Heat Treatment Oven Build Process
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Posted 10th February 2025
I did a few tests this evening, using various different thermocouples (including a low temperature rated one for some low temperature testing). It was quite interesting. The main thing I've learnt so far is that the temperature varies quite a bit and, in particular, the insulating fire bricks insulate the thermocouples from the chamber's ambient temperature.
I know that sounds a bit obvious (insulating fire bricks are insulating, who knew?!), but I didn't expect the effect to be as pronounced. The thermocouples as designed stick out into the chamber by about 5 mm, so the entirety of the welded blob on the end (where the two metals join) is in the cavity rather than being surrounded by brick. I thought that would mean they measure the air temperature quite effectively, but look at these two measurements (Channel 1 and Channel 2):
Channel 2 is one of the thermocouples at the back, which sticks out about 5 mm and looks a bit like this earlier picture of the door thermocouple:
In that test, the door thermocouple was replaced by a new (uncut) one looking like this:
For that to account for that much of a temperature difference was quite surprising to me (especially considering that the door one previously under-read relative to the one at the back by about 20°). I'd guess that this would explain why the attempt at tempering went wrong: the thermocouples were under-reading by enough to make a mess of any measurement accuracy.
I'll replace all the thermocouples with ones that stick out a bit more (although not as much as the test one shown in the last photo above, as that'll probably get in the way) and do some more testing.
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