February 19, 2020

Study on the Delay and Hold Characteristics of Leakage Gas in Vacuum Mixing Leakage

Study on the Delay and Hold Characteristics of Leakage Gas in Vacuum Mixing Leakage
The core tips: If there is a mixed leak in the vacuum container, even if the leak rate reaches 1X103P, L/S level, it is impossible to find the leak by using the conventional blowout leak gas. The schematic diagram of the process of leaking the leaked gas is shown in the diagram. The leaked gas enters the mezzanine space from the channel A on the atmospheric side of the wall and undergoes a leakage and pumping.

If there is a mixed leak in the vacuum vessel, even if the leak rate reaches 1X103P, L/S level, it cannot be detected by the conventional blowout leak gas.

The schematic diagram of the process of leaking air from a mixed leaking hole shows that the gas leaks from the channel A on the atmospheric side of the wall into the mezzanine space, and then goes through a balancing process of leaking and pumping, as well as leaking gas and the original mezzanine space. The mixed alternative process of stored air, finally through the channel B into the vacuum chamber. Since both channels A and B are tiny leaks, even if the volume of the sandwich space is small, the pumping balance process will still be very slow. Therefore, the concentration equilibrium time of the leaked gas entering the vacuum chamber becomes the reaction time becomes Very long. Only by using the gas method, it can be ensured that the leaked gas is continuously injected for a long time, so that a certain concentration of leaked gas can be established in the vacuum chamber.

After the gas hood is dismantled, the leakage gas that was originally stored in the leaking channel will also be lost for a long time after the leaked gas is stopped. This means that there is a long retention time. The space and the channel for the actual mixed leaky hole may be cavities, capillaries, or welds, etc. The channels and the interlayer space may be equivalent to a large number, indicating that the leaked gas enters the interlayer space through the leaked hole channel and is then stored in the interlayer space. It takes a certain amount of time for the air phase to mix. In particular, these channels and cavities tend to be under high pressure, and they must rely on slow gas diffusion to mix. This results in the extension of the leakage gas flow. Multi-stage concatenation and delay will create a long total delay time. Obviously, during the delay period, there is basically no leaking gas in the vacuum chamber, so even if a high-sensitivity leak detector is used, no leaking gas can be measured. This kind of long delay characteristic is not found in ordinary leaking holes. The phenomenon is easily overlooked by people. It is mistaken to think that there are no leaks.

The sandwich leaks shown, channels A and B are not in one position, even if A is found, it is difficult to find B. Therefore, it is difficult to repair welding. Blocking the weld A only caused a "dead space" and became a source of air release for the vacuum system.

When the method is closed after the two-compacted method is completed, the ionization vacuum gauge indicates the vacuum degree as KT10a, and the cut-off indication is turned on, and the pressure is increased to UxlO'a. After many times of opening and closing, the stable residual gas mass spectrum shown is obtained, and the solid line is the valve closed. The mass spectrum shows the open spectrum.

The difference between the two is mainly (28) and 02 (32). The other peaks have almost no change in intensity, which proves that the sensor under test actually leaked into the air. The ion flow is a relative value.

A resolution thin film vacuum gauge is used to determine the static pressure time curve. To find the actual leakage rate value, the curve is as shown. The curve shown is basically a good straight line, which also shows that it is caused by the leak factor. After careful study of this straight line, we found that in the initial OA segment, the average leakage rate was 1.63xl (T3PaL/S, while in the termination segment BC, the average value was 1.42xlO3PaL/S. If the sensor under test was first vacuumed If the test is repeated overnight, then the slope of each segment of the straight line is close to the value of the original BC segment.In the vacuum system shown, the total volume of the sensor and the diaphragm meter on the side of the stop valve is 10cm3. The pressure value used is higher, the effect of static deflation can be ignored, and the change of the slope of the curve reflects the process that the original stored air pressure in the interlayer space gradually decreases from the original atmospheric pressure to the dynamic balance value that is leaked and pumped away.

