Helium mass spectrometry leak detection method accumulation method
In the process of helium mass spectrometry leak detection, if the suction gun method introduced by the vacuum pump manufacturer can not meet the requirements due to the small leakage rate, another method appears, that is the accumulation method.
The sensitivity of the cumulative method is very high. When used correctly, it can be two orders of magnitude higher than the sensitivity of the instrument. Therefore, it is very suitable for the leak detection of helium mass spectrometry when the leak rate and outgassing rate are small. There are several kinds of accumulation methods, including helium boosting accumulation method, helium cap peak accumulation method and nozzle peak accumulation method.
In the helium boosting accumulation method, the sample is vacuumized, and the pumping speed control valve is closed; the helium in the helium cover enters the mass spectrometer chamber through the leakage hole and accumulates gradually, so that the helium partial pressure is continuously increased, so as to improve the leak detection sensitivity. The leakage rate of the tested part is obtained by calculating the cumulative time, the change of the output value of the leak detector within the accumulated time, the system volume from the pumping valve to the inspected part, and the partial pressure sensitivity of the leak detector.
The peak accumulation method of helium hood is similar to that of helium boosting accumulation method, but a fast valve is added between the mass spectrometer chamber and the sample. After vacuumizing, the fast valve is also closed. The helium leakage from the helium cover to the tested part will accumulate in the tested piece first. After the accumulation reaches a certain time, the fast valve will be opened. At this time, the helium flowing into the mass spectrometer chamber of the leak detector will increase significantly, thus improving the leak detection sensitivity. Its final leakage rate calculation is different from that of helium boost accumulation method, involving three parameters, including leak detector calibration of peak method, maximum variation of leak detector output and accumulation time.
In other words, the helium is accumulated in the leak detection chamber through the accumulation time. Then open the nozzle valve, and the helium with high partial pressure will flow into the leak detector through the suction nozzle, which improves the leak detection sensitivity. The peak accumulation method of nozzle can calculate the final leakage rate through the comparison method of standard leakage. The parameters involved include the nominal leakage rate of the standard leak and the peak value variation of the detector output between the tested piece and the standard leak under the same detection conditions.
No matter what kind of accumulation method is adopted, the working state of the leak detector should be kept stable to prevent the change of instrument noise and working pressure. At the same time, the rubber, grease, plastic and other materials which are easy to absorb helium should be used less or not in the leak detection system. In the accumulation time, it is necessary to ensure that the leak detector can obtain obvious signal output, and for some materials with helium permeability, the time should not be too long.