Servicing XTx00 RGA probes, SN 1-999

Ionizer Replacement

There are 2 reasons to replace the ionizer, filament failure and contamination resulting from sustained normal operation at high (>10-6 Torr) pressures. A probe can also be contaminated in a matter of minutes due to misuse. The filament will eventually fail after years of operation even in a clean UHV system due to the thoria coating on it eventually decomposing and evaporating. Unlike other RGA’s the Extorr filament will not fail due to operation at very high pressures because it is protected by the pirani gauge output. A filament failure can be verified by looking at the DIAGNOSTICS readings on the OUTPUTS page. If there is an open filament, the filament voltage will read > 5 volts and there will be < .1 amp filament current. The filament light icon will not stay yellow on the main window.

Always handle the probe with clean gloves, or you will see the results of your contamination on the next scan. A fingerprint will outgas and cause peaks at virtually every amu until it is evaporated or turned into low vapor pressure compounds in your vacuum system. Of course other sources of contamination such as diffusion or mechanical pump oil could cause similar problems. Even after the volatiles are gone, the residue may cause the probe to operate improperly by leaving behind insulating compounds, or high secondary electron yielding surfaces where there is contamination. This can cause charge from electrons to build up on these insulators and disturb the electric fields in the ionizer or the mass filter.

The results of contamination could cause low sensitivity for partial pressure readings. It generally take a very severe contamination to cause the total pressure readings to be low because the B/A type gauge uses higher voltages and works with more energetic ions that are less susceptible to stray fields. If you have a total pressure reading of 10-7 for example, your partial pressure readings should add up to about that. A number of factors such as the relative sensitivities of different compounds and the cracking of molecules into fragments in the ionizer makes it unlikely that they will add up exactly, but as the ionizer contamination increases, you may see a an order of magnitude or more difference between the total pressure and the sum of partial pressures. Cleaning the ionizer is difficult if not impossible in most cases, so replacement is the remedy. It is also likely that some contamination will make it to the rods of the quadrupole, as a direct result of the principle of operation of this type of analyzer. So when replacing the ionizer due to low sensitivity, it is a good idea to clean the quadrupole rods also.

At the factory, all parts are mechanically cleaned, then vacuum baked, and then plasma cleaned. If you are doing UHV work and demand the best cleanliness, you can send the analyzer to the factory for refurbishment, but many customers have had good success with the following method. The rods are best cleaned with an abrasive method, such as 1200 grit abrasive paper polishing, followed by an Alconox ultrasonic bath and several rinses with distilled water. The Extorr probe was designed to allow easy cleaning and ionizer replacement, the only tool required is an allen wrench to loosen the set screws that hold the ionizer to the probe. This is supplied with all new ionizers.

The following steps will usually restore the XT system to proper operation:

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