Helium mass spectrometry leak detection method -inhalation method
Several helium mass spectrometry leak detection methods introduced by the vacuum pump manufacturer are all helium gas with a certain pressure applied to the outside of the tested piece, while the inside is in a vacuum state. Next, we introduce a reverse helium mass spectrometry leak detection method, inhalation method.
Inhalation method is also called suction gun method. The suction gun is connected with the helium mass spectrometer leak detector. When the sample to be inspected is evacuated, helium is filled (higher than atmospheric pressure), and then the nozzle of the suction gun moves at a certain speed in the suspected leakage on the outer surface of the inspected piece and sucks point by point. If there is a leak, the internal helium will leak through the leak hole and be pumped into the suction nozzle together with the surrounding air by the suction gun, and then it will enter the mass spectrometer chamber of the helium mass spectrometer leak detector, thus producing the output indication of the leakage. If the specimen filled with helium is placed in a hood or vacuum chamber, the inhalation method can also be evolved into gas chamber method and pressurized vacuum method.
The actual leak detection sensitivity of inhalation method is much lower than the limit sensitivity, so it is usually used for qualitative leak detection. The influence factors of leak detection sensitivity are similar to that of injection method, mainly including the pressure of helium filled, the shape and size of leakage hole, the shape of suction nozzle, the distance between suction nozzle and leakage hole, the angle of suction gun relative to the leak, moving speed and helium concentration in the atmosphere, stability, etc.
Therefore, attention should be paid to: the distance between the suction gun and the tested surface should not exceed 1 mm, and the moving speed of the suction gun should not exceed 2 cm / S; the sensitivity of the leak detection can be improved by increasing the pressure of helium filled inside, but it is necessary to prevent the waste of helium and the interference to the background of the leak detector. In general, the coarse inspection is carried out by using low charging pressure and low helium partial pressure. In case of large leakage, repair it in time, and then carry out fine inspection with high charging pressure and high helium partial pressure. When calibrating the leak detection sensitivity, pay attention to the partial pressure of helium in the atmosphere
If the indication of helium leak detector in the atmosphere is greater than 30% of that indicated by the leak detector when helium passes through the standard leak hole, it is necessary to reduce the helium partial pressure in the atmosphere, otherwise, it is of no practical value to continue the leak detection. Finally, although increasing the internal helium charging pressure can improve the leak detection sensitivity, it is necessary to pay attention not to exceed the allowable value of strength or stiffness of the tested part, and pay attention to Helium recovery and ventilation of the site.