I posted this question on another forum, but it probably applies more to topics here.  It is more of an electrical question than blacksmithing.

A friend has a small oven about the size of a microwave. Where he got it, I don't recall. The oven was a high temperature type used by a company to test the residual ash content of coal after baking out the volatile components. He indicated it has only one heat, and that is HIGH...about 2000 deg. F or somewhere in that neighborhood. Beyond that, I know nothing else about it. He would like to use it to heat treat and temper knives and other steel to temps of anywhere from 400 deg to 600 deg instead of the one high heat. Is there a type of rheostat or controller that he can place inline to regulate it to a lower temperature?

 

That's a "muffle furnace." I'm thinking you should take note of the manufacturer and model number and contact the company and see what they say. I'm thinking turning one of those down to tempering range is like using the engine from a AA fuel dragster in your Civic. It'd lose so much performance detuning it that far . . . I don't know what.

What I do  know if using a rheostat would be a B-A-D idea, they control current through resisters and get hot themselves. Let's say we take 1,600*f out of the oven and turn it loose in something sitting on the counter. Hmmmmm?

I suppose he could put the elements in a larger furnace till the volume was large enough it could only get to the desired temps but I think my toaster oven is more practical. Heck, check with the scientific supply close by and ask what they have on the used or discontinued oven shelf in the desired volume and temp range.

Oh one last thing about muffle furnaces, his electric meter will be spinning so fast you'll swear it's crying in anguish.

Frosty The Lucky.

Frosty, thanks for the insight on the furnace.  I haven't seen the beast, but your analogy of the AA fuel dragster and Civic might be a real problem for him.  He does have a toaster oven, but wanted to try to put this big one to use.  There is a first class electrical shop and supplier in town that I suggested he drop by and talk to as well.  I recall in labs in school that we used large rheostats or voltage controllers that had a dial and big black knobs on them (how scientific is that, fer ya???) to control some equipment, but I don't think they were ovens...too many years ago.

Heating to critical before quenching Yes, you could probably find a controller that world work with a thermocouple on it.  Tempering---best to source a different oven for that

Thomas, he is a blacksmith who does his heating and quenching via his forge, but apparently wants to use this old oven/furnace to temper.  I was given some links for heat treating oven controllers, but I don't know if he is willing to spend the $ for some of the equipment.

use a toaster oven, I agree with the analogy of top fuel dragster in a civic... I have never tried to quench in a forge

Edited March 15, 201511 yr by Steve Sells

PID Controller $46.50 http://auberins.com/index.php?main_page=product_info&products_id=1&zenid=90b32ac5f8da355dc0ed454b403f29f8

Thermocouple $27.75 http://auberins.com/index.php?main_page=product_info&cPath=3&products_id=39

Solid State Relay $15.00 http://auberins.com/index.php?main_page=product_info&cPath=2&products_id=9

Heat Sink for SSR $9.95 http://auberins.com/index.php?main_page=product_info&cPath=2&products_id=45

Grand total: $99.20

Run the power to the oven through the SSR, the PID controller can use the relay to turn the oven on and off. They can be used to control a propane forge within a few degrees, so they shouldn't have a problem turning an electric element on and off fast enough to control an electric oven at any temp you want.

PID Controller $46.50 http://auberins.com/index.php?main_page=product_info&products_id=1&zenid=90b32ac5f8da355dc0ed454b403f29f8

Thermocouple $27.75 http://auberins.com/index.php?main_page=product_info&cPath=3&products_id=39

Solid State Relay $15.00 http://auberins.com/index.php?main_page=product_info&cPath=2&products_id=9

Heat Sink for SSR $9.95 http://auberins.com/index.php?main_page=product_info&cPath=2&products_id=45

Grand total: $99.20

Run the power to the oven through the SSR, the PID controller can use the relay to turn the oven on and off. They can be used to control a propane forge within a few degrees, so they shouldn't have a problem turning an electric element on and off fast enough to control an electric oven at any temp you want.

​Great info, thanks!  I'll pass it on to him.

use a toaster oven, I agree with the analogy of top fuel dragster in a civic...

​He is currently using a toaster oven.  I believe this is a new project he has undertaken.

For PID, SSR and Heat sink check online for Homebrew beer or aquarium suppliers, some great deals are available

I've seen the PID for £15 ($22) direct from Hong Kong.on Ebay, these have been used by Microbrewers  for a while to control heat/cool temps. worth a try perhaps?

PID Controller $46.50 http://auberins.com/index.php?main_page=product_info&products_id=1&zenid=90b32ac5f8da355dc0ed454b403f29f8

Thermocouple $27.75 http://auberins.com/index.php?main_page=product_info&cPath=3&products_id=39

Solid State Relay $15.00 http://auberins.com/index.php?main_page=product_info&cPath=2&products_id=9

Heat Sink for SSR $9.95 http://auberins.com/index.php?main_page=product_info&cPath=2&products_id=45

Grand total: $99.20

Run the power to the oven through the SSR, the PID controller can use the relay to turn the oven on and off. They can be used to control a propane forge within a few degrees, so they shouldn't have a problem turning an electric element on and off fast enough to control an electric oven at any temp you want.

​That's what I would do also. Heat treating ovens / kilns/... should use a PID/SSR solution. The self learing algorithm of the PID is perfect for the application.

I would go the pid route.. you can find them dirt cheap.. Grab a type K thermocouple, I like the long ones with the threaded mounts. Just run the SSR into whatever type outlet the oven takes and drill a hole through the oven in a safe location to mount the thermocouple ie whereever there aren't any wires running, probably the door if you don't know for a certainty. I use them for everything from my forge, heat treat oven and even my coal stove in the shop.. and I don't think I've ever spent more than $45 for all the needed parts.

J

For what it is worth it would be good to estimate the current draw on the heating element so you buy a big enough solid state relay..or just get a monster 40A one..

Nicole