How long do Extorr filaments last?
The Extorr filaments are protected against overpressure by the ion gauge function of the probe. The ion gauge pressure is read at all times whenever the Pirani gauge indicates a low enough pressure. Any harmful pressure excursion will result in the filaments being turned off. The filaments are thoria coated iridium. The thoria allows for a high electron emission without ever approaching the melting temperature of iridium. When operated in a benign vacuum with pressures always below 10-7 torr, the filament is expected to last far over a year. After an extended period of operation, the filament may experience evaporation, become thinner, and eventually fail. Accelerated failure could be brought on by heavy use on corrosive gases at pressures in the 10-4 torr region and above. Overuse of the degas function may also lead to a decreased filament lifetime.
