Helium leak detector

The helium leak detector is a common laboratory device for locating minute leaks in vacuum systems and other gas-tight devices. It is attached to the vacuum system under test a helium stream is played on the suspected leak and any leakage gas is passed into a mass spectrometer focused for the helium-4 peak. The lack of nearby mass peaks simplifies the spectrometer design the low atmospheric background of helium yields high sensitivity helium s inertness ensures safety and its high diffusivity and low adsorption make for fast response. 

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]). 

Object to be tested 2, helium leak detector L, slit with a fixed conductivity (Fig. 10 from [2.27]). 

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).

Adjustment or tuning refers to the correct setting of an instrument. For example, setting 0 and 100 % in THERMOVACs or setting the mass spectrometer to mass 4 in the helium leak detector. 

Partial pressure analysis using a mass spectrometer or the pressure rise method may be used to differentiate between these two causes. Since the pressure rise method will only prove the presence of a leak without Indicating Its location In the apparatus. It Is advisable to use a helium leak detector with which leaks can, in general, also be located much more quickly. 

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. 

Freon F12 loss per year (g/a) Time taken to form a gas bubble (s) Equivalent leak rate (cm3[STP]/s) Detection time using helium leak detector (s)... 

Helium leak detectors with 180° sector mass spectrometer (UL 200, UL 500)... 

These units are the most sensitive and also provide the greatest degree of certainty. Flere certain is intended to mean that there is no other method with which one can, with greater reliability and better stability, locate leaks and measure them quantitatively. For this reason helium leak detectors, even though the purchase price is relatively high, are often far more economical in the long run since much less time is required for the leak detection procedure itself. 

A helium leak detector comprises basically two sub-systems in portable units and three in stationary units. These are ... 

Where the size of the vacuum vessel or the leak makes it impossible to evacuate the test specimen to the necessary inlet pressure, or where this would simply take too long, then supplementary pumps will have to be used. In this case the helium leak detector is operated in accordance with... 

The partial flow concept is usually used in making the connection of a helium leak detector to vacuum systems with multi-stage vacuum pump sets. When considering where to best make the connection, it must be kept in mind that these are usually small, portable units which have only a low pumping speed at the connection flange (often less than 1 l/s). This makes it all the more important to estimate - based on the partial flow ratio to be... 

Leak detection techniques using helium leak detectors... 

The helium content of the air can also be detected with helium leak detectors where large leaks allow so much air to enter the vessel that the 5... 

Here the points suspected of leaking at the pressurized test specimen (see Fig. 5.4, d) are carefully traced with a test gas probe which is connected with the leak detector by way of a hose. Either helium or hydrogen can be detected with the INFICON helium leak detectors. The sensitivity of the method and the accuracy of locating leaky points will depend on the nature of the sniffer used and the response time for the leak detector to which it is connected. In addition, it will depend on the speed at which the probe is passed by the leak points and the distance between the tip of the probe and the surface of the test specimen. The many parameters which play a part here make it more difficult to determine the leak rates quantitatively. Using sniffer processes it is possible, virtually independent of the type of gas, to detect leak rates of about 10 mbar l/s. The limitation of sensitivity in the detection of helium is due primarily to the helium in the atmosphere (see Chapter 9, Table VIII). In regard to quantitative measurements, the leak detector and sniffer unit will have to be calibrated together. Here the distance from the specimen and the tracing speed will have to be included in calibration, too. 

N cuum envelope tests are integral leak tests using helium as the test gas, in which the test specimen is enclosed either in a rigid (usually metal) enclosure or in a light plastic envelope. The helium which enters or leaves (depending on the nature of the test) the test specimen is passed to a helium leak detector, where it is measured. Envelope tests are made either with the test specimen pressurized w/ith helium (Fig. 5.4c) or with the test specimen evacuated (Fig. 5.4a). In both cases it may be necessary to convert the helium enrichment figure (accumulation) to the helium standard leak rate. 

When the test specimen, pressurized with helium, is placed in a rigid vacuum chamber, connected to a helium leak detector, the integral leak rate can be read directly at the leak detector. 

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

Investigations were carried out on a small turbomolecular pump intended for use in a helium leak detector (see Chapter 4). Measurements at 24 °C of KmdX He for a suitably backed nine-stage pump with a blade circumferential speed of 200 m s 1 yielded a value of 254. 

A very convenient way of detecting and measuring leaks is to use a tracer gas (such as He) and a selective tracer gas detector, such as a helium leak detector. In addition to the mass spectrometer, MSLD include a high vacuum pumping system of definite pumping speed. 

A portable helium leak detector has a pumping speed for He of 1 L s 1 at its inlet. It is connected directly to a chamber which has its own pumping system. To reduce the test time and also maintain a low enough pressure for the connection of the leak detector, the chamber s pumping system He = lOOLs-1) is used in the test. If the chamber has a leak equivalent to lO mbarLs"1 He, calculate approximately the leak rate (qind) which would be shown on the MSLD. 

Silicone Fluids (IO 6 - IO 9) Are thermally stable and are resistant to oxidation and reasonably resistant to chemical attack. Are also reasonably priced. Form insulating polymeric layer when irradiated by electrons, so they cannot be used where physical electronic equipment is being pumped, such as in helium leak detectors. 

Avoid using pipe threads whenever possible because they create long leak paths. Long, thin leak paths require long time periods for leak verifications. If pipe threads must be used, do not use Teflon tape because it voraciously absorbs helium and will later thwart the use of a helium leak detector. Rather, use a small amount of thread sealant paste that contains Teflon (see Fig. 7.54). Glyptol, a common temporary leak sealant, is sometimes used as a pipe sealant. This sealant is not recommended because it ages and, after some six months or so, can develop a leak. 

Imagine a room full of people. Now imagine you want to find the one(s) named Bill. If there was no way to easily identify the Bills, the process would become significantly laborious. However, if you called out and asked all those named Bill to please raise their hands, the task of identification becomes easy. The helium leak detector has the ability to isolate and count the helium atoms from a vacuum system full of many other atoms and molecules. In reality, the helium leak detector is nothing more than a customized mass spectrometer. The mass spectrometer was developed in the 1920s, but became a leak detection tool in the 1940s when the need for ultra-vacuum systems became critical on the Manhattan project. ... 

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