helium mass spectrometry leak detection common methods
There are many leak detection methods of helium mass spectrometry, which can be divided into two types according to the measurement purpose of the tested piece, one is the leakage point type, the other is the leakage rate type; in the actual inspection process, the most reasonable method should be selected according to the purpose of the inspection, and various leak detection methods should be flexibly applied according to the specific conditions of the tested device.
Determination of leakage type
The determination of leakage point type is not only to determine the specific leakage point or leakage location of components to be inspected, but also common in large parts or large parts, such as satellite, missile body, warhead, gas pipeline, gas tank, oil tank, boiler, etc.
This is the most commonly used method. It is generally used to detect parts with relatively small volume, and connect the device to the instrument. After vacuumizing, spray helium with a spray gun where there may be a leak in the device to be tested (such as sealing joints, welds, etc.). If there is a leak in the tested device, when helium is sprayed on the leak hole, the helium will be immediately sucked into the vacuum system and diffused into the mass spectrometer room, The output of helium mass spectrometer leak detector will respond immediately. When using this method, attention should be paid to: helium is a light inert gas, which will rise automatically after being ejected. In order to accurately inject helium at the leak location, helium injection should be from top to bottom and from near to far (relative to the position of leak detector). This is because helium may be inhaled by the upper leak when spraying below, so it is difficult to determine the location of the leak; In addition, the distance between the leak hole and the mass spectrometer chamber is also different, so helium injection should start from the side near the leak detector and proceed from near to far.
When testing large parts, vacuum pre pumping with mechanical pump can improve the efficiency and time of leak detection. Helium injection method is used to inspect small containers with complicated structure, more sealing openings and welding seams, and are squeezed together. Because helium will diffuse quickly after the helium is ejected, it is not easy to accurately determine the position of the leakage, so helium injection should be taken from different angles, By carefully observing the difference in reaction time and temporarily sealing the found leaks with vacuum sealing mud, the leakage can be detected one by one.
Helium absorption method
Helium absorption method is mainly used to check some large sealed containers. For example, for missile body, warhead, gas tank, oil tank, etc., vacuum the container first, and then fill the container with helium (low concentration helium can be used to save the amount of helium). A rubber tube is connected to the inlet flange of the helium mass spectrometer leak detector. The front end of the rubber tube has a capillary tube with a small diameter, which makes the capillary move at the welding seam and sealing joint of the helium filled container, Helium seeping through the leak will be sucked in by the capillary tube, and the leak detector will respond.
Determination of leakage rate type
The leak rate type is mainly used to detect the components with strict sealing requirements, such as spacecraft, rocket liquid fuel storage tank, satellite, electronic components, etc. This method can only test the leakage rate of the specimen, but can not determine the location and number of leaks.
Helium filling method
Helium filling method is also called negative pressure method. As shown in Fig. 7, a closed gas chamber is connected with vacuum pump and helium tank. The parts to be inspected are put into a gas chamber, and the valve of helium tank is closed. The valve of vacuum pump is pumped into vacuum by mechanical pump, then the valve of vacuum pump is closed, and the valve of helium tank is opened to fill the chamber with helium, and it is left for a period of time. If there is a leak in the small device surrounded by helium, the helium will slowly fill into the small device. Then open the air chamber, take out the tested device, and blow off the helium that may be adsorbed on its outer surface with compressed air.
Then put these devices one by one (or in groups) into the containers connected to the inlet flange of the leak detector. After the container is evacuated, the throttle valve is opened. At this time, the helium originally entering the device under test will be discharged through the leakage hole, and the leak detector will give a leakage rate response. If the helium mass spectrometer leak detector is used, the instrument can directly give the digital display of leakage rate. In this way, the satellite can be put into a large annular mold chamber. The vacuum system of the ring mold equipment is used to pump the ring mold into a vacuum. First, the auxiliary pump is used for rough pumping and pre pumping. In the leak detection, the satellite and the gas filling pipe are vacuumed by mechanical pump, and then helium is filled into the satellite. The pressure is consistent with the working pressure of the satellite, Finally, turn on the helium mass spectrometer leak detector and calibrate the instrument with the standard leak, and then the leakage rate of the satellite can be measured directly.