression of diffusion and turbomolecular pumps for lighter gases as compared to heavier gases. Finally a helium leak detector is described which is equipped with a double-entry vacuum pump. The test object is connected to the inlet with a high volume flow rate and a mass spectrometer is connected to the second inlet. The forevacuum connection lies between the two pumps which are arranged on-axis.