Helium detection, the full name of helium mass spectrometry leak detection, its core is to use of mass spectrometry to measure the partial pressure of helium. Helium is used as the exploration gas during leak detection. When helium enters the leak detector through the leak hole, the helium in the instrument The partial pressure of helium increases significantly, which indicates the existence of leaks. Due to its high leak detection sensitivity, fast response and strong adaptability, helium detection has been widely used in the leak detection of vacuum products.
When using a helium mass spectrometer leak detector for leak detection, due to the different structure, size and air tightness requirements of the tested parts, the connection method between the tested part and the leak detector and the method of applying helium to the tested part are also different. Different, thus producing a variety of specific leak detection methods.
1. Injection method, the tested part is connected with the helium mass spectrometer leak detector through the pipe, and the inside of the tested part is evacuated. During leak detection, use a spray gun connected to the helium gas source to spray helium gas on the suspected leaking part of the outer wall of the tested part. If there is a leak, the helium gas will enter the leak detector through the leak hole and be detected.
2. Helium cover method and helium chamber method, cover a certain part of the inspected part with a cover or put the whole inspected part in a helium chamber, and fill helium into the cover or helium chamber, which is equivalent to using Numerous spray guns spray helium at countless suspicious points at the same time. This not only speeds up the leak detection speed, but also increases the helium application time due to the increased helium concentration at the leak hole, thus greatly improving the leak detection sensitivity. However, the helium cover method and the helium chamber method can only measure the total leak rate value of the covered part, but cannot determine the location of the leak hole.
3. Vacuum chamber method, place the inspected piece in a vacuum chamber, the vacuum chamber is connected to the leak detector, and the inspected article is connected to the helium gas source. When the vacuum chamber is evacuated to a certain degree of vacuum during leak detection, a certain pressure of helium or a mixture of helium and air is filled into the object to be inspected. If there is a leak hole on the wall of the object to be inspected, the helium gas will enter the vacuum chamber through the leak hole. , randomly enter the leak detector, and the output meter of the instrument indicates the leak rate value. This method is suitable for leak rate testing of small containers, such as helium vacuum leak detection boxes.
4. Helium inhalation method: After filling the container under inspection with a certain pressure of helium, use a special structure suction nozzle connected to the leak detector through a hose to kiss the outer wall of the container point by point. When there is a leak on the container, the helium will escape through the leak. When the suction nozzle is facing the leak, the helium will be sucked by the suction nozzle together with the surrounding air and sent to the leak detector. The output meter of the leaking meter is indicated. The helium absorption method has been widely used in pressurized leak detection. The mass spectrometry chamber is isolated from the atmosphere by the flow limiting effect of the suction nozzle, so the flow conductance of the suction nozzle is very small, and the helium gas leaked through the leak hole diffuses rapidly to the surrounding space, so what the suction nozzle sucks is only the helium gas leaked from the leak hole. Part of it, so the leak detection sensitivity is low. In addition, because the connecting pipe is relatively long, the flow state in the pipe is generally molecular flow or transitional flow, so the reaction time is relatively long.
5. Leak detection box method. A special leak detection box that fits well with the surface to be tested is buckled on one side of the wall of the object to be tested, and the leak detection box and the wall are sealed with a rubber sealing ring or vacuum mud. The leak detection box is connected with the leak detector and the auxiliary pump through the hose. When leak detection, first use the auxiliary pump to evacuate the leak detection box, then close the auxiliary pump valve, and open the throttle valve to connect the leak detector with the leak detection box. If there is a leak at the part locked with the leak detection box, helium will enter the leak detection box and the leak detector through the leak hole, and the output instrument of the leak detector will indicate the leak rate value. This method is especially suitable for inspected parts that cannot be pumped and pressurized. It is often used in leak detection of puzzle welds in the process of large container processing. The leak detection box and the wall of the tested device are often not well matched. In order to improve the sealing condition, a double-layer sealed leak detection box structure can be used. Since the annular space between the two sealing rings is evacuated, the leakage rate of the inner sealing ring can be greatly reduced.
6. Countercurrent method. Generally, when performing helium mass spectrometry leak detection, the test piece is connected to the high vacuum side of the instrument, and the mass spectrometer chamber is connected to the test piece through a throttle valve. In order to ensure the normal working pressure of the mass spectrometry chamber, not only the main pump of the leak detector is required to have a certain pumping speed, but also the leak rate and degassing rate of the tested parts are also limited. For those inspected parts with a large air leakage rate, it is often impossible to detect the leak because the throttle valve cannot be opened. The countercurrent leak detection method is to connect the tested part to the low vacuum side of the leak detector. Diffusion pumps and turbomolecular pumps have different compression ratios for different gas molecules, and the compression ratio decreases with the decrease of the molecular weight of the pumped gas. Gases with smaller molecular weights such as helium are easy to flow back into the mass spectrometer chamber from the front stage of the main pump.
7. Accumulative method, the conventional vacuum method for leak detection, part of the helium leaked into the leak detection system from the leak hole is pumped away, and part of the helium partial pressure is established on each section of the leak detection system to achieve dynamic balance. In order to increase the partial pressure of helium in the mass spectrometry chamber, a valve can be used to isolate the leak detector from the container under test. The helium leaked from the leak hole is first stored in the volume between the container under test and the valve, and the helium in the cumulative volume The partial pressure increases linearly with time. After accumulating for a period of time, open the accumulating valve, and the accumulated helium will be quickly pumped into the leak detector so that the helium partial pressure in the mass spectrometry chamber will rise sharply, so as to obtain a larger output indication. This is the accumulative method. 8 Back pressure method, for the non-destructive leak detection of some closed containers that can neither be evacuated nor inflated, the back pressure method is generally used. The back pressure method operates in three steps:
an Adding helium, immersing the inspected piece in a high-pressure helium atmosphere for several hours or several days, if there is a leak hole on the inspected piece, helium will enter the inspected piece through the leak hole, so that the Helium partial pressure rises;
b Purification, take out the tested part from the pressurized cover, and blow off the remaining helium on the surface of the tested part with nitrogen or air, during this process, some helium will be lost from the tested part through the leakage hole;
c Leak detection put the inspected part into a vacuum container for leak detection. When the leak hole of the inspected piece is too large, misjudgment may occur when using the back pressure method for leak detection, because the helium in the inspected piece may be exhausted during the waiting time for purification. The inspected parts that are not leaked should be re-inspected by other rough inspection methods.
