Picked up a couple kilns to convert to HT ovens

[Eric Morgan]

Bought these off of FB marketplace this morning. I feel like it was worth the drive and the $100 for the pair. Now to get all the parts for the PID temp controller. 
 

Apparently the bigger one was still being used until the control box got torn off moving it somehow. The smaller one may need some firebricks replaced if I can’t figure out a good way to keep the coils separated, but it apparently was working as well. 

[timgunn]

I don't know how familiar you are with PID temperature control generally. I set up PID controllers in my day job: I'm certainly no expert, but I'm not completely clueless. There were a couple of things I didn't fully understand about the process when I built my first HT oven. 

 

With the benefit of hindsight, I would have appreciated being told.

 

Pottery kilns tend to provide "Heat-Work" and the important parameter is the average temperature maintained over several hours. The workpieces usually have considerable thermal mass which helps damp out temperature fluctuations.

 

HT ovens need to provide quite precise temperature control, with minimal deviation from setpoint.

 

This is best provided by switching the element power through SSRs with a 2-5 second output cycle time. Most pottery kilns switch through relays/contactors on a 30 second-plus cycle time and this tends to give a saw-tooth temperature profile with relatively large teeth. With the reduced thermal mass of the work, and reduced damping, when Heat-Treating blades, this can give sizeable temperature swings, particularly near the cutting edge. Look for a DC-pulse output PID controller that will work with SSRs. If you are running on 220V in the US, I gather 2 SSRs are needed, so you'll want at least 40 mA of DC output current to switch 2 together.

 

I assume you'll be using an industrial PID controller, primarily because they are much cheaper than dedicated kiln controllers. Maybe even a controller with ramp/soak programming capability? Part of the reason they are cheap is because they don't have the same level of segregation between "technical" and "non-technical" settings. 

 

The dedicated kiln controllers usually allow the operator to set up the ramp/soak segments of the program, but deny access to any of the things that could result in a failure of the kiln itself: things like thermocouple type, the actual P. I & D settings, etc.

 

When you look for PID controllers, DOWNLOAD and thoroughly read the manual for each one you are considering. If you can't download the manual without jumping through hoops, look for another controller. If you can't understand at least most of the manual, look for another controller. 

 

The reason for this is as follows: you will need to set the controller up to do what it needs to do. You will therefore need to understand what the settings mean. If you find yourself out of your depth, you can either contact the suppliers tech support (in which case you will need to understand your process well enough to explain it clearly), or you can call for help, here or on another forum, with a link to the manual, hoping that someone familiar with PID controllers in general can look at the manual and offer advice. 

[timgunn]

The industrial controllers generally allow the operator to adjust the setpoint in "vanilla" PID control (no ramp/soak), but anything beyond that often requires a God-level passcode that allows access to absolutely everything. 

 

I have had a knifemaker, with an oven I'd built, call me because he'd hit a wrong button whilst working through a menu and changed the thermocouple type from N to R or S. He was smart enough to recognize that he was getting temperatures in the tempering range when set for the Austenitizing range and called me. I'd made a similar mistake in the past, so quickly identified the problem. It was fixed during a 5-minute phone call. 

 

You will need to consider the level of technical expertise that can be expected from the end-user: on his own, I don't know whether he would ever have sorted the problem.