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Leak detection knowledge

Detection principle, advantages and disadvantages and detection standards of four common helium mass spectrometry leak detection methods

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Leak detection by vacuum helium mass spectrometry


When vacuum leak detection is used, the auxiliary vacuum pump or leak detector should be used to vacuumize the internal sealing chamber of the inspected product, and helium gas is applied to the outer surface of the inspected product by helium cover or injection. When there is a leak on the surface of the inspected product, the helium will enter the internal part of the inspected product through the leak hole, and then enter the helium mass spectrometer leak detector, so as to realize the leakage

measurement of the inspected product. According to the different methods of leaking gas, vacuum method can be divided into vacuum injection method and vacuum helium cover method. Among them, the vacuum injection method uses the spray gun to inject helium to the outer surface of the inspected product, which can realize the accurate location of the leakage hole; the vacuum helium cover method uses a helium cover with certain sealing function to cover all the inspected products, and fill the cover with a certain concentration of helium, which can realize the measurement of the total leakage rate of the inspected product.


The advantages of vacuum method are high sensitivity, accurate positioning and leak detection for large vessels or complex products.


The disadvantage of vacuum method is that it can only detect the leakage rate of one atmospheric pressure difference, and can not accurately reflect the real leakage state of the product under pressure.


The detection standards of vacuum method mainly include qj3123-2000 helium mass spectrometry vacuum leak detection method and GB / T 15823-2009 helium leakage inspection, which are mainly applied to products with vacuum sealing performance requirements but without pressure, such as space moving parts, liquid hydrogen tank cars, environmental simulation equipment, etc.


Leak detection by positive pressure helium mass spectrometry


When the positive pressure method is used for leak detection, the inner seal chamber of the inspected product needs to be filled with helium at a higher atmospheric pressure. When there is a leak on the surface of the inspected product, the helium will enter the surrounding atmospheric environment of the external surface through the hole, and then the helium concentration increment in the surrounding atmospheric environment of the inspected product is detected by the suction gun, so as to realize the leakage measurement of the inspected product. According to the different ways of collecting helium, the positive pressure method can be divided into positive pressure suction gun method and positive pressure accumulation method. The positive pressure suction gun method uses the leak detector suction gun to scan and detect the outer surface of the inspected product, which can realize the accurate location of the leakage; the positive pressure accumulation method uses the helium cover with certain sealing function to cover all the tested products, and uses the leak detector suction gun to measure the helium concentration change in the helium cover before and after a certain period of time, so as to realize the accurate measurement of the total leakage rate of the inspected product.


The advantage of the positive pressure method is that it does not need an auxiliary vacuum system, and can accurately locate and detect under any working pressure.


The disadvantage of the positive pressure method is that the detection sensitivity is low, the uncertainty of the detection results is large, and it is greatly affected by the environmental conditions.


The detection standards of positive pressure method mainly include qj3089-1999 helium mass spectrometry positive pressure leak detection method, qj2862-1996 test method for helium mass spectrometer suction gun cover box of pressure vessel welds, which are mainly used for leak detection of large volume and high-pressure closed container products, such as high-pressure helium cylinder, cabin door leak detector, etc.


Leak detection by vacuum pressure helium mass spectrometry


When the vacuum pressure method is used for leak detection, it is necessary to put the inspected product into the vacuum sealing chamber as a whole. The vacuum sealing chamber is connected with the auxiliary evacuation system and leak detector. The inflation interface of the inspected product is led out of the vacuum sealing chamber through the connecting pipe, and then connected with the helium gas source. When there is a leak on the surface of the inspected product, helium will enter the vacuum sealing chamber through the leakage hole, and then enter the helium mass spectrometer for leak detection In order to measure the total leakage rate of the inspected products.


The advantage of vacuum pressure method is that it has high detection sensitivity, which can detect the leakage rate of any working pressure and reflect the real leakage state of the inspected part.


The disadvantage of the vacuum pressure method is that the leak detection system is complex, so it is necessary to design the vacuum sealing chamber according to the volume and shape of the inspected product. It needs to be explained here that in the leak detection process, it is required to ensure that there is no leakage at the gas filling pipe interface, or special structural design shall be adopted to place all the gas filling pipe connection interfaces outside the vacuum sealing chamber.


The detection standard of vacuum pressure method is helium leakage inspection (GB / T 15823-2009), which is mainly used for sealing products with simple structure and not particularly high pressure, such as solenoid valve, high-pressure inflation pipeline, propellant tank, antenna, transponder, whole satellite products, etc.


Leak detection by helium mass spectrometry with back pressure method


When the back pressure method is used for leak detection, the tested product is first placed in a high-pressure helium chamber and soaked for hours or days. If there is a leak on the surface of the inspected product, helium will be pressed into the internal sealing chamber of the inspected product through the leakage hole, so that the helium partial pressure in the internal sealing chamber will rise. Then take out the inspected product, blow off the residual helium on the surface, and then put the inspected product into the vacuum container connected with the leak detector. The helium in the inner sealing cavity of the inspected product will leak to the vacuum container through the leakage hole, and then enter the helium mass spectrometer leak detector, so as to realize the total leakage rate measurement of the inspected product. The leakage rate value given by the leak detector is the measured leakage rate, and the equivalent standard leakage rate of the inspected product needs to be calculated by conversion formula.


The advantage of back pressure method is high detection sensitivity, which can realize the leakage detection of small sealed container products, and can carry out batch detection.


The disadvantage of back pressure method is that it can't leak large sealed vessels, otherwise the pressure time will be too long due to the large volume of sealing chamber. In addition, each measurement leakage rate corresponds to two equivalent standard leakage rates. After the fine inspection, other methods should be used for rough inspection to eliminate the possibility of large leakage.


The leak detection standards of back pressure method mainly include qj3212-2005 helium mass spectrometry back pressure leak detection method, GJB360A-1996 test methods for electronic and electrical components 112 sealing test, which are mainly used for leak detection of various electronic components.

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