From the residual gas mass spectrum, it can be inferred that the 28 and 32 peaks change significantly, while the residual M=40 peak intensity is very small. If the leak is an ordinary fast-response leak, M=40 is used to blow the leak and the mass spectrometer can fully detect the leak. We also used helium as a leak gas, and the mass spectrometer did not show peaks. In order to further confirm the conclusion that there was no leak gas reaction, after the sensor was removed, the leak was detected with a helium mass spectrometer leak detector. In order to avoid the leakage of the leak, the gas method was used to fill the crucible or argon, because the hood was short and no leak was found.

Through the above experiment, we assume that the sensor's leak is a hybrid leak. Due to this long leak time and delay time, the conventional detection method failed. For this reason, an experiment of injecting leaked gas for a long time using the gas method was performed. The sensor was wrapped in a plastic bag, filled with gas, and the residual gas mass spectrum was measured with a quadrupole mass spectrometer. It was found through experimentation that there was no change in the argon peak of M=40 on the mass spectrometer within 5 hours of argon charging. However, there was a long reaction time measured after the argon gas was charged for the eighth hour although K13 was reported, but the leak was shown in the leak. The delay and hold time of the gas flow have not caused attention.

The long reaction time of the leaky hole only results in a slow increase in leaking gas concentration. If the leak detector sensitivity is high enough, theoretically, the existence of a leak hole can still be quickly found. Only the leak rate value is difficult to calculate. For example, in this experiment, lxl (T3PaL/S large leaks should be measured with a helium mass spectrometer leak detector, because this kind of leaky hole generally has a delay characteristic, during which the leak does not occur. Gas, it can not find the existence of the leak, which is the reason for the failure of the conventional method to leak the mixed leak.

When we use a mass spectrometer to perform a mixed gas ratio experiment, two liter glass bottles are equipped with two kinds of gas, each with a pressure of several thousand Pascals. After being mixed by a 6-glass tube, it takes 3-4 hours. The peak-to-peak ratio of the two gases on the mass spectrum is stable, and is usually stored for 12 hours before being used. This mixing process, which relies on gas diffusion rather than free movement of the molecules under high vacuum, is slow. This experiment is similar to the physical process of mixing leaks studied.

Our study of the delay characteristics of the leaky hole remains to be further studied. There are several unexplained phenomena in our experimental data. First, the rate of decline of the argon peak represented by M=40 is much greater than the rise of air represented by M=29. The second problem is that if the delay time is calculated in 5 hours, the total amount of air stored in the leak hole should be 0.25cm3atm. Obviously, the leak detection sensor is unlikely to have a 0.25cm3 leak volume. The above contradiction may be caused by the leakage gas that we have filled into the air due to loose packing of plastic bags. Also, "5: Other leakage holes can exist, and it is worth further confirmation.

IV. Conclusions The leakage response of mixed leaks due to leaking gas concentrations is very slow, and there is a delay effect. Therefore, the existence of leaks and the value of measured leak rate cannot be ascertained by the conventional leak gas injection method.

In the production of vacuum components, only the mass spectrometer leak detection method is used. It is possible to miss the detection of mixed leaks. It is recommended to use the mass spectrometry contrast method to solve the product's rapid measurement requirements. The mass spectrometer comparison method is to connect the isolating valve to the mass spectrometer vacuum system of the element to be inspected and observe the change of the air mass spectrum peak after the opening and closing of the isolating valve to determine the existence of the leak. Another auxiliary method can also be adopted, that is, storing the batch of components in a vacuum chamber, first pumping the gas, and then filling the leaked gas helium so that there is sufficient time in the leaking hole to remove the original storage air and re-storage leaks. The gas is then taken out one by one, and the helium concentration is measured during the holding period. Due to the long enough holding time, the leakage failure of the leak gas is not detected.

The mixed leaking hole is easy to appear in the complex vacuum sealed structure body, and its long reaction time and attendant delay and retention characteristics are grasped, which will help to find the existence of such leaking hole, thereby improving the reliability of leak detection work. degree. The above comments are for leaking workers and discussion.

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