JPH0159531B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a leak detection device. Specifically, the present invention relates to a leakage detection device used for leakage testing of airtight containers, piping, etc., and in particular, a probe gas is pressurized and sealed inside a test object, and the probe gas flowing out from the leakage point to the atmosphere is collected by a probe nozzle. (Snitzhua)
This invention relates to a highly sensitive leak detection device that detects leaks by inhaling them together with the atmosphere.

Recently, vacuum devices such as fusion devices and accelerators have become larger and more complex in structure. There have been significant advances in welding technology for manufacturing vacuum containers and sealing technology using metal gaskets, etc.
At present, it is difficult to completely eliminate leakage, and leakage testing has become an indispensable process when manufacturing vacuum containers. Furthermore, even after the device is completed, it is not uncommon for leaks to occur due to stress fatigue, corrosion, etc. of the material. For this reason, it has become necessary to conduct leakage tests on large, complex-shaped devices on-site, where there are many restrictions.

These leak tests are based on the premise of repairing or replacing the leak, so they are meaningless unless the leak can be identified to a few square centimeters (area) or several centimeters (length). This is relatively easy in the case of small vacuum containers, but generally takes a long time and a lot of effort in the case of large vacuum equipment, so increasing this efficiency is extremely important.

Probe gas is usually used when searching for minute leaks of 10 ^-5 Torr./s (~ ^{10 -5} cm ³ (NTP)/s) or less. Probe gas methods can be broadly divided into internal vacuum methods and internal pressurization methods. In the former method, the inside of the test object (vacuum container) is evacuated using a vacuum pump, and a probe gas (for example, helium) is partially sprayed from the outside of the test object to search for leaks. When the probe gas is blown, the probe gas flows into the vacuum container depending on the size of the leak, and is sensed by a detector (for example, a helium detector) attached to the vacuum container. The latter is the opposite of the former, in that a probe gas of several atmospheres is sealed inside the test object (vacuum container), and the probe gas that has leaked into the atmosphere from the leak is sucked in through the probe nozzle (sniffer) along with the atmosphere. The purpose is to detect. Both methods have their own advantages and disadvantages, but especially when the internal vacuum method is applied to leak testing of large equipment, there are problems in that equipment such as vacuum evacuation equipment is bulky and it takes a considerable amount of time to start the test. On the other hand, in the internal pressurization (sniffer) method, the leak detection device is simple and easy to use regardless of the size of the test object, but the sensitivity is relatively low, and the sensitivity is 10 ^-7 Torr./s (~
Detecting minute leaks of less than 10 ^-7 cm ³ (NTP)/s was difficult.

In view of these points, the inventors of the present invention have proceeded with the study of increasing the sensitivity of the internal pressurization (sniffer) method, and as a result, they have developed a probe nozzle (sniffer) and probe gas detection using one of helium, neon, or hydrogen as the probe gas. By inserting in series in this order a long capillary tube that maintains a constant flow rate and a container filled with porous adsorbent that can be cooled to below the liquid air temperature in the middle of the tube, the method is more sensitive than the conventional internal pressurization (sniffer) method. They also found that it can be increased by about 10,000 times.

The present invention was made based on this knowledge, and has a simple configuration and is easy to use.
The present invention provides a leak detection device that can detect extremely small leaks of 10 ^-10 Torr./s ( ^{~10 -10} cm ³ (NTP)/s).

To explain the present invention in detail, this involves sealing a probe gas such as helium of several atmospheres inside a test object such as a large vacuum container, and then using a probe nozzle (sniffer) to collect the probe gas that has leaked out into the atmosphere from a leakage point. This is a device that detects the air that is inhaled from the air along with the air, and is generally constructed as follows.
For example, as shown in Fig. 1, a flexible stainless steel capillary tube 2 with an inner diameter of about 0.6 mm, an outer diameter of about 1.1 mm, and a total length of 10 m is connected to a probe nozzle (sniffer) 1 that is open to the atmosphere. It is connected by a flange 3 to one end of a container 5 filled with molecular sieves 4, and the other end of the container 5 is connected via a throttle valve 6 and a liquid nitrogen cooling trap 7 to an inner diameter of 20 mm.
It is connected to a diffusion pump 8 through a pipe of about 50 mm, and the diffusion pump 8 is further connected to an oil rotary pump 9. The container 5 filled with molecular sieves can be cooled with liquid nitrogen contained in a thermos flask 10, and near the liquid nitrogen cooling trap 7 there is a total pressure gauge 11 that is sensitive only to probe gases such as helium. Mass spectrometry tube 12 adjusted to
is installed. 13 is an electronic device for displaying the partial pressure of the probe gas in the mass spectrometry tube 12 on a meter or recorder. In addition, 14 is a hand-held opening/closing mechanism, 15 is a safety valve that opens automatically when the pressure inside the container 5 rises, and 16 is a housing that houses the main body of the leak detection device.

