Leak detector spray

Leak detector spray. After pressurisation with air, point leaks can be visualised by the application of a low surface tension liquid which forms bubbles where the gas is emitted. 

Uses Detergent for carpet and upholstery cleaners esp. aerosols leak detector spray... 

Chamber with a leak 2, helium leak detector 3, pressure bottle with helium 4, vacuum pumpsystem, evacuating the chamber 5, helium spray pistol (Fig. 3 from [2.26]). 

Fig. 2.36. Helium leak detector in operation at an industrial vacuum plant. The leak detector stands behind the hand rail, the flange is sprayed with He, the mobile indicator is held in the left hand (photograph Balzers und Leybold Holding AG, D-63450 Hanau).

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. 

Today leak tests for vacuum systems are usually carried out with helium leak detectors and the vacuum method (see Section 5.7.1). The apparatus is evacuated and a test gas is sprayed around the outside. In this case it must be possible to detect (on the basis of samplings inside the apparatus) the test gas which has passed through leaks and into the apparatus. Another option is to use the positive-pressure leak test. A test gas (helium) is used to fill the apparatus being inspected and to build up a slight positive pressure the test gas will pass to the outside through the leaks and will be detected outside the device. The leaks are located with leak sprays (or soap suds, 5.4.5) or - when using He or H2 as the test gas - with a leak detector and sniffer unit (5.7.2). 

The fact that the pressure reading at vacuum gauges (see Section 3.3) is sensitive to the type of gas involved can, to a certain extent, be utilized for leak detection purposes. Thus it is possible to brush or spray suspected leaks with alcohol. The alcohol vapors which flow into the device - the thermal conductivity and ionizablity of which will vary greatly from the same properties for air - will affect and change pressure indication to a greater or lesser extent. The availability of more precise, easy-to-use helium leak detectors has, however, rendered this method almost completely obsolete. 

Figure 5.14 shows the course of the signal after spraying a leak in a test specimen attached to a leak detector, for three different configurations ... 

Fig. 2.54. Helium leak detector in operation sprayed with He, the mobile indicator is held...

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. 7.65 A suggested pattern for spraying helium on a vacuum system when using the tracer-probe technique. The pattern is a compromise between proximity to the leak detector and spraying high before low areas.

For example, say your setup takes one minute from the time you introduce helium to a leak until it is acknowledged by the helium leak detector. In addition, say your spray probe is being moved at four inches per minute (which is pretty slow if you think about it). You will therefore be four inches away from a leak when it is first identified. Generally, moving the spray probe at a rate of one foot per minute provides an adequate time response, which of course can be made faster or slower as conditions warrant. However, if you know the response time, you will know about how much to back up to retest the area in question to allow pinpointing the specific leak spot. 

The signal response time is important for the safe localization of leaks. It states the time period after spraying helium on the leak until the leak detector indication reaches a defined level (63% of final equilibrium value). Figure 8.3 shows the typical exponential trace of the signal rise. The response time can be calculated from the parameters of the system under test. The critical factors are the volume of the system and the total pumping speed of the installed vacuum pumps. Typical response times of chemical plant systems are in the range of several seconds up to a few minutes. 

Mass Spectrometers Leak detectors using mass spectrometers are built to measure the flow rate of helium past the sensor. Helium is relatively rare in the air, so it registers only when there is a leak in the system being tested and when helium is being sprayed near the leak. By using a sensitive system with a very fine and slow helium probe, leaks as small as 1 X 10 ° torrL/s can be located. 

Should a leak occur, evacuate personnel from the area, shut off all ignition sources, ventilate the area, and reduce vapors with fog or fine water spray. Shut off the main source of gas supply. Flammable and toxic vapors may spread. The atmosphere should be checked with an appropriate detector while using self-contained breathing apparatus. For large leaks, personnel must wear special personal protective suits for fire/chemicals and positive-pressure self-contained breathing apparatus. 

---