Helium mass spectrometry vacuum leak detection and precautions of advantages and disadvantages
Helium mass spectrometer leak detection means that helium is blown from the outside of the vacuum chamber to the detected part, and then the helium mass spectrometer connected to the vacuum chamber is used to detect the invaded helium, which is the most commonly used vacuum leak detection method.
The basic principle of helium mass spectrometer leak detector. The hot electrons emitted from the filament accelerate the flow to the anode and ionize the gas molecules when they meet. When a magnetic field is set on the way to the ion collector, the ion moves in a circular path by Lorentz force. If the mass of the ion is m and the charge is e, the radius of the circular trajectory is related to the mass charge ratio m / E. The position of the exit gap should be reasonably set so that M / E = 4, i.e. he + can pass through the gap. Other ions with different mass charge ratio of helium can not pass through the exit slit because of the different deflection radius and slit setting of the instrument. Therefore, helium mass spectrometer leak detector can only detect monovalent helium ions. When helium mass spectrometer leak detector is used, the vacuum degree of the detected space is usually less than 10-3 PA, and the detection accuracy can reach 10-13 PA · m3 · s-1.
Helium mass spectrometer leak detector has the characteristics of stable performance and high sensitivity. Sensitivity, reaction time, clearing time, working vacuum degree, limiting vacuum degree and pumping speed at the inlet of the instrument are the main performance indexes of helium mass spectrometer leak detector.
The main reasons for using helium in the detection gas are as follows:
(1) It has no toxicity to human body and natural environment;
(2) No burning, safe operation;
(3) Inert gas will not react with the tested object;
(4) Small molecular diameter and mass, easy to pass through the small gap, easy to leak detection;
(5) In general, the gas state is maintained in the environment, and the micro leakage is not blocked;
(6) There is almost no helium in the gas released from the vacuum chamber itself;
(7) The amount of helium in the air is only 5ppm, so it is easy to find out the location of the leakage.
Since helium is much lighter than air, it is necessary to operate from top to bottom when helium is injected into the vacuum chamber for leak detection, otherwise it is not conducive to determine the leakage point. When the cavity structure is complex or the leakage rate is very small, the plastic bag can be used to cover the key parts, and then helium gas is filled into the plastic bag to realize the leak detection of specific key parts.
In general, vacuum leak detection should focus on the following parts:
(1) Flange interface part;
(2) Welding position;
(3) Bellows part;
(4) Other sealing parts.