What is the operating principle of leak detectors with a mass spectrometer
Far and away the most sensitive leak detection method and the one most widely used in industry.
The MS (Mass Spectrometers) leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending across many powers of ten. The lower limit lies around 1·10-12 mbar·l/s. This even allows for the quantitative measurement of the inherent gas flow through solids (permeation).
In principle, it is possible to detect all gases using mass spectrometry. Of all the available options, the use of helium as a test gas has proved to be especially practical. Helium is:
- unequivocally detectable with a mass spectrometer,
- chemically inert,
- non-explosive,
- non-toxic,
- present in normal air in a concentration of only 5 ppm ( = 5 · 10-4 volume % ) and quite economical.
Two types of mass spectrometer are used in commercially available MSLD’s:
a) the quadrupole mass spectrometer
b) 180° sector field mass spectrometer (due to its simple design).
Every mass spectrometer consists of three fundamental assemblies:
- ion source,
- separation system and
- ion trap.
The ions must be able to travel along the path from the ion source and through the separation system to the ion trap, to the greatest possible extent without colliding with gas molecules. This path amounts to about 15 cm for all types of spectrometers and thus requires a medium free path length of
at least 60 cm, corresponding to pressure of about 1·10-4 mbar; in other words, a mass spectrometer will operate only in high vacuum. In modern leak detectors, turbomolecular pumps are used to create the high vacuum.
Associated with the individual component groups are the required electrical and electronic supply systems and software which, via a microprocessor, allow for the greatest possible degree of automation in the operating sequence, including all adjustment and calibration routines and measured value display.
The operating principle for a MSLD
Operating principle of a leak detector with mass spectrometer (main flow leak detector)
Above figure is provided to explain the operating principle of a leak detector with mass spectrometer: This sketch shows the most commonly found configuration for leak detection using the test gas spray method at a vacuum component (= test object).
If gas enters the component through a leak, it is pumped through the interior of the leak detector to the outlet, where it leaves the detector again. Assuming, that the leak detector is properly sealed, the gas flow q is always the same at any point between the inlet and the outlet of the leak detector. The following applies directly at the pumping port of the vacuum pump:
Q = p . S
with:
- p = Inlet pressure directly at the pumping port of the vacuum pump in mbar
- S = Pumping speed of the vacuum pump directly at the pumping port of the vacuum pump in l/s
At any other position x, the following applies while taking the line losses into account:
qx = q = px · Sx
with:
- px = pressure at position x in mbar
- Sx = pumping speed of the vacuum pump at position x in l/s (Sx < S !)
Above equation applies to all gases which are pumped by the vacuum pump and thus also for the test gas TG (e.g. TG = helium).
At the mass spectrometer (x = MS), the following applies:
qMS, TG = qTG = pMS, TG · SMS, TG = qL
In this case, the test gas flow qTG equals the leak rate qL being sought. It must be noted that, in case of equation above, the partial test gas pressure pMS, TG is present at the mass spectrometer.
The measuring value for pMS, TG is provided by the mass spectrometer which must be set to the mass M of the test gas (e.g. M = 4 for TG = helium). The value of SMS, TG is an experimentally determined constant for each leak detector.
The value for pMS, TG provided by the mass spectrometer is multiplied by the value SMS, TG which is stored in the microprocessor of the leak detector. The result of this multiplication is then displayed as leak rate qL.
Fundamentals of Leak Detection
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