This morning after completing one an a half calibration cycles in Helium at 1 bar, we looked at the comparison between the first run and the second.
Cell 1.1 had a very close tolerance of 1.5C max variation between the first cycle and the second. InCell 1.0, though,the second run was almost 4 degrees cooler on the second run, mostly at the higher temperatures.
That lead us to wonder if the resistance had changed significantly enough to alter the power in. Instead, we had a mystery.
When we looked a the pressure, it got even more mysterious. What would cause the pressure to go up when it was already above atmospheric?
The answer become both more obvious and more mysterious, yet, when we inspected the cell.
A crazy, black coating on the Isotan wire!
Our best interpretation is that, despite the baking of the mica before assembling the cell, there must have been something left to off gas. Last time we noticed off gassing from the mica the material formed a sticky, hazy film on the glass and flanges - it seemed to like the cool places. This time, whatever this material is, it seems to have been catalytically broken down on the hot surface of the Isotan. This made more gas. The higher pressure gas conducted better and the residue on the wires insulated them, making them run hotter, and at a higher resistance.
Next step: Disassemble the cell, again, and replace the Isotan. Any other suggestions?