Four commonly used methods for leak detection of high-purity helium gas by mass spectrometry
Helium, high-purity helium
The helium mass spectrometer leak detection method uses the helium partial pressure measurement principle of the helium mass spectrometer leak detector to measure the helium leakage of the tested object. When there is a leak on the sealing surface of the inspected part, high-purity helium gas and other components of the gas will leak out from the leak. After the leaked gas enters the helium mass spectrometer leak detector, due to the selective recognition ability of the helium mass spectrometer leak detector, only the partial pressure signal value of high-purity helium gas in the gas is given.
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Based on the different storage positions of the leak gas during the leak detection process and their relationship with the tested object, helium mass spectrometry leak detection methods can be divided into vacuum method, positive pressure method, vacuum pressure method, and back pressure method. The following summarizes the detection principles, advantages and disadvantages, and detection standards of these four helium mass spectrometry leak detection methods.
Helium mass spectrometry leak detection positive pressure method is primarily used for leak detection of large volume and high-pressure sealed container products, such as high-pressure high-purity helium cylinders, cabin door leak detectors, etc. Positive pressure method is a vacuum system that does not require assistance and can accurately locate and complete detection under any working pressure. However, this method has low detection sensitivity, high uncertainty in detection results, and is greatly affected by measurement environmental conditions.
When using the positive pressure method for leak detection, it is necessary to fill the sealed chamber inside the tested product with high-purity helium gas at a pressure higher than one atmosphere. When there is a leak on the surface of the tested product, the high-purity helium gas will enter the surrounding atmosphere through the leak hole. Then, a suction gun is used to detect the increase in high-purity helium gas concentration in the surrounding atmosphere of the tested product, and the leakage measurement of the tested product is completed. According to the different methods of collecting high-purity helium gas, the positive pressure method can be divided into positive pressure suction gun method and positive pressure accumulation method. During this process, the positive pressure suction gun method uses a leak detector suction gun to scan and explore the outer surface of the inspected product, which can accurately locate the leakage hole; The positive pressure accumulation method uses a helium cover with a certain sealing function to cover all the tested products, and uses a leak detector suction gun to measure the change in high-purity helium concentration inside the helium cover before and after a certain period of time, completing the accurate measurement of the total leakage rate of the tested products.
Helium mass spectrometry leak detection back pressure method is primarily used for leak detection of various electronic component products. This method has high detection sensitivity and can complete leak detection of small sealed container products, enabling batch testing. But leakage of large sealed containers cannot be carried out, otherwise the pressurization time will be too long due to the large volume of the sealed chamber. In addition, each measurement leakage rate corresponds to two equivalent standard leakage rates. After the detailed inspection is completed, other methods need to be used for rough inspection to eliminate the possibility of major leakage.
When using the back pressure method for leak detection, the first step is to place the tested product in a high-pressure high-purity helium chamber and soak it for several hours or days. If there are leakage holes on the surface of the tested product, high-purity helium gas will be pressed into the internal sealing chamber of the tested product through the leakage holes, causing the helium partial pressure in the internal sealing chamber to rise. Then take out the inspected product, blow off the remaining high-purity helium gas on the surface, and place the inspected product in a vacuum container connected to the leak detector. The high-purity helium gas inside the sealed chamber of the inspected product will leak into the vacuum container through the leak hole, and then enter the helium mass spectrometer leak detector to complete the measurement of the total leakage rate of the inspected product. The leakage rate value given by the leak detector is the measured leakage rate, and it is required to calculate the equivalent standard leakage rate of the tested product through a conversion formula.
Helium mass spectrometry leak detection vacuum method is primarily used for products that require vacuum sealing function but do not work under pressure, such as space moving parts, liquid hydrogen tankers, environmental simulation equipment, etc. The vacuum method has high sensitivity for detection, can accurately locate, and can complete leak detection of large containers or disordered structural products. But it can only complete the leakage rate detection of one atmospheric pressure difference, and cannot accurately reflect the actual leakage situation of the product under pressure.
When using the vacuum method for leak detection, it is necessary to use an auxiliary vacuum pump or leak detector to evacuate the sealed chamber inside the tested product. High purity helium gas should be applied to the outer surface of the tested product using a helium cover or spray. When there is a leak hole on the outer surface of the tested product, the high-purity helium gas will enter the interior of the tested product through the leak hole and then enter the helium mass spectrometer leak detector to complete the leak measurement of the tested product. According to the different methods of gas leakage, vacuum method can be divided into vacuum injection method and vacuum helium hood method. The vacuum spraying method uses a spray gun to spray high-purity helium gas onto the outer surface of the inspected product, which can accurately locate the leakage hole; The vacuum helium hood method uses a helium hood with a certain sealing function to cover all the tested products, and fills the hood with a certain concentration of high-purity helium gas, which can measure the total leakage rate of the tested products.
Helium mass spectrometry leak detection vacuum pressure method is primarily applied to sealed products with simple structures and low pressure, such as solenoid valves, high-pressure inflation pipelines, propellant storage tanks, antennas, responders, and whole satellite products. This method has high detection sensitivity and can detect the leakage rate of any work pressure, reflecting the actual leakage situation of the inspected item. However, this leak detection system is chaotic and requires the design of a vacuum sealed chamber based on the volume and shape of the tested product. During the leak detection process, it is required to ensure that there is no leakage at the inflation pipeline interface, or to adopt a special structural design to place all inflation pipeline connection interfaces outside the vacuum sealed chamber.
When using the vacuum pressure method for leak detection, it is necessary to place the inspected product as a whole in a vacuum sealed chamber, which is connected to the auxiliary vacuum system and leak detector. The inflation interface of the inspected product is led out of the vacuum sealed chamber through a connecting pipeline, and then connected to a high-purity helium gas source. When there is a leak on the surface of the inspected product, high-purity helium gas will enter the vacuum sealed chamber through the leak hole, and then enter the helium mass spectrometer leak detector to measure the total leakage rate of the inspected product.
