BLP Gas-Phase Replication Effort - Run 4….14JAN98 ………………..(best viewed with 1024 pixel display width)
Legend
Total Input Power (electrical)
Output Power (heat)
Chamber Wall Temperature (C)
Gas Temperature (C)
Gas Pressure (torr)
Filament Resistance (ohms)
Room Temperature (C)
Inlet water Temperature (C)
This was a messy run with lots of problems but there is some useful information present. The run started at 14:45 on 13JAN98 with the chamber at high vacuum and both the filament and the cartridge heater at 15 watts. About 2.5 hours later (marked by a small step drop in Rfil) we turned the filament down to about 10 watts and the cartridge heater up to 20 watts to keep the overall input power at 30 watts. We also closed off the main valve to the vacuum system and shut it down for the night. This left the chamber under a nominal vacuum but that vacuum surely degraded steadily due to the outgassing going on in the hot chamber. Note the frenetic oscillations in Troom as the heating system in the lab struggled against the cold night. These room temperature variations produce a small corresponding variation in the Pout signal. More importantly, the Rfil trace indicates that the filament lost about half its cross-sectional area during the night! The resistance increase caused the filament power to drop significantly because the power supply is constant voltage. About 17 hours into the run (marked by the vertical spike in Rfil) we resumed pumping on the chamber, turned off the filament entirely, and adjusted the cartridge heater power to 25 watts (and turned off the heating system in the lab). About 1 hour later we filled the chamber with 2 torr of H2 gas (marked by a step increase in Press). About 2 hours after that we turned off the cartridge heater and applied 25 watts to the filament. The filament lasted only about an hour. During that time, it's resistance increased dramatically. We attempted to keep the filament power constant at 25 watts by periodically adjusting the voltage but we didn't do a very good job of it.
During this hour, however, the filament was definitely hot (>2000° C), there was 2 torr of H2 gas in the chamber, the gas temperature was about right…240° C, and there was a small vial of KNO3 in the chamber. If the experiment had started producing an extra, say, 20 watts we should have seen the Pout trace heading upwards. Instead, the Pout trace dribbled downwards pretty much as you might expect from the <25 watt average Pin value during that period.
Interestingly the gas pressure decreased slightly during the period of filament operation. We certainly have solved the outgassing problem. Perhaps the next experiment should be a repeat of this one except with a substantially shorter bakeout time…so that most of the filament is still left when the H2 gas is admitted to the chamber.
As usual, comments and suggestions are most welcome.