Månadens Tips

TIP OF THE MONTH 02

QUESTION: How do I find out whether a dry helium leak detector is "fit for purpose"?

TIP OF THE MONTH 02

ANSWER: With the leak detector warmed up and a blank flange fitted, spray the exhaust with the helium spray gun and read off the background signal. The lower the background signal, and the more slowly it climbs, the better the leak detector.

REASONING: The pump which delivers the helium test gas from the leak detector is the mechanical roughing pump. With dry-running roughing pumps, the gaps between rotor and stator are not sealed with oil, as is the case for example with rotary vane pumps. Therefore dry leak detectors have an "inbuilt leakage" from the leak detector "exhaust" back to the spectrometer cell, in which the test gas is detected. Thus helium from the ambient air gets into the analyser cell via the pump system and is detectable as a background signal, which can make the measurement of small leak rates difficult or even impossible.

Low cost dry-running systems such as piston or scroll pumps attempt to improve gap sealing by using dry "lubricants" in the form of plastics. However these plastics, usually a form of Teflon, are subject to a considerable degree of wear. As a consequence, the "inbuilt leakage" gets gradually larger during operation, which means that the background performance of the leak detector deteriorates. In addition, the abrasion process produces particles which reduce the working life of the leak detection equipment (e.g. when they are deposited in valve seats) and can potentially cause yet further leaks.

So, depending on the time and expense which the manufacturer has invested in the dry-running pump system, the dry leak detector will be more or less "fit for purpose". This property cannot be judged from the general technical data. If small leaks need to be detected, or if a high concentration of helium in the ambient air is anticipated, it is essential that this property is tested before selecting the equipment. Adixen dry leak detection models with outstanding background stability are, for example, the ASM 122D and the ASM 182TD+. Both models have pump systems without abrasive, dynamically loaded seals between the gaps (diaphragm pump or 5-stage Roots pump). The primary pumps are each supplemented with two high vacuum pumps. This expense pays off in terms of the exceptional long-term stability of the background signal, which is broadly independent of the service life of the pumps.

Glossary

Helium: At room temperature, helium is a colourless and odourless gas. After hydrogen, it is the chemical element with the lowest density and has the lowest melting point of all elements. As with all other noble gases (neon, argon, krypton, xenon) helium is very inert. Therefore there are no known helium compounds in nature. Helium (helium I) changes state at low temperatures (helium II). Helium II is the best conductor of heat known and has a heat conductivity which is 300 times higher than silver. In addition it flows without internal friction and penetrates even the tiniest of openings. Because of these properties, helium II is also referred to as a superfluid.

In normal air, sound travels at a speed of around 330 metres per second, but in helium gas it travels at twice the speed. This explains the Mickey Mouse effect on the voice when helium is inhaled.

Occurrence: After hydrogen atoms, helium atoms are the second most frequently occurring atoms in the universe, with a proportion of 7.19%. While they only represent a low proportion (0.0000004%) of the earth's lithosphere, their presence in the composition of the air is 0.00046%. The noble gas is also found in larger volumes as a radioactive breakdown product in natural gas and crude oil, which can contain up to 10% helium. In the sun, the element is produced by nuclear fusion processes from hydrogen atoms.

Many radioactive substances emit so-called alpha rays, consisting of doubly ionised helium nuclei which have lost their electrons. Helium atoms with mass number 4 (helium 4) have 2 protons, 2 electrons and 2 neutrons. Natural helium consists of a mixture of two different isotopes. However, with a proportion of 0.00014%, the helium 3 isotope (with only one neutron) occurs relatively rarely.

Piston pump: A piston pump is a machine for delivering liquids or gases, whereby gases may also be compressed. In the first part of the cycle, the medium to be pumped is drawn up through an inlet valve by a piston moving inside a cylinder. This is then discharged through an outlet valve. This process is also referred to as positive displacement pumping. Piston pumps enable high pressures to be achieved when pumping liquids. In addition, the volume of liquid delivered can be precisely determined. The pump can be driven manually, by solenoid or by motor.

Scroll pump: A scroll pump is a vacuum pump which works on the displacement principle. It consists of two interleaved Archimedes spirals. One of these spirals is fixed, the other is driven in an eccentric orbit. Thus individual closed pockets are produced between these spirals and their volumes become smaller and smaller. In this way, the gas to be pumped is drawn in from the outside, compressed in the pump and expelled through a connection in the centre of the spiral.

Diaphragm pump: The diaphragm pump is a particular form of piston pump in which the medium to be delivered is drawn up and expelled by a diaphragm instead of a piston.

Multistage Roots pump: A rotary positive displacement pump in which two impellers of equal cross-section rotate in opposite directions inside the pump casing, usually in a figure of eight movement, without touching each other or the casing wall. The two rotary pistons are synchronised so that they pass each other with close tolerances. No compression takes place in the suction chamber of the pump.


Överst på sidan
 

Kontakta oss Företaget Nyheter Event Site map Nyhetsbrev  


Välj hemsida :

Adixen Worldwide | Adixen China | Adixen Germany | Adixen France | Adixen India | Adixen Italy | Adixen Japan
Adixen Korea | Adixen Netherlands | Adixen Taiwan | Adixen UK | Adixen USA | Adixen Singapore
Copyright © 2010 Alcatel Vacuum Technology. All right reserved Legal Notices