Getting ready to put together a kiln

[Huntrick64]

Hey all, my first post here.  I've been building and hunting with selfbows since 1991.  The last few years I have been wanting to hunt with stone points, but want to use my own.  I have tried knapping off and on since I was a kid, but never stuck with it long enough to make a decent hunting point.  I am now breaking rock a couple times per week and finding the need to heat treat a little more than what my turkey roaster has been doing.  A friend of mine and I just secured an old pottery kiln with inside dimensions of 17.5" diameter by 18" deep.  That works out to almost 19 gals of volume (or 4 buckets).  I recently ordered all the stuff from Auber's to build a ramp/soak PID using a SSR and "K" type thermocouple.  I'm going to try to use some of the existing coils to get to enough resistance to generate 1200 watts on a 120V circuit.  I'm hoping to get two wraps around the inside of the kiln with the proper resistance.

My question is, do you think 1200 watts will be sufficient to heat this thing full of spalls, or will I need to cut the kiln down in size?  I want to stay with a dedicated 15A 120V household circuit.   Most of my rock with be Kay County (550), but I can see wanting to get some novaculite as well (850-900).

thanks,
Rick

[iowabow]

How many ramps or segments can your program into the pid

[Huntrick64]

This particular one has 30 programmable steps some have 50 steps.  You would use about 5 or 6 steps for each recipe so I could basically keep about 5 recipes stored.  You can start at any step you want so here would be an example:

Step1  200 degrees soak for 24 hours
Step2  Ramp 200 degrees to 550 degrees over 10 hours
Step3  550 degrees soak for 8 hours
Step4  Ramp 550 degrees to 200 degrees over 10 hours
Step5  shut off
Step6
Step7  Another recipe

Let's say that step1 is for Kay County, I could start at step one and it would stop at Setp5.  Novaculite could start at Step7, etc.  The really cool part in the ramp feature is that it calculates and uses a linear progression for the increase in temp over whatever time you put in.  So you don't have to use up steps for each hour if you wanted to raise the temp 30 degrees per hour, just put in beginning temp, ending temp and the duration of the ramp.

Anyway, this is how I'm hoping it works.

[iowabow]

You will need to program rate/ hour up and down

[iowabow]

Pm your number and we can talk

[iowabow]

Pm me for perfect ramp for novaculite. It has to be done just so or it will crack

[iowabow]

Ok I missed the rate =temp to temp time. I was wondering how you would set rate.

[Huntrick64]

IOWABOW, PM Sent

[Huntrick64]

An Update:

Measured voltage at the dedicated 15A receptacle is 124V.
 
The old kiln has 4 elements and 4 switches.  I removed all of the electronics (kiln-sitter, switches, etc.)  To my surprise, all 4 elements were still good.  Each one makes 3 complete wraps around the inside of the kiln and measured 9 OHMS.  I think I'm going to try and use one or two of the existing elements in the center of the kiln with our setup.  The existing elements are too brittle to modify without breaking ( I proved this on the kiln extension elements).  The 9 Ohms is too little resistance and would provide 1700 watts (I like), but would draw 13.8 Amps ( a little too close to 15A for me).  If I wire two of them parallel, then total resistance drops to 4.5 Ohms, and Amps jumps to about 28 (going the wrong way).  I actually wired two of them serially and they measured 17 Ohms.  Even though 7.3 amps was well within the safety zone, we are dropping to 905 watts.  I'm hoping that this will still be enough heat given the insulating abilities of the kiln.  If I can ramp up a full kiln at 20-30 degrees per hour with 900 watts, we should be fine.  The inside dimensions of the kiln are 17.5" diameter X 18" high. 

Does anyone have any experience with a kiln of similar size using only 900 watts?

Do you think this will work?

On kilns that hold 3 or 4 buckets of rock, other people talk about using two separate Robert Shaw switches and separate elements with 16.5 Ohms each.  So they would be looking at two elements of 900W each where I would only have one, but I have no idea how quickly their kilns heat up.  I'm thinking that their 16.5 Ohms is more of a function of making the Robert Shaw switch function properly instead of generating the correct heat. 

Man, I'm not sure how this qualifies as "primitive", but I guess I also drive a pickup truck to the quarry to get my rock, wear modern clothes, eat processed food, etc.

[Huntrick64]

UPDATE!

Got everything assembled and tested.  Last night we set her off on her maiden voyage, and so far, everything is working perfectly.  The ramp/soak feature is awesome.  Now we have to see if the two serial elements can heat it up at 20 degrees per hour all the way up to 575.  It had no problem ramping at that rate, completely full of cold rock, to 200.  It is holding between 3 and 4 buckets of rock depending on their shapes.  The PID is soaking it within +or- 1 degree F.  You could actually watch it "auto program" itself to get better at activating or deactivating the element.  When it first started it would be +or- 5 degrees.  We used the 3M Fire Block FB-136 (Lowes) to caulk up all of the holes in the kiln and to plug the gap around the thermocouple.

Time will tell.

[iowabow]

Very good thread

[iowabow]

Something to keep in mind, a type K is good for about 50 normal firings but the heat treatment is longer and will wear on it faster. Watch for a crack to develop at the tip. This crack will be an indicator that it needs to be changed. Also the first firing should be done emply to oxidize the element without contamination. This will insure an accurate reading. Just something to remember when you replace it.

[Huntrick64]

I never knew that, but will watch for it.  I have a k-type thermocouple on my PID for boolit casting and I have fired it about 200 times at 750 degrees, but it stays submerged in lead even when it is cold so that probably matters.  I'm thinking my hurdle might be getting to 575 degees with just 920 watts, but so far it appears to change the temp pretty quickly and holds well.  My element makes six complete wraps around the inside of the kiln covering about the middle half of the kiln.  Will know by morning how it did getting to 575.  If all goes well, then I need to round up some novaculite to see if it goes to 850.

[iowabow]

Hey how did your cook go

[Huntrick64]

It ramped up exactly like it was supposed to at 20 degrees per hour.  It is holding at 575 right now and is varying no more than 1 degree +or-.  This thing is amazing.  Sometime I will list my materials that I bought from Auber, but it is pretty simple.  The programming stumped me a little until I emailed Auber.  They are very knowledgeable and helpful.  Once they explained a really simple concept on the PID programming, it was very easy.  Keep in mind that the power doesn't flow through the PID, it flows to the SSR and the PID simply sends a DC current to open and close the SSR. 

This PID has 30 available steps and can also be programmed to trigger alarms and other things if needed.  I have enough space for 5 separate rock recipes and can simply start the PID at any one of the recipes and it shuts off at the end of that recipe.  Their instructions on wiring were somewhat vague, but I found a great schematic (in laymen terms) on the castboolits website.  I will build another one of these as soon as I find another kiln about the same size.  If my math is correct, assuming the coil fires about 20 percent of the time, and electricity is 10 cents per KWH, then it costs less than $2.00 to cook a batch of kay county for 3 days (soaking to dry, ramping up, soaking at high temp, and ramping down).  You can't buy much charcoal for $2.00.