BLP Gas-Phase Replication Effort - Run 2….8JAN98 ………………..(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 Temperature (C)
Room Temperature (C)
There are 10 vertical divisions. For each trace the vertical scale information (max/min) is given in parentheses after the color-coded digital displays in the upper portion of the screen.
(the particular digital values shown are for the last data point in this run and are of little importance…concentrate on the plot!)
The first two hours of this run were spent getting the chamber up to temperature with the cartridge heater only. During the third hour, the input power was left constant at about 30 watts to demonstrate that the water-flow calorimetry was accurate (note agreement between Pout and Pin traces). Note the gradual rise in chamber pressure from the starting value of 2 torr to about 8 torr after 3 hours. At about 3.2 hours into the run, 10 watts of filament power was applied and the cartridge heater power was turned down to 20 watts, keeping the total input power at about 30 watts. This event is marked by the sudden appearance of the Tfil trace, which zoomed up to about 1500 ° C. The hot filament caused a rapid increase in Press to about 14.5 torr. At 3.7 hours (the first step-function drop in Press) we decided to evacuate the chamber briefly and refill to 2 torr of H2 in an effort to stay closer to the given BLP parameters. At 4.3 hours we raised the filament power to 20 watts and lowered the cartridge heater power to 10 watts (and flushed the chamber again). The filament temperature rose to about 1750° C. At 5.0 hours we increased the filament power to 30 watts and turned off the cartridge heater. Filament temperature rose to 2000° C and Tgas reached about 260° C (the filament heats the gas more efficiently than the cartridge heater). During all this time, the Pout trace stayed within 1 watt of the Pin trace…i.e. there was no sign of any anomalous heat generation. At 5.7 hours, in an effort to get the gas temperature up to the target temperature of 280° C, we raised the filament power to 40 watts and flushed the chamber again. Apparently this high power level damaged the filament because the reported filament temperature dropped sharply and the filament current rose abnormally. However, the filament was still conducting so we maintained 40 watts until the gas temperature reached 290° C and then backed down to about 34 watts. We stayed at ~34 watts until about 7.3 hours and observed that Pout was again closely matching Pin…no sign of excess heat. At 7.3 hours we reduced filament power to 0.1 watts to end the run.
Upon disassembly of the cell, we see that the filament is indeed damaged. Sections of the tight little coil are melted together in the center, reducing the overall length of the filament wire. The cold resistance is now 0.40 ohms and it was 0.51 ohms before the run. Since the filament temperature is calculated from the ratio of the measured resistance to the cold resistance, this damage to the filament explains the apparent drop in filament temperature that occurred at 5.7 hours when we applied 40 watts of filament power.
Before the run we weighed the KNO3 in its new holder: 4.57 grams (empty holder is 4.46 grams). After the run, the holder w/ KNO3 weighs 4.56 grams. The apparent 0.01 gram weight loss could easily be a weighing error. This time the appearance of the KNO3 is unchanged…still a loose granular white salt.
12 hours after the end of the run, with everything cooled back down to the 40° C base temperature imposed by the water-flow calorimetry, the gas pressure was about 5 torr. The chamber was last filled to precisely 2 torr at hour 5.7 when it was at about 250° C. If no additional gas was added to the chamber, the 40° C pressure should have been about 1.2 torr. Where is the extra gas coming from? Maybe from the KNO3…if 0.01 grams of the KNO3 dissociated into KNO2 and O, it would raise the 140 cm3 chamber pressure by about 10 torr.
What's next? I think we're going to have to do a run with no KNO3 present to see if the gas pressure behaves as expected.
As usual, comments and suggestions are most welcome.