VS Series Helium Mass Spectrometer Leak Detectors
DRAFT 4/23/15
B-1
Appendix B. Introduction to Leak Detection
B.1 Leak Testing—Why is it Needed?
Helium is a superior choice of tracer gas used to find leaks for a multitude of reasons.
Helium is:
❑ Non-toxic
❑ Inert and non-condensable
❑ Normally not present in the atmosphere at more than trace amounts
❑ Relatively inexpensive
❑ Readily passes through leaks due to its small atomic size
❑ Non-flammable
❑ Available in various size cylinders
❑ Available in purities appropriate for medical usage
The only molecule smaller than Helium (mass 4) is Hydrogen (mass 2), which is not inert.
Helium is much lighter than the next heavier inert molecule, Neon (mass 20) which is much
more expensive. Helium is present at a concentration of only 5 ppm in normal atmospheric
conditions.
B.2 Agilent's Helium Leak Detection Technologies
A Mass Spectrometer Leak Detector (MSLD) is a complete system for locating and/or
measuring the size of leaks into or out of a device or a container. This method of leak
detection is initiated when a tracer gas, helium, is introduced to a test part that is connected
to the MSLD system. The helium leaking from the test part diffuses through the system, its
partial pressure is measured, and results are displayed on a meter.
The MSLD operating principle consists of ionization of the gases in a vacuum and their
acceleration across a voltage drop and a magnetic field (Figure B-1). The helium ions are
separated and collected, and the resulting ion current is amplified and indicated on the
meter or display.
A mass spectrometer leak detector consists of the following components:
❑ A spectrometer tuned to detect the mass of helium
❑ A vacuum system to maintain adequately low pressure in the spectrometer
❑ A mechanical pump(s) to evacuate the part to be tested
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