Helium leak detectors with

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. 

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

This level of leak detection is not inexpensive A new mass spectrometer helium leak detector with the accompanying equipment can easily cost between 20,000 and 30,000. Purchasing used equipment can significantly reduce the initial costs, but one should not enter the level of helium leak detection because it seems like a good idea. On the other hand, if you need a helium leak detector, you cannot afford not to have one. 

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).

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. 

INFICON builds leak detectors with quadrupole mass spectrometers to register masses greater than helium. Apart from special cases, these will be refrigerants. These devices thus serve to examine the tightness of refrigeration units, particularly those for refrigerators and air conditioning equipment. 

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. 

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... 

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. 

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.

Even if there is no reason to quantify any given leak, it is still important to establish a leak limit for the helium leak detector before leak hunting begins. This limit is set with a standard leak. 

Standard leaks come from the factory with their calibrated leak rate imprinted on their sides. The approximate annual attenuation is also indicated. After a certain amount of time (every 5 to 10 years) they can be returned to the factory for either recharging and/or recalibration. Recalibration is not critical if the helium leak detector is being used only for leak location and not leak calibration. 

Because of the many different brands and types of helium leak detectors, it is impossible to explain how to operate your particular model use your owner s manual. If you cannot find the owner s manual for your machine, contact the manufacturer and obtain a replacement, possibly for a nominal charge. If you have been using your leak detector on the fly with apparent success, it would still be worthwhile to obtain the manual. 

Probably the biggest problem with helium leak detector use comes from the construction material of the vacuum system or apparatus being tested. If the materials within the tested piece readily absorb helium, a false and/or confusing reading will exist for a considerable time afterward and will remain until the helium has left the piece. 

A second complication can come from materials that helium can easily permeate such as the elastomers used in O-ring connections. Helium readings from these materials can distract you from other true leak readings. Once an elastomer has become saturated with helium, it slowly desorbs the gas, which creates a large background noise. This background noise interferes with a helium leak detector s sensitivity and performance. All elastomers readily absorb, and slowly desorb, helium. Thus the fewer elastomers used within a system, the less of a problem there will be. Silicone tends to absorb helium the most, followed by nat-... 

One additional complication with elastomers has to do with the connection between a vacuum system and a helium leak detector. If it is possible, connect the two with a nonelastomeric connection such as a stainless steel bellows. If this connection is not possible, the best elastomeric connection that you can use is a high-vacuum, thick-walled (V2 in I.D., 3/16 in. wall) vinyl tube of the shortest length possible. Do not use rubber or neoprene tubing. If you have several systems that may need leak detection, it is unlikely that one length of tube will accommodate all systems. It is best to have several connecting tubes, one for each system (or several systems) being tested, to allow you to select the shortest one that can be successfully used. 

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. 

Response time can be easily explained with a hot water analogy When you turn on the hot water in your shower, water immediately comes out of the shower fixture, but there is a time lag before hot water arrives. This lag time is simply a function of water pressure, the amount you have the valve open, the pipe diameter, and the distance between the hot water tank and the shower. It is important to keep in mind this delay factor, or response time, when operating a helium leak detector. 

The response time for a helium leak detector is the time required for a known leak rate to be indicated on the leak detector from zero to 63% of its maximum equilibrium level. 87 Of course the helium leak detector must be calibrated with a standard leak so that the maximum reading can be properly set on the leak detector. The response time is dependent on the quality of vacuum, the size of the leak, physical barriers (such as constrictions, bends, or traps) between the leak and the leak detector, and the proximity of the leak to the leak detector. Because of so many factors, there can be no set specific response time. 

If the leak rate is unacceptable, the leaks have to be located by a helium leak detector (HLD), a mass spectrometer that measures only the amount of helium atoms. The freeze-drying plant is evacuated and the HLD with its pumping system is operated parallel to the main VPS. The HLD can measure He coming from a leak down to 2 X lO" mbar L/s, but its He sensor can only operate at a pressure of 10 mbar. The pressure in a freeze-drying plant will always be higher. Therefore a pressure reducer has to be placed between the HLD and the freeze-drying plant. If the pressure reducer has a conduetance L 5 and L . becomes... 

Helium leak tests were carried out under load pressure up to 4 MPa and surface temperature of the models up to 500°C. All cormections were installed in a vacuum electric furnace, and leak rates were measured with a helium leak detector. Table 2 shows the leak rates obtained at 500°C in the third times at the heat cycle up to 5()()°C, and Table 3 obtained under 2()°C after the leak measurement at 500°C shown in Table 2. In these tables, line loads and seating stresses (tightening forces) are also indicated. As seen these tables, although the tightening forces under 500°C decreased down to less than... 

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