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help needed evening out my kiln temp differences


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#21 R Fraser

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Posted 28 January 2013 - 12:43 PM

One thing I also have that may provide some guidance is an oximeter made from a Ford fuel injection oximeter. The thing works pretty well and is easy to makeHow about giving us the build details on this?Mark



I read about plans for the same thing on an Roger Grahm's web site from Australia. All you need is a 2 lead type oxygen sensor and an Ohm/Voltmeter. Apparently you can buy used oxygen sensors fairly cheap from junk yards if you remove them yourself, but I priced new Bosch sensors with an extended probe at around 100.00USD.
There is a thread about this on clayart but the original (as far as I know) discussion and link to the PDF with the how to set up and "calibrate" is found here:

http://members.optus...ust_sniffer.pdf

The Main page that is filled with Ooodles of good stuff is here;

http://members.optus...u/~rogergraham/

I deleted a hobby and used the proceeds from the sale of the stuff related to said hobby to buy a fireball Oxyprobe, but was in the planning stages of using an oxygen sensor in the kiln exhaust when I remembered the RC airplane kits gathering dust in the attic...

Richard
So I built a light saber, am I a Jedi now?


#22 Diz

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Posted 28 January 2013 - 12:57 PM

Yes - I use the same thing for an oxyprobe. It works great - so easy and cheap too when compared to Oxyprobe prices!

Diz

#23 docweathers

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Posted 28 January 2013 - 12:58 PM

yes, that is the source of my information on how to build the oximeter. The only thing I would add is that I got a new Ford probe off of eBay for $35. It is one of the five wire models it works fine by hooking your multimeter to the black and gray leads

Larry

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#24 Mark C.

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Posted 28 January 2013 - 01:11 PM

Thanks for all those links -I did read about this years ago on clayart-but forgot about it.
I own and use two oxyprobes but I could use a third one and this amy fit the bill.
Mark
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#25 docweathers

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Posted 28 January 2013 - 01:35 PM

I was surprised that a salty old 40 year veteran of pottery did not know of this. :rolleyes:src="http://ceramicartsda.../rolleyes.gif">

Larry

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#26 Mark C.

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Posted 28 January 2013 - 06:32 PM

I was surprised that a salty old 40 year veteran of pottery did not know of this. :rolleyes:src="http://ceramicartsda.../rolleyes.gif">


I'm at the point where I have forgotten more than I know-or put another way the older I get the less I know
You get the idea
Mark
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#27 docweathers

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Posted 11 February 2013 - 01:01 AM

I tried Diz's suggestions of putting a baffle shelf just under the exhaust port and baffles made of kiln post over half of the burners. Though this kept the top and bottom temperatures within 15° all way up to 1785°F, I had to abort at that point because even though I was running 7 inches of water manifold pressure ,my temperature rise had dropped to 40° per hour and falling. iI was obvious that there is no way that I was going to get to cone 6

suggestions would be appreciated.

Larry

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#28 Mark C.

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Posted 11 February 2013 - 02:29 AM

Diz posted a pretty extensive post on how to with that kiln-I would stack it his way-My guess is too much burner was blocked or diverted but its just a wild guess. His stacking method is spelled out great along with a small stack for draw-I would recreate his methods .
More draw is good
Sounds like you were stalling out-that can be from to much damper closing or not enough fuel or underpowered burners or blocked burners or outflow blocked up to much(his was 2 inches?)-no simple fix just have to experiment. These torch bearers sound like they have burner issues -are yours all 1/2 from floor underneath as he mentions?
Mark
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#29 Diz

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Posted 11 February 2013 - 11:49 AM

Hi Doc,

I often have more that 15 degrees between top and bottom early in the firing but it evens out later and once I got to about 2000 then my increase really slowed down. I was told that I needed to have more air action within my kiln so I used bricks to make a chimney - just place 4 bricks to form a box around the top port (about 5" high) and make sure the primaries are fully open. I also use a broken shelf placed across the top of the chimney to help keep heat in the kiln. I start closure about 1000 degrees and although I use this method to later induce reduction, for most of the time the kiln is in oxidation even with the top port partially closed. I can have the port closed 1/3-1/2 and it will still be in oxidation with temperature rising. Sorry but I don't know what my water pressure manifold reads. What Mark mentions about too much or too little fuel is a problem and a PAIN as it is all experiment.

I have found that 1.5 to 2" is the ideal space for the baffle below the top port. Again I use a 1/2 shelf or broken shelf for the baffle it is not necessary to have a 'full' shelf there.

Good luck.

