HT oven build questions.

[Nick EBK]

Over the past week or so, I've really been delving in to the stickies, and I've learned quite a bit about what I've been doing wrong to date. In order to take my knives to the next level, I need precise heat treat control, so I want to build a 120v electric oven. I'm planning on basically mimicking Evenheat's kh418. I'm not the most electrically inclined, but I think I've got my head wrapped around the whole PID/SSR/TC system. In the stickies, Stacy links to a preferred PID, but that link is dead. Is there a preferred PID, or at least is there something it needs that I'm missing? I know it needs to run on 120v, have a range up to 2000f, and be programmable, but thats it.

Also, for the heating element, I will probably buy the replacement coil for the kh418, but in theory lets say I took out the wire from a hair dryer. Would twisting the wire tightly on itself have the same effect as running a thicker wire at half the distance? I don't know if kanthal works that way.

Thanks in advance.

 

[timgunn1962]

You can get precise heat control using a very basic PID controller. It's plenty good enough for many Carbon steels, but once you start getting into hardening Stainless Steels you'll want ramp/soak capability.

The cheapest Ramp/Soak controllers are much more expensive than the cheapest basic PID controllers.

The most popular among US builders seems to be the Auber SYL2352P http://www.auberins.com/index.php?main_page=product_info&products_id=4

I use either the AutomationDirect Solo SL4848VR http://www.automationdirect.com/adc...ers/1-z-16_DIN_Size_(SL4848_Series)/SL4848-VR

Or the Omega CN7823 http://www.omega.com/pptst/CN7800.html

The AutomationDirect and the Omega are effectively the same instrument with different badges, so I use whichever works out cheapest at the time. Auber do not have a presence over here, so I've not tried them.

All 3 of the above will work on a 110VAC supply. The input range is largely determined by the input type selected and you will probably use a type K thermocouple, though I prefer Type N (Type N was developed to be more stable than type K above about 1000 degC / 1832 degF. My preference is based on experience in an altogether different application, so may not be an improvement on type K in this application). All 3 will take type K or type N inputs and display/control to any temperature you might realistically need for HT in either degF or degC.

It's worth mentioning that industrial controllers are aimed at industrial users with an understanding of both the process and process control in general. The ramp/soak profile programming is usually *much* less user-friendly than the controllers used on big-name kilns aimed at the end-user.

Doubling the wire cross-sectional area will halve the resistance, so it will effectively work like a wire of 1.41 times the diameter, but the math involved in lengths, resistances and power outputs is pretty complex. Once you start adding in surface loadings, you are looking at migraine territory. I quite like that stuff but for most folk I'd suggest that if you need to ask, you are probably best either going to someone that sells elements and letting them do the calculations, or buying "known" elements and building the rest of the oven to suit.

The usual element material is Kanthal A1 or equivalent. I'd advise specifying 16AWG wire as an absolute minimum. Thicker is better. Thin wire equates to short element life.

 

[Nick EBK]

I will look into those PIDs, thank you.

It's worth mentioning that industrial controllers are aimed at industrial users with an understanding of both the process and process control in general.

I suppose I should start studying up then.

I quite like that stuff but for most folk I'd suggest that if you need to ask, you are probably best either going to someone that sells elements and letting them do the calculations, or buying "known" elements and building the rest of the oven to suit.

Exactly why I'll be buying the element, too many times in the past I've tried to jury rig things in that manner. This kanthal scenario is totally hypothetical.

My understanding (please correct me if I'm wrong!) is that the thinner the wire, the hotter it will be, at the same amperage. So by doubling up the wire, it acts as a larger wire of the same length in that sense. Where I'm curious, but not curious enough to go actually try this and kill myself is, does Kanthal braided in this manner works the same way? I suppose it should, for some reason in my head there would be arcing or the wire would melt itself.

 

[Nick EBK]

Suppose I get the Aurber SYL-2352P (SSR output model), is their 25A SSR (MGR-1D4825) what I should be looking at as well?

 

[Kramwit]

Ok I must be tired of reading, but I want to put a temperature controller in with the pid controller. I have fuses, switches, ssr's mapped out, ground also but I can't figure where to install the temp. Control.......

 

[Stacy E. Apelt - Bladesmith]

Do you mean "Thermocouple"? The TC mounts in the oven and the leads are attached to the PID controller. Use a TC mounting block to mount the TC and connect the wires. The wiring connecting them needs to be TC wire that match the TC type. Both the TC and the wires are polarized, as is the connection to the PID.