7.3.2 Comparison of test results with leak detector and quadrupole mass spectrometer
Quadrupole mass spectrometers are primarily used to analyze the composition of gas mixtures. They require test pressures in the high vacuum range. Mass spectrometers can be used with almost any tracer gas, as their use does not necessarily depend on the classic tracer gas helium. Residual gas analyzers can detect leaks in a vacuum system without any special test gas. They analyze the mass of the gases in the air.
Figure 7.9: Mass spectrum of a recipient with air leak
Leak detectors for the tracer gas helium are not currently designed on the basis of quadrupole mass spectrometers. Their greater robustness and long-term stability as well as the easier quantification of results and data interpretation have resulted in the fact that above all mass spectrometric detectors on the basis of magnetic sector mass spectrometer are mainly used for this purpose.
Units with detectors based on a quartz window sensor are used too for applications which do not require the superior sensitivity of mass spectrometric detectors. Units with quartz window sensors make it possible to work at very high test pressures and high water vapor exposure.
|Leak Detectors||RGA / QMS|
|Permeation||Rough indication possible, no differentiation between permeation and desorption||Yes, data interpretation possible|
|Desorption||Yes, conclusion about desorbing gases|
|Detection limit||5 · 10-13 Pa m3 s-1
(sector field MS)
5 · 10-9 Pa m3 s-1
(quartz window detector)
|Ion current dependent on rod system and detector|
|General||Quantitative leak rate measurement possible||Qualitative instrument, quantification complex, requires more expertise, provides more information|
Table 7.3: Comparison of leak detector and quadrupole mass spectrometer