This is an Ion Source Thoriated Iridium Filament for the Agilent Varian 979, 959, 936, 936SP, 938-41 Portatest, Helium Leak Detectors. This Thoria long life ion source is for the newer model 979 Leak Detector, but can be used in older models 936 and 936SP, 938-41 Portatest models. For the Older 936 Cabinet Models with enhancement kits, redirect the flexible metal tube away from the spectrometer tube. For 936SP Cabinet Models, if the unit has a spectrometer cooling fan, unplug it. For models without a spectrometer cooling fan, the IRIDIUM/TUNGSTEN switch must be in the IRIDIUM position. For the 938-41 Portatest Model with serial numbers between 62E001-62E171 contact Agilent Varian. For 938-41 Portatest with serial numbers later than 62E171, the IRIDIUM/TUNGSTEN switch must be in the IRIDIUM position. This is the Ion Source Filament ONLY, leak detectors sold separately on this website.