What are leak detectors with quadrupole or 180° sector field mass spectrometer
What are leak detectors with quadrupole mass spectrometer?
Leak detectors with quadrupole mass spectrometers (QMS) are mostly build to detect masses greater than helium. Apart from special cases, these will be refrigerants or lamp filling gases. Therefore, leak detectors with QMS are mostly used to inspect refrigeration units for leaks.
Fig. 12 below shows the vacuum diagram of a leak detector with QMS. One may recognize the twofold gas inlet system which consists of the (direct flow) inlet for refrigerant via flow divider 1 and the (counter flow) inlet for helium into the turbomolecular pump via flow divider 2. This way, the device may be used to sniff for refrigerants as well as for helium.
Another special feature is the use of a two-stage diaphragm vacuum pump for evacuation of the QMS as well as for delivery of the sniffer gas flow (1st stage of the diaphragm vacuum pump).
Fig. 12: vacuum diagram of a leak detector with quadrupole mass spectrometer (QMS)
What are leak detectors with 180° sector field mass spectrometer?
Helium leak detectors with 180° sector field mass spectrometer are the most sensitive and reliable leak detection devices.
There is no other leak detection method with which one can, with greater reliability and better stability, locate leaks and measure them quantitatively. Therefore, helium leak detectors – even though the purchase price is relatively high – are very economical in the long run since the required for the leak detection procedure itself is very short.
A helium leak detector with 180° sector field mass spectrometer basically consists of three assemblies:
- the mass spectrometer
- the high-vacuum pump system
- the roughing pump system.
Fig. 13: Structure of a 180° sector field mass spectrometer
- Ion source flange
- Cathode ( 2 cathodes: r + Yt2O3)
- Anode
- Shielding of the ion source with discharge orifice
- Orifice
- Ion traces for M > 4
- Ion traces for M = 4
- Intermediate orifice plate
- Magnetic field
- Suppressor
- Shielding of the ion trap
- Ion trap
- Flange for ion trap with preamplifier
The mass spectrometer (see Fig. 13 above) consists of the ion source (1–4), the separation system (5–10), and the ion trap (11–13).
The ion beam is extracted through the orifice (5). The ions always enter the magnetic field (9) at a certain speed. Inside this magnetic field the ions move along circular paths whereby the radius for low masses 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 helium ions move exactly on the circular arc which is required for them to pass through the intermediate orifice plate (8). This way, the helium ions reach the ion trap (12).
The ion source features a simple and rugged design. It can be replaced without much effort. During operation, the ion source is permanently heated and therefore unsusceptible to contamination.
The two selectable yttrium oxide coated iridium cathodes have a long service life. These cathodes are largely in-sensitive to air ingress, i.e. the quick-acting safety cut-out will keep them from burning out even if air enters. However, as for any annealed metals, 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.
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