I don't want to hijack this thread but since all the pros on self made heat treatment ovens are gathered here I thought it might be a good place to ask.
So far all the self made ovens I've seen use the bricks laying flat down. Wouldn't it benefit the insulation to turn the bricks by 90° and therefor have a 4.5 inch wide wall instead of a 2.5 inch one. I would need more bricks and the outer dimensions and the weight would be larger but that wouldn't bother me in my construction.
Any suggestions on that?
Also what's your experience with using cement to connect the bricks instead of just putting them side by side? I thought about just cementing each side to make one larger and sealed "brick" and then put the walls, top and bottom just together by placing them next to each other. Therefore it would be easy to disassemble if I needed to replace the heating elements.
Thanks.
Yes, but....
Assuming all else is equal and both have the same heat input from the elements, an HT oven with better insulation will need less power to maintain any given temperature. However, an HT oven with more thermal mass will need a greater heat input to reach that temperature.
I'm guessing most real-world HT cycles expend more power heating to temperature than holding at Austenitizing temperature.
The calculation for heat-input-to-reach-temperature vs heat-input-to-maintain-temperature is fairly complex and will be different for each temperature. If you regularly run a particular cycle, you could do the math and work out the best approach for your process.
If building more-or-less in-situ, you don't have quite the same issues with shipping and handling that the Evenheats and Paragons of this world face. Nor do you have to consider some of the commercial factors they do (competitive market, selling at a price-point, etc).
I get the impression that most knife makers would like shorter time-to-temperature than they have, pretty much regardless of the speed their HT oven heats up. I think most would happily take a hit on electricity cost to reduce the time, if they were given the choice.
Cementing is a skill. I do not have that particular skill and I do not feel it would be worthwhile putting in the time to learn it. I tried using jointing compound on my first HT oven, but gave up because the IFBs sucked all of the moisture out of the jointing compound before I could make the joint. The compound was then too stiff to move. Soaking the bricks helps, but then you need to dry everything out completely before using the HT oven.
I've certainly seen noise on the thermocouple signal. It wasn't too bad on the controller (AD Solo), but showed up bigstyle on the datalogger I was using (ABB SM3000) at 1-second sampling intervals. I assumed it was mains-frequency noise, but did not put a scope-meter on it to check. The Solo has adjustable input damping/filtering, as do most PID controllers, so it should be easily manageable.
I use a completely separate control box with separate sockets on to accept the cables for the power cable to the elements, the door switch and the thermocouple. This gives longer cable runs than most builders will have to contend with, and an increased likelihood of noise problems. I've had enough niggling problems with noise in thermocouple and sensor circuits in my day job that I automatically tend to follow best practice: use screened cable where possible, keep power and sensor wiring apart, etc.
The main reason I build the separate box is so that I can use it for other stuff as well. I have already built a small salt pot and still want to build a tempering oven, heating jacket (for making diffusion-bonded Mokume Gane in an hydraulic press) and a few other things when life stops getting in the way.
The thermocouple connection is on the front (I use type N thermocouples and IEC 584-3 cable/connectors, hence the pink). If you use copper cable instead of thermocouple extension (or compensating) cable, you'll get a cold junction error equal to the difference between the temperature of the controller and the temperature of the join between the thermocouple legs and the copper cable. It's best to use the correct cable.
On the bottom of the box, the gland is for the incoming mains power cable, the black plug/socket is for the door switch and the blue plug/socket is for the power out to the elements.
The cable trapped in the door is the comms cable from the controller to a laptop. I could not find drivers that would get the cheap Chinese RS485 converter to work with Windows XP, so never actually got the comms working.