The principle of the present invention will be explained according to the configuration shown in FIG. Normally, air is sucked in through the probe nozzle (sniffer) 1, but as shown in Figure 2, when the tip of the nozzle approaches a location where probe gas such as helium is leaking, the probe gas leaking along with the air can be seen. Some of it is also inhaled. The long capillary tube 2 plays the role of keeping the amount of air inflow constant and allowing the probe nozzle to move freely. In this example, the air inflow rate is approximately 0.6 Torr./s (0.8 (NTP)/s).
Most of the suction gas that has reached the container 5 filled with molecular sieves cooled with liquid nitrogen, except for helium, neon, and hydrogen, is adsorbed and exhausted by the molecular sieves. That is, almost 100% of helium, neon, etc., passes through the container 5 and reaches the liquid nitrogen cooling trap 7 and diffusion pump 8, but less than 1% of the main components of the atmosphere, such as oxygen and nitrogen. As a result, the concentration of the probe gas is relatively increased in the mass spectrometry tube section, and the sensitivity is increased about 100 times compared to when the container 5 is not used. The time it takes the probe gas to reach the mass spectrometer tube from the probe nozzle is called the time constant of the measurement system, and is an important value that determines the moving speed (probing speed) of the probe nozzle. In this example, the time it takes to pass through the capillary tube 2 is about 3 seconds, and the time it takes to reach the mass spectrometer tube 12 from the container 5 is 5 seconds.
The time constant is short, about seconds, about 8 seconds in total, and this time delay does not pose a problem in practice.

Fig. 3 shows a 5.5x10 ^-9 Torr using the device of the present invention.
This is the result of measuring a known helium leak of /s. In this case, in order to reduce the influence of the helium background (approximately 5 ppm) present in the atmosphere, the test object was covered with a polyethylene bag as shown in Figure 4, and the atmosphere was replaced with pure nitrogen. . It can be seen that when the probe nozzle is brought closer to the leak, the reading on the recorder immediately increases, and when it is moved away, it immediately decreases. Since the zero point indication is stable and the S/N ratio is good, this measurement yields 10 ^-10 Torr./s ( ^{~10 -10} cm ³
(NTP)/s) (corresponding to one scale on recording paper) is considered to be sufficient.

By the way, in the conventional internal pressurization method using a probe nozzle and a helium leak detector, the minimum detectable probe gas content in the atmosphere taken in from the probe nozzle was 1 to 10 ppm. Using the device of the present invention, this was reduced by about 4 orders of magnitude to 0.1 to 1 ppb. In addition, in the conventional method, the air inflow rate required to suck a large proportion of the leakage into the probe nozzle is 1x10 ^-4 Torr./s ( ^{~1x10 -4} cm ³
(NTP)/s), but by using the device of the present invention, it can be increased to about 1 Torr./s (~1 cm ³ (NTP)/s), which is about 10,000 times. In the end, the conventional internal pressurization method had a practical limit of detecting leaks of 1x10 ^-6 Torr./s, but the device of the present invention has made it possible to detect leaks of 1x10 ^-10 Torr./s.

There are three main reasons why the device of the present invention exhibits the above-mentioned excellent characteristics.

(1) A long flexible capillary tube was used. In the conventional method, the nozzle has an extremely small opening, making machining accuracy and operation of this part extremely difficult, but the use of a capillary tube stabilizes the suction flow rate and makes leak detection extremely easy. Stabilization of the flow rate contributes to reducing the minimum probe gas content by two orders of magnitude to 10 to 100 ppb.

(2) A container filled with porous adsorbent cooled to below liquid air temperature is placed between the capillary tube and the gas detection tube. Since the porous adsorbent selectively adsorbs and exhausts oxygen and nitrogen, which are the main components of the atmosphere, the concentration of the probe gas in the detection tube section can be relatively increased by about two orders of magnitude.

(3) Optimal design that takes full advantage of gas flow properties. The gas flow rate through the capillary varies depending on the inner diameter and length of the capillary. In the range where the flow within the pipe conforms to the conditions of viscous flow, the flow rate is proportional to [inner diameter] ⁴ /[length], but this value is preferably set in the range of 1 to 0.1 Torr./s. Further, the flow delay time (time constant) is proportional to [length] ² / [inner diameter] ² , but it is desirable to set this value to 5 seconds or less. It is also necessary to ensure that the Reynolds number of the gas passing through the flow path does not exceed 1200. In order to satisfy all of these conditions, the inner diameter of the capillary tube must be 0.75 mm or less.

In addition to molecular sieves, various activated carbons and silica gels can also be used as the porous adsorbent.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of one specific example of the leak detection device of the present invention. FIG. 2 is a diagram (part 1) for explaining the leakage detection method. In the figure, 17 is a probe gas, 18 is a test object, and 19 is a leak location. FIG. 3 shows that by the device of the present invention,
1 is a graph showing the results of measurements made using a known helium leak of 5.5x10 ^-9 Torr./s. Figure 4 is a diagram explaining the leak detection method (Part 2)
It is. In the figure, 18 is a test object, 20 is a leak location, 21 is a polyethylene bag, and 22 is sealed nitrogen.

Claims (1)

[Scope of Claims] 1. A probe nozzle (sniffer) open to the air or gas atmosphere, a long capillary tube, a container filled with a porous adsorbent that can be cooled to a temperature below liquid air temperature, helium, neon, A leak detection device characterized in that a gas detection tube that detects one or more hydrogen gases and a vacuum evacuation device are connected in series in this order. 2. The leak detection device according to claim 1, wherein the long capillary tube is flexible. 3. The leak detection device according to claim 1, wherein the capillary tube has an inner diameter of 0.75 mm or less. 4. The leak detection device according to claim 1, wherein the porous adsorbent is a molecular sieve.