Helium leak detection techniques

There are four different approaches to helium leak detection. They are displayed in Table 7.17. A graphical representation of these techniques is shown in Fig. 7.59 through Fig. 7.61. 

Some of the other leak detection methods explained earlier in this section are based on these techniques. Methods such as bagging parts of a vacuum system and filling the bagged parts with a probe gas are similar to the outside-in technique. However, it is not possible to quantify a leak with this approach. Alternatively, filling a container with gas and either submerging it in a liquid or covering it with a bubbling solution is similar to the detector-probe technique. 

When examining a vacuum system for leaks in the laboratory, calculating the size of a leak is often unnecessary for two reasons  

The techniques used to find the size of a leak entail the use of the inside- 

Although on occasion it may be useful to know the relative size of one leak to another, the specific size of a leak is irrelevant. If the leak is large in relation to the standard leak, it should be repaired. On the other hand, if the leak is close to the size of the standard leak (and your system does not get below 10 7 torr), there is little reason to repair the leak. 

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Where a vacuum is present inside the test specimen (p < 1 mbar), atmospheric pressure outside, and helium is used at the test gas, one refers to standard helium conditions. Standard helium conditions are always present during helium leak detection for a high vacuum system when the system is connected to a leak detector and is sprayed with helium (spray technique). If the specimen is evacuated solely by the leak detector, then one would say that the leak detector is operating in the direct-flow mode. If the specimen is itself a complete vacuum system with its own vacuum pump and if the leak detector is operated in parallel to the system s pumps, then one refers to partial-flow mode. One also refers to partial stream mode when a separate auxiliary pump is used parallel to the leak detector. 

Leak detection techniques using helium leak detectors... 

Although there are other leak detectors and leak detection techniques that rely on detection of a change in gas, none have enjoyed the success of the mass spectrometer tuned to helium. There are other types of tuned mass spectrometers that specifically look for oxygen or halogen, but they are not as common. Their opera-... 

Aside from permeability and absorbency complications, other universal concerns of helium leak detection are factors such as source operating pressure, spraying patterns (for tracer-probe technique), response time, clean-up time, and cold trap usage. Pump use and general helium leak detector maintenance operations are also fairly universal. 

Fig. 7.60 Using the inside-out technique, the piece in question is filled with helium and then placed in a covering of some type that can be evacuated. Any helium that is released within this covering will be detected by the helium leak detector. Note that the tested piece can have either end lifted up so that all surfaces are exposed to the vacuum. From Introduction to Helium Mass Spectrometer Leak Detection, Figs. 3.3 and 3.4, by Varian Associates, Inc. 1980, reproduced with permission.

The source operating pressure is the vacuum necessary to operate the leak detection device. This pressure is not specific, rather it is a pressure range within the leak detector which works. Optimistically, we want the helium leak detector, and the system to which it is connected, to have the greatest possible vacuum. This gives the tracer-probe technique the maximum sensitivity with the quickest response time. As an added benefit, when one is operating at a very high vacuum,... 

Most helium leak detectors will not operate with pressures above lO"4 torr to 10 5 torr. At these greater pressures, the main element to the mass spectrometer will bum out. Fortunately most, if not all, helium leak detectors have various safety check mechanisms that automatically shut off the current to the main filament if the pressure goes above a set limit. So, you must depend on alternate leak detection methods, or use the detector-probe technique to discover large leaks. Once large leaks have been discovered and closed, you can concentrate on the smaller leaks that can be found with the tracer-probe technique. 

Fig. 7.59 In the detector-probe technique, the tested piece is filled with helium and the sniffer probe sniffs the areas in question to detect leaks. Always sniff from bottom to top.

Chemical Reactions. Products from gas-phase chemical reactions can also be trapped in rare gas matrices, and those products which absorb light can be studied by optical spectroscopic techniques. For example (31), the products from a low pressure 1 mm. of Hg) atomic flame of oxygen atoms plus acetylene were allowed to leak through a small oriflce in a borosilicate glass reaction chamber, where they were mixed with an excess of gaseous krypton at 1(H mm. of Hg pressure. The mixture was condensed on a quartz window cooled to liquid helium temperature. The only detectable small free radical found was HCO, but it was present in considerable quantities. Similar experiments by Harvey and Brown (23) showed that HNO could be easily produced and trapped from the gas-phase reaction of hydrogen atoms plus nitric oxide. 

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