Part of the process of commissioning the stainless cell with the insulation jacket that we blogged about a few posts ago is to heat the cell and burn off anything stinky in the insulation jacket. Once it was up to temperature of 400 some and slowly rising, we left it to bake overnight. That was a bit of a mistake.
When we came back the next morning, we noticed the cell was cool and no current was flowing through it. It looked fine, but I figured it burned out a wire. When we pulled it out of the insulation, we could see it had been quite hot. And then it got stuck and wouldn't slip out of the aluminum sleeve even with a great deal of force. That meant we had to cut the insulation jacked open so we didn't damage the thermocouples. Once we opened it up we saw why it got stuck.
The aluminum sleeve had been melted! That means we had over 630 C there. Looking at the core of the cell, we learned more of the story.
The wire was burned off in at least 2 places. What happened was that the copper wire bowed out as it got hot and shorted against the heating wire. The shirt made a low resistance path and it drew way more power. As it heated up faster, now, it shorted out again! This time it got hot enough to melt the aluminum and, eventually, to burn out what was left of the heating wire. The Macor was discolored near the bottom where it had been the hottest, especially around the corresponding wire notches. Unfortunately, the epoxy in the pass throughs had melted out, too, so the cell was no longer tight.
All together, it means a lot of re-work.
Notice the vacuum hose to remove the dust in the air from the buffing wheel.
And below, is the re-furbished product with clean looking macor and new pass-throughs being glued in.
Time to make a new aluminum sheath.
Hopefully we can reuse the insulation shell since it is already baked out, but almost everything else will have to be redone before we can get this together and set into the air flow calorimeter for calibration. And then we'll keep a much better eye on it.
The good news is that the cell was showing about 8 C/Watt before I left it alone, so it should be nice and sensitive to excess heat generation.