Thank you for your suggestions. I finished the installation last night. It was really easy and uneventful. For the completeness sake (in case other people find it helpful), let me describe the procedure.
I installed my thermocouple at, roughly, the same level with the Sitter, but on the adjacent wall. My peep holes are to the left of the control box, and the thermocouple is to the right of it.
I measured the distance between the upper rim of the chamber to the midpoint of a brick, transferred the measurements outside and drilled from the outside in. Before drilling, make sure the termocouple is not going to be at the same level as your shelf when the shelf is rested on your posts. This will keep the thermocouple away from the large mass of the shelf that heats and cools slowly and may therefore affect reading.
A smaller pilot drill was followed by a 15/32" drill. As always with drilling stainless steel, you need to use slower speed and a sharp drill (preferably, with a 135 degree point) and apply definitive pressure so that the drill grabs the metal right away. If you rub the drill against stainless steel, the drill will dull, stainless steel will work harden and make further drilling much more difficult.
Then I inserted the thermocouple with a nice reasonably tight fit. A made it so that it protrudes into the chamber a healthy 1 3/4" (approximately, as much as the Sitter does). Then I tried inserting a kiln shelf to check if the thermocouple doesn't interfere with its insertion. Such a significant protrusion of the thermocouple will insure a more true temperature reading. Besides, it played well with the lengths of the thermocouple insulation rings (by the way, I needed to remove two one them to fit my wall thickness). Then I attached the thermocouple to the terminal block to mark the block holes location and took the block away. Then I drilled very small holes and re-attached the block with sheet metal screws. Finally, I cut extra protruding thermocouple wires using an angle grinder with a cutting wheel attached.
This is a permanent installation to automatically control firing cycles. I did use peep holes for taking occasional temperature readings before though. But thank you for your suggestion anyway.
I'm in the process of finishing a DIY programmable controller box for my kiln. The kiln has the Sitter, and I'm not going to make any changes to the kiln. However, I need to install a thermocouple, so drilling a hole for it will be my next step. The kiln is an Evenheat 18"dia x 20" octagonal one. It has a stainless steel outside shell and 4"-high extension ring.
First, I thought about drilling a hole just below the Sitter cone holder. Then I decided not to attempt getting inside the control box and, instead, drill a hole in the ajacent wall. It would be easier to drill the lid, but I don't think measuring temperature on the top where it's higher is a good idea (no downdraft vent yet). Yes, I can make adjustments, but I'd rather know the actual temperature in the middle of the kiln (approx., the Sitter cone level). Besides, the lid is not covered with a metal shell, so attaching terminal plate to it will be very problematic.
Have you ever done it? What area did you choose for the thermocouple?
Did you use any refractory compound to seal around the thermocouple's insulation rings or went with just a reasonably close fit? I think sealing the space around is not a good idea since the thermocouple may need to be replaced at one point.
What are your thoughts, ideas and hints? It sounds like an easy and very straight forward procedure, but it's always better to get a second opinion.
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The same thing about "natural products" quackery and other "easy money" enterprises. I find it impossible to shake beliefs of those who are affected by this mass hypnosis, so I just stopped arguing with them. When I have patients who try to coerce me into going this route, I simply explain to them that I don't share those views and refuse to be a part of this. I'd stop respecting myself if I exploit people's ignorance to make money.