Most leak testing today is carried out using special leak detection devices. These can detect far smaller leak rates than techniques which do not use special equipment. These methods are all based on using specific gases for testing purposes. The differences in the physical properties of these test gases and the gases used in real-life applications or those surrounding the test configuration will be measured by the leak detectors. This could, for example, be the differing thermal conductivity of the test gas and surrounding air. The most widely used method today, however, is the detection of helium used as the test gas.
The function of most leak detectors is based on the fact that testing is conducted with a special test gas, i.e. with a medium other than the one used in normal operation. The leak test may, for example, be carried out using helium, which is detected using a mass spectrometer, even though the component being tested might, for example, be a cardiac pacemaker whose interior components are to be protected against the ingress of bodily fluids during normal operation. This example alone makes it clear that the varying flow properties of the test and the working media need to be taken into consideration.
Gaseous chemical compounds whose molecules contain chlorine and/or fluorine – such as refrigerants R12, R22 and R134a – will influence the emissions of alkali ions from a surface impregnated with a mixture of KOH and Iron(III)hydroxide and maintained at 1472°F to 1652°F (800°C to 900°C) by an external Pt heater. The released ions flow to a cathode where the ion current is measured and then amplified (halogen diode principle). This effect is so great that partial pressures for halogens can be measured down to 10-7 mbar.
Whereas such devices were used in the past for leak testing in accordance with the vacuum method, today – because of the problems associated with the CFCs – more sniffer units are being built. The attainable detection limit is about 1 · 10-6 mbar · l/s for all the devices. Equipment operating in accordance with the halogen diode principle can also detect SF6. Consequently, these sniffer units are used to determine whether refrigerants are escaping from a refrigeration unit or from an SF6 type switch box (filled with arc suppression gas).
The detection of a test gas using mass spectrometers is by far the most sensitive leak detection method and the one most widely used in industry. The MS leak detectors developed for this purpose make possible quantitative measurement of leak rates in a range extending across many powers of ten (see Leak types and rates) whereby the lower limit ≈ 10-12 mbar · l/s, thus making it possible to demonstrate the inherent gas permeability of solids where helium is used as the test gas. It is actually possible in principle to detect all gases using mass spectrometry. Of all the available options, the use of helium as a tracer gas has proved to be especially practical. The detection of helium using the mass spectrometer is absolutely (!) unequivocal. Helium is chemically inert, non-explosive, non-toxic, is present in normal air in a concentration of only 5 ppm and is quite economical. Two types of mass spectrometer are used in commercially available MSLD’s:
a) The quadrupole mass spectrometer, although this is used less frequently due to the more elaborate and complex design (above all due to the electrical supply for the sensor), or
b) the 180° magnetic sector field mass spectrometer, primarily due to the relatively simple design.
Regardless of the functional principle employed, every mass spectrometer comprises three physically important sub-systems: the ion source, separation system and ion trap. The ions must be able to travel along the path from the ion source and through the separation system to the ion trap, to the greatest possible extent without colliding with gas molecules. This path amounts to about 15 cm for all types of spectrometers and thus requires a medium free path length of at least 60 cm, corresponding to pressure of about 1 · 10-4 mbar; in other words, a mass spectrometer will operate only in a vacuum. Due to the minimum vacuum level of 1 · 10-4 mbar, a high vacuum will be required. Turbomolecular pumps and suitable roughing pumps are used in modern leak detectors. Associated with the individual component groups are the required electrical- and electronic supply systems and software which, via a microprocessor, allow for the greatest possible degree of automation in the operating sequence, including all adjustment and calibration routines and measured value display.
The basic function of a leak detector and the difference between a leak detector and mass spectrometer can be explained using Figure 5.6. This sketch shows the most commonly found configuration for leak detection using the helium spray method (see Local leak detection) at a vacuum component. When the sprayed helium is drawn into the component through a leak it is pumped thorough the interior of the leak detector to the exhaust, where it again leaves the detector. Assuming that the detector itself is free of leaks, the amount of gas flowing through each pipe section (at any desired point) per unit of time will remain constant regardless of the cross section and the routing of the piping. The following applies for the entry into the pumping port at the vacuum pump:
At all other points
applies, taking the line losses into account.
The equation applies to all gases which are pumped through the piping and thus also for helium.
In this case the gas quantity per unit of time is the leak rate being sought; the total pressure may not be used, but only the share for helium or the partial pressure for helium. This signal is delivered by the mass spectrometer when it is set for atomic number 4 (helium). The value for Seff is a constant for every series of leak detectors, making it possible to use a microprocessor to multiply the signal arriving from the mass spectrometer by a numerical constant and to have the leak rate displayed directly.
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