Evening All,
So today I tightened up and sorted all the wiring and double checked everything. I am still waiting for two ceramic standoffs and two split bolts for the element lead connections because I just want to make sure they cant touch the case if the brick inside shifts for any reason. I drilled holes large enough to keep a buffer with the steel case but I just want to be double sure there is no fault to case ground. In the mean time, I wanted to do a test fire and see how the element may expand and shrink over a heat and cool cycle. I wanted to do three cycles but had to drive to Charleston and back so I didn't have time but for one cycle. I don't smoke anymore but I do vape and have used kanthal to make vape coils and learned a long time ago to do several burn in cycles on a newly wound coil and then adjust individual coils for peak performance and even heating. I can't imagine its any different with a kiln element. I had the control box sideways and set several inches away from the case so I could monitor everything as well as take some current flow readings on the element leads while the PID was allowing the SSR to pass full current cycles during heating which ended up being slightly over 11 amps (perfect buffer on a 15 amp circuit). Surprisingly, the 120V oven heated to over 900 F in under 8 minutes. The PID started to taper off my 1000 F setting and it was another two min to reach 1000 F. I thought it would take longer at the voltage and wattage I built it to and that was nice surprise. I let it soak for about 10 min and then stopped the cycle and observed the cool down. It all went well except for one thing I was a bit surprised about. The cabinet sides both got to about 130-140 degrees on a very short 10 minute full heat cycle and then remained quite warm, (over 125 F during cool down). The top, bottom, and back of the oven case stayed just barely above ambient temperature. The majority of the element coils are on each side. Since my design has the control box mounted on the right side of the case frame and the fact that the left side of the control box is open, I've decided to make a design change and close that side of the control box with a steel plate as well as install standoffs to move it at least an inch away from the side of the case. No need to allow all that heat inside the control box to shorten the life of the SSR and PID. I may even sandwich some kaowool there to assist in keeping the control box components as cool as possible while working. My little two brick never gets quite that hot on the outside. Maybe 90-100 F but definitely not 140+ and no idea how much higher it would go on a longer heat cycle! I should have the rest of the parts I need sometime next week so in the meantime, I'll work on the design change. Once I have this lil guy done, I'll post a refined schematic with the little changes I made to the wiring as well as some photos. As a side note, I followed the guidelines from DanCom's kanthal calculator and came up with 29.63 feet at the coil diameter I wound and the gage of wire used. To my surprise, the resistance was too high at almost 11.8 ohms when done. I very carefully measured the length before cutting it and winding the coil. I ended up having to cut off about 20" or so to get it at my target which was 9.6 ohms. I ended up with 9.4 ohms and I can live with that. I'm not sure if it's an anomaly with the wire I bought or just a space/time continuum issue here in Darlington SC
Anyway, I think I will do another calculation with DanCom's .xls file for a shorter element and wind it with some of the remaining wire I have so I can experiment and hammer that math down for future coil windings, maybe even two or three different lengths... I don't know the "inside baseball" math but can guess that the change does not take a linear path with kanthal the longer you go, probably a complicated logarithmic curve involving math even Martians don't understand. Either way, I'll reverse engineer a test coil or three if necessary to get the correct resistance according to the math of the calculator and see what comes of it as far as correct length on that sample of kanthal I bought. No bets on any different suppliers though. I hope all this info will possibly help someone tinkering with a HT oven build someday.
Cheers!
Steve.