Diz

#30 docweathers

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Posted 11 February 2013 - 08:16 PM

My primaries are fully open.

I don't understand the point of this "I used bricks to make a chimney - just place 4 bricks to form a box around the top port (about 5" high) ... I also use a broken shelf placed across the top of the chimney to help keep heat in the kiln." You are increasing the draft by building a 5 inch chimney and then restricting it again with a shelf across the top of this chimney. It would seem as if these two things are contradictory, though maybe I don't understand the intent here. Please clarify.

in a prior post on this topic you mentioned that you cut off the burner tubes so they were just 1/2 inch lower than the bottom of the kiln. I want to make sure I understand how you are measuring this. Mine are about a half-inch below the inside of the bottom, as opposed to the underside. Is this what you meant?

I have one of the older six burner versions of this kiln. I am running propane with my jets drilled at 3/64ths.

Back to it a prior topic in this thread.. the oximeter.

attached is a picture of my version of this gizmo

By pivoting it on a piece of strap steel, I can also calibrate my exhaust port opening with numbered marks on the top of the kiln to the right in the picture. This is the five wire version of the probe, which works fine if you measure voltage across the gray and black leads.

Attached Files


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#31 Mark C.

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Posted 12 February 2013 - 02:29 AM

Lets see what Diz answers on your points but I thought he meant the burners where 1/2 below bottom of kiln. All the burner setups I have been around never protrude into kiln there is some space to pick up what's called secondary air.
On the draft issue-

Let me see if I can verbalize this so its clearer-As far as the short chimney I will say that in any natural draft kiln as soon as kiln is hot the induced draft from a chimney helps draw the heat/flow thru the kiln and before you get to a reduction temperature this draw will help with the temp climb. Think of it as a breathing creature-needs air and draw to breath.So before say 1800 degrees the draw of a chimney is already helping get to that temp before you even push in the damper. If the damper was below the chimney then its even better but thats hard on this trashcan kiln(this is what I have always called them and Its not a dig.).
I like your damper set up but for one fire take it off- stack up some soft brick as Diz said (lighter than hard brick) and use a broken shelve on top to see if this helps.
I never have fired one of these small round flat top kilns but you should be getting to temp better than you are-
The burners sticking into floor do not sound right as well.
You have 6 burners which to me mean you should have no issues gaining upward temps-Is there any chance you are reducing to much stalling out the climb?
Mark
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#32 docweathers

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Posted 12 February 2013 - 01:26 PM

Let me ask one specific question.

How should I interpret the fact that my kiln temperature generally rises faster with the damper about two thirds closed? My procedure is to watch my pyrometer and slowly tweak the position of the broken kiln shelf I put over the exhaust port. Often I will spend 15 minutes observing the temperature response to slightly different kiln openings until I get one that seems to cause the fastest increase. Usually this comes out to around two thirds closed.

I have tried opening the exhaust port up to half way and the temperature will actually begin to drop whether I have the baffles in or not.

Does this mean my jets are too big or too little?

I don't know if I reducing too much or not the above is the procedure I have been using.

I understand how the extra draft produced by the small chimney would increase the airflow through the kiln. The part I am fumbling with is why one would then put a restriction over the top of it. This would seem to undo the increased draft.

In my particular case, I'm not sure how increased draft would be helpful since, as described above, my temperature rises actually better if I restrict the draft.

I'm beginning to think that your "trashcan" name for this kind of kiln has some reality.

I know I'm going to have to keep experimenting but the more focused my experiments the fewer I may need to do.

Thanks to all for the helpful input

Larry

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#33 docweathers

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Posted 12 February 2013 - 03:58 PM

I know I'm going to have to do a lot of testing to get this thing to work right. I would like to make these tests as quick and efficient as possible, without trashing anymore pots. If I only have my shelves and baffles in the kiln, how fast can I raise the temperature without doing any damage?

Larry

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#34 Mark C.

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Posted 13 February 2013 - 12:00 AM

Empty kilns fire different than ones with pots--300 per hour is about right for a fast fire on furniture.Puts in some junker pots and fire it with something in it-you cab fill it with bricks as well that will fill space.
Mark


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#35 Diz

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Posted 13 February 2013 - 10:11 AM

Doc - I know your frustration and I have been there for a LONG time - this kiln is a bear!

Although the shelf across the chimney around the exhaust port does inhibit the air flow , as you have said you can see the temp climb when it is inhibited. There is an extremely fine tap...tap...to the exhaust shelf that can change the rise/fall of temp or in/out of reduction. There is no easy way to fire this kiln that I have found. My thought is that 'if it doing what I want (climbing, reducing) than I don't fiddle with the exhaust port'. Once I get to 2000 my climb is very slow 75-100/hour at most but if temp is climbing I leave it alone. Usually I have to fiddle with the 1/2 shelf to keep temp rising and in reduction but during several firings by the time I set the kiln into reduction it walked itself up to temp and go into and out of reduction at the proper times all by itself. Now I would love to know the good gremlin that is helping me that day! He doesn't appear too often but I love it when he does.

This kiln is a true trial and error. I keep notes about stacking and loading the shelves. After incredibly good fires I have reloaded the kiln exactly the same way and fired with completely different results. The only good thing I have found about the slow rise of temp at the end (<100 per hour) is that the heat work comes into l play and using visible cones by the peeps, the heat work reaches temp before the actual pyro reaches even 2300. My firings run about 12-14 hours to cone 10 reduction.

Regarding the 1/2" between the pipes and base - when I adjust that I put a 1/2" board across the top of the pipe and slide the pipe up so the board covers the hole in the bottom of the kiln - then I remove the board.

Hope this makes some sense - every firing with a Torchbearer is an experience. Good luck!

Diz

#36 docweathers

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Posted 13 February 2013 - 10:44 PM

Trying to put all the good advice together the game plan is:

1. Modify Diz's burner baffles to still redirect three burners to under the bottom shelf but so that the flow pattern is smoother. I am going to saw up an old broken shelf into three 4 x 6 squares. Above each burner I will put a six-inch side against the outside wall and lean it in to touch the outside edge of the bottom shelf. I'm hoping this will cause a smoother and thus less restrictive gas flow through the burner but still retain the temperature balancing effects of directing the flames under the bottom shelf.

2. I'm going to drill out my jets a couple of drill sizes.

3. Add an enhanced version of Diz's chimney above the exhaust port. And, this is where I need some suggestions. I would like to stick about a 2 foot section of 7 inch ID tube above the vent. My question is, what kind of inexpensive tube can I use that will withstand the heat?

I figure this should move a lot more air through and if I mix more gas with it then I should get more heat, I hope. I am really tired of screwing with this thing. I want to do an overkill solution that, if necessary, I can dampen down on both the gas and air side.

Then I should have a truly Diz-y kiln.

Any suggestions would be appreciated.

Larry

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#37 Diz

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Posted 13 February 2013 - 11:22 PM

Doc,

I have never drilled out my jets - they are the same ones that arrived with the kiln. So have no idea what help that will be. I do know that if I turn my gas valve open too far that my temp does not rise, it falls.

My gas valve is like a swivel that will turn 90 degrees from closed to open. In all the times I have used this valve I know that I have never opened it up 20 degrees total and when I make adjustments they are minimal - as soon as I feel the switch move I stop. (My change to the kiln this year will be to make an extension on the length of the valve handle so that the arc traveled when I open the gas valve will be easier to document).

Truly tiny adjustments in the amount of gas make large changes, be they good or bad. That is why this kiln is such a bear.

Hang in there as there is no easy fix. The only faith that I had in Olympic was that the kiln could be fired so I did not want to change too many physical things as then it would not be the kiln they built - I just fought to manage what they offered me.

Diz

#38 docweathers

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Posted 14 February 2013 - 12:15 AM

I have already read drilled my Jets three times. I went from the factory recommended way to big, to too small but much better, to a bit bigger yet and even better. now I'm going to give it a shot with even a little larger but not as big as the factory recommended corncobs size.

With the original jetting the valve was so sensitive that it was impossible to move the valve in any purposeful way. The only way that I can tell that I had moved the valve was that the burner sound would change slightly. I have the same gas valve you do. I welded an 18 inch rod on the strap metal lever that not only made it longer but stuck up higher so I didn't have to crawl around the floor to fiddle with the thing. I really recommend a manifold pressure gauge or a manometer. It make your gas adjustments much more reliable and repeatable. I got a very nice one for $15 off of craigslist.

I'm fantasizing building a heat exchanger that will let me recycle some of the exhaust heat back into the air intake of the burners.

Thanks for your help

Larry

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#39 Kally

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Posted 23 February 2013 - 09:59 PM

I have already read drilled my Jets three times. I went from the factory recommended way to big, to too small but much better, to a bit bigger yet and even better. now I'm going to give it a shot with even a little larger but not as big as the factory recommended corncobs size.

With the original jetting the valve was so sensitive that it was impossible to move the valve in any purposeful way. The only way