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Application of helium mass spectrometry leak detection technology with low helium concentration

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In order to solve the sealing performance detection problem of sealing device which can not be vacuum pumped and pressurized, a helium mass spectrometer leak detection simulation experiment with low helium concentration was carried out. The experimental results of the experiments are as follows: the leakage rate of helium through the mixed gas is measured at 0.5, 1, 3 and 5 of 1000 respectively. Based on the assumption that the mixture gas passes through the molecular flow hole with the same score, the leakage rate results measured at different concentrations are compared with the theoretical calculation; The time required for the free diffusion of helium to the basic uniform distribution is obtained by experiments. On the basis of experiments, a special calibration device for low helium concentration detection can reduce the uncertainty of the results of leak detection due to the large difference between the helium concentration of the standard leak and the helium concentration of the leak detection.

The extremely toxic substances test is carried out in the sealing device. The experimental environment of extremely toxic substances is required to be in the atmospheric pressure state. The sealing device is set with a unique opening. The flange sealing method is adopted. After the experimental device and flange are installed in place, the sealing state of the flange shall be tested and ensured: 1. to ensure the integrity of the sealing device and the sealing reliability of the working state, No air filling and vacuum suction channels are set for leak detection; 2. in order to maintain the atmospheric pressure state in the sealing device, a large number of leakage indicating gases cannot be released inside the sealing device; 3. the leakage detection sensitivity shall be better than 10-7pa.m3/s. The helium mass spectrometry leak detection method is used to control the release of helium gas regularly by using built-in helium source. The increase value of air pressure after helium gas is released in the sealing device is no more than 100 Pa. The influence can be ignored. In this case, the helium concentration in the container is about 1/1000 (volume ratio). Therefore, the relationship between concentration and leakage rate and sensitivity analysis of low concentration tracer gas leak detection were carried out.

1. Theoretical hypothesis of helium mass spectrometry leak detection with low helium concentration


In the leak detection with helium as tracer gas, helium enters the mass spectrometer chamber through the leak hole and enters the mass spectrometer through vacuum channel under the action of pressure difference. The leak detector can measure the helium ion flow signal which is linear with the helium partial pressure through mass spectrometer analysis. The ion flow signal is compared with the calibration signal of the standard leakage (known leakage rate) to calculate the leakage rate. The gas chamber of standard leakage is generally pure helium. In the leak detection, pure helium is generally applied to the inspected part, but the mixture gas containing helium is also used, and the helium concentration is often more than 20%. If the helium concentration is as low as 1 in 1000, there are two hypotheses to accurately determine the leakage rate: one is that a small amount of helium in the helium filled space is evenly distributed in the mixed gas; the other is that helium passes through the leakage hole with the same score, that is, the helium concentration in the gas leaked from the leakage hole is consistent with the theoretical concentration of helium in the mixed gas. Under this assumption, there are

2. Conclusion

(1) As long as enough waiting time is guaranteed, the leakage rate has quantitative relationship with the concentration under the condition of low helium concentration.

(2) The sensitivity of the standard leak calibration system with 1 / 1000 concentration standard gas is used to make the calibration state basically consistent with the leak detection condition, and further reduces the uncertainty of the leak detection results due to the difference between the helium concentration of the standard leakage and the helium concentration of the leak detection.

(3) For the overall leakage detection of the inspected parts that cannot be under pressure, the low concentration built-in helium source method can be considered for detection; in the leak detection, the helium concentration can be properly reduced according to the technical specifications, and the resources can be saved.


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