BLP Gas-Phase Replication Effort - Run 13….18FEB98 ………………..(best viewed with 1024 pixel display width)
A Platinum Filament…
This photo shows the arrangement of the catalyst vial (left), filament (center) and cartridge heater (lower center) before Run 13. You can barely see the thermocouple wires leading up from their feedthrus to the catalyst vial where the junction was strapped onto the vial with a piece of 30ga Nichrome wire.
The Pt filament was made from a 5" long piece of 0.005" dia Pt wire. It was wound around a 3/32" dia rod and then carefully stretched to produce uniformly spaced turns. Room temperature resistance of the filament was 1.18 ohms.
As in previous runs, the filament temperature was computed from the observed resistance of the filament during operation. This provides only a mean value for the temperature and there are sure to be significant temperature variations along the filament. The melting point of Pt is 1768° C. We determined empirically (by burning out a few) that the filament would usually burn out (melt, actually) when the mean temperature was around 1400° C. Therefore, for this run we limited the mean filament temperature to around 1200-1300° C.
For this run, 0.23 grams of KNO2 was placed in the catalyst vial.
Results:
Legend
Total Input Power (electrical)
Output Power (heat)
Chamber Wall Temperature (C)
Catalyst Temperature (C)
Gas Pressure (torr)
Filament Temperature (° C)
Room Temperature (C)
Inlet water Temperature (C)
The run duration was 20 hours (each horizontal division is 1 hour). The run starts with the usual equilibration period (30 watts in the cartridge heater) for the calorimetry, during which time we experimented with putting H2 in the chamber and running the filament under vacuum. We were concerned about the catalyst temperature (Tcat) which threatened to exceed 300° C with the chamber evacuated so we filled the chamber with ~10 torr of H2 for much of this period (hour 1.8 to hour 4.3). At hour 4.5 we managed to crash the computer and had to restart, which produced the bipolar excursion in the Pout trace. At hour 4.6 we filled the chamber with 2 torr of H2 gas. At hour 5.2 we put 10 watts on the filament and adjusted the cartridge heater down to maintain a 30 watt total Pin. Thankfully, the Pt filament resisted the H2 gas beautifully. The Tfil trace showed no signs of the rapid erosion seen with the W filaments. In fact, it trended downward slightly to hour 7.5, due probably to the increasing chamber pressure (Press). We have no idea why the chamber pressure was increasing. During this 2 hour period of proper filament operation in a nominally 2 torr atmosphere of H2, with a vial of KNO2 at ~300° C sitting very near the filament…there was no sign of excess heat generation. The Pin and Pout traces run very close together and almost on top of each other.
At hour 7.5 we decided to try a higher H2 pressure: 10 torr. The increased H2 pressure allowed us to raise the filament power considerably and still stay in the 1200° -1300° C "safe" range. We raised the filament power to 20 watts and lowered the cartridge heater power correspondingly. These conditions persisted for 3.5 hours until hour 11, when the filament suddenly melted open. During this period there also was no sign of excess heat…but there was a noticeable decline of the Pout trace. By the 11th hour, it was about 1 watt below the Pin trace…perhaps due to the precipitous decline in Troom that was occurring at that time.
After hour 11, the system slowly equilibrated at the new Pin of about 9.3 watts, due only to the contribution from the cartridge heater.
Upon disassembly of the chamber we found that the catalyst vial weighs maybe 0.01 grams less than it did before the run (our balance has 0.01g resolution). Since the KNO2 is visibly moist, this could easily be water loss. Thus, we continue to observe little or no catalyst consumption, even at a catalyst temp of 300° C.
Conclusions
In this run we had conditions similar to those reported by BLP in the chamber for nearly 6 hours with no sign of excess heat. Judging by the performance of the calorimeter, our detection limit for excess heat should be put at about 1 or 2 watts…i.e. 3-6% of the total input power.