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Calibrating a leak detector is to be understood as matching the display at a leak detector unit, to which a test leak is attached, with the value shown on the “label” or calibration certificate. The prerequisite for this is correct adjusted? Sniffer units or configurations will, as a rule, have to be calibrated with special, external test leaks in which there is a guarantee that on the one hand all the test gas issuing from the test leak reaches the tip of the probe and on the other hand that the gas flow in the sniffer unit is not hindered by calibration. In the special case where helium concentration is being measured, calibration can be made using the helium content in the air, which is a uniform 5 ppm world-wide. The “calibration” with helium content in air is very inaccurate. A calibration leak is always recommended. ent of the ion paths in the spectrometer, also known as tuning. Often the distinction is not made quite so carefully and both procedures together are referred to as calibration.
In the calibration process proper the straight-line curve representing the numerically correct, linear correlation between the gas flow per unit of time and the leak rate is defined by two points: the zero point (no display where no emissions are detected) and the value shown with the test leak (correct display for a known leak).
In vacuum operations (spray technique, see the page on local leak detection) one must differentiate between two types of calibration: with an internal or external test leak. When using a test leak built into the leak detector the unit can itself be calibrated but it can only calibrate itself. When using an external test leak not just the device but also a complete configuration, such as a partial flow arrangement, can be included. Internal test leaks are permanently installed and cannot be misplaced.
Test leaks (also known as standard leaks or reference leaks) normally comprise a gas supply, a choke with a defined conductance value, and a valve. The configuration will be in accordance with the test leak rate required. Figure 5.9 shows various test leaks. Permeation leaks are usually used for leak rates of 10‑10 < QL < 10‑7, capillaries, between 10‑8 and 10‑4 and, for very large leak rates in a range from 10 to 1000 mbar · l/s, pipe sections or orifice plates with exactly defined conductance values (dimensions).
a Reference leak without gas supply, TL4, TL6
b Reference leak for sniffer and vacuum applications, TL4-6
c (Internal) capillary test leak TL7
d Permeation (diffusion) reference leak, TL8
e Refrigerant calibrated leak
Leak detectors can be built 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.
Figure 4.2 shows a functional diagram for a quadrupole mass spectrometer. Of the four rods in the separation system, the two pairs of opposing rods will have identical potential and excite the ions passing through along the center line so that they oscillate transversely. Only when the amplitude of these oscillations remains smaller than the distance between the rods can the appropriate ion pass through the system of rods and ultimately reach the ion trap, where it will discharge and thus be counted. The flow of electrons thus created in the line forms the measurement signal proper. The other ions come into contact with one of the rods and will be neutralized there.
Figure 5.10 shows the vacuum schematic for an ECOTEC II. The mass spectrometer (4) only operates under high vacuum conditions, i.e. the pressure here must always remain below 10-4 mbar. This vacuum is generated by the turbomolecular pump (3) with the support of the diaphragm pump (1). The pressure PV between the two pumps is measured with a piezo resistive measuring system (2) and this pressure lies in the range between 1 to 4 mbar while in the measurement mode. This pressure must not exceed a value of 10 mbar as otherwise the turbomolecular pump will not be capable of maintaining the vacuum in the mass spectrometer. The unit can easily be switched over at the control unit from helium to any of various refrigerants, some of which may be selected as desired. Naturally the unit must be calibrated separately for each of these masses. Once set, however, the values remain available in storage so that after calibration has been effected for all the gases (and a separate reference leak is required for each gas!) it is possible to switch directly from one gas to another.
These units are the most sensitive and also provide the greatest degree of certainty. Here “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:
The mass spectrometer (see Fig. 5.11) comprises the ion source (1–4) and the deflection system (5–9). The ion beam is extracted through the orifice plate (5) and enters the magnetic field (8) at a certain energy level. Inside the magnetic field the ions move along circular paths whereby the radius for a low mass is smaller than that for higher masses. With the correct setting of the acceleration voltage during tuning one can achieve a situation in which the ions describe a circular arc with a defined curvature radius. Where mass 4 (helium) is involved, they pass through? the aperture (9) to the ion trap (13). In some devices the discharge current for the ions impinging upon the total pressure electrodes will be measured and evaluated as a total pressure signal. Ions with masses which are too small or too great should not be allowed to reach the ion trap (13) at all, but some of these ions will do so in spite of this, either because they are deflected by collisions with neutral gas particles or because their initial energy deviates too far from the required energy level. These ions are then sorted out by the suppressor (11) so that only ions exhibiting a mass of 4 (helium) can reach the ion detector (13). The electron energy at the ion source is 80 eV. It is kept this low so that components with a specific mass of 4 and higher – such as multi-ionized carbon or quadruply ionized oxygen – cannot be created.
The ion sources for the mass spectrometer are simple, rugged and easy to replace. They are heated continuously during operation and are thus sensitive to contamination. The two selectable yttrium oxide coated iridium cathodes have a long service life. These cathodes are largely insensitive to air ingress, i.e. the quick-acting safety cut-out will keep them from burning out even if air enters. However, prolonged use of the ion source may eventually lead to cathode embrittlement and can cause the cathode to splinter if exposed to vibrations or shock.
Depending on the way in which the inlet is connected to the mass spectrometer, one can differentiate between two types of MSLD. These are known as Direct-flow and Counter-flow leak detectors.