Cheers. BTW what wattage does yours run at? How much current/Voltage through/across the elements?
I'm in the UK and we have 230V mains here. Our domestic outlets are rated 13A and the standard plug is fitted with a 13A fuse. This gives 3 kW of available power.
I use 2 elements, each wound for 1500W at 120V and wired in series. Coil OD is .380".
The elements are mounted in routed grooves in the sides with 4 horizontal runs per side.
The first few I built were 18"L x 7"W x 6"H internally and used the 16 AWG elements. By about number 5, I'd decided that thicker (and therefore longer) elements would give better reliability. and felt that longer grooves would be wise to accommodate them. I was happy that the power input would cope with a longer oven and added an extra brick to bring the length up to 22.5".
That worked fine.
I'd had a couple of conversations with smiths. One of them had one of my early, 18", ovens and wanted something longer. He only works in Carbon steels and therefore did not need stainless temperatures.
Another came up with the bright idea of building 2 ovens. Normally one would be for Austenitizing, the other for tempering. Each would just plug into a common control box and this would eliminate the need to wait while one oven cools from Austenitizing to below tempering temperature. However, for occassional long stuff, both ovens could be bolted together.
The first smith being happy with Carbon steel temperatures made it pretty much risk-free going to 27" long: if it wouldn't reach the highest Stainless temperatures, it wasn't going to mean a scrapped oven. I was absolutely certain it would easily achieve Carbon Steel temperatures.
I built one, tested it, found it would actually reach 1300 degC/ 2372 degF quite comfortably (this was the upper measuring limit of the Type N thermocouple. The temperature was still rising) and then built another to test the joined-together setup. That also worked rather well.
I think a 31 1/2" oven built the same way and from the same materials would probably reach at least 2200 degF on 3 kW and would therefore be able to handle all but the most demanding of the modern steels.
My preferred thermocouples are Mineral-Insulated assemblies from Omega, using their "Super Omegaclad XL" sheath material. This is a proprietary material rated for use to 1335 degC/2440 degF. I feel it is worth the (fairly small) premium, though I have also used Nicrobell-sheathed MI thermocouples from other suppliers without problems. Nicrobell is rated to "only" 1250 degC/2282 degF.
I usually use transition-joint assemblies to eliminate any potential wiring issues.
For HT ovens, I use type N because long-term accuracy is the priority.
For measuring forge temperatures, I use type K because the extra 70 degC/126 degF of range at the top end gets well into the welding temperature range and that seems more important than the long-term accuracy for the application.
