Vacuum leak detection by helium mass spectrometry
Helium mass spectrometry leak detection refers to blowing helium gas from the outside of the vacuum chamber to the detected site, and then detecting the invaded helium gas by the helium mass spectrometry leak detector connected to the vacuum chamber. This is the most commonly used vacuum leak detection method at present.
The basic principle of a helium mass spectrometer leak detector is shown in Fig. 1. Hot electrons from the filament accelerate to the anode, where they encounter gas molecules and ionize them. When the magnetic field is set on the way of the ions moving towards the ion collecting pole, the ions are subjected to the Lorentz force and move in a circular path. If the mass of the ion is m and the charge is e, then the radius of the circular trajectory is related to the mass charge ratio m/e. The outlet slit position is reasonably set so that M/E =4, that is, He+ can pass through the slit. Other ions different from helium mass charge ratio cannot pass through the outlet slit due to their deflection radius different from the slit setting of the instrument. Therefore, the helium mass spectrometer leak detector can only detect univalent helium ions. The use of helium mass spectrometer leak detector usually requires the vacuum degree of the detected space to be below 10-3 Pa and the detection accuracy to be up to 10-13 Pa·m3·s-1.
The helium mass spectrometer leak detector has the characteristics of stable performance and high sensitivity. Sensitivity, reaction time, zero clearing time, working vacuum, ultimate vacuum and inlet pumping speed are the main performance indexes of helium mass spectrometer leak detector.
The main reasons for using helium in gas testing are as follows:
(1) It is not toxic to human body and natural environment
(2) can not burn, safe operation
(3) The inert gas will not react with the detected object
(4) the molecular diameter and mass is small, easy to pass through tiny gaps, easy to leak detection
(5) usually keep the gas state in the environment, will not plug the micro leak hole
(6) The gas released by the vacuum cavity itself contains almost no helium
(7) The amount of helium in the air is only 5ppm, so it is easy to pinpoint the location of air leakage
Because helium gas is much lighter than air, it is necessary to operate from top to bottom when spraying helium gas into vacuum chamber for leak detection, otherwise it is not conducive to determine the location of leakage point. When the cavity structure is complex or the leakage rate is extremely small, plastic bags can be used to cover the key parts, and then helium gas is filled into the inside of the plastic bags to realize the leak detection of the specific key parts.
In general, the following parts should be paid attention to when vacuum leak detection:
(1) flange interface parts
(2) Welding site
(3) the bellows part
(4) other sealing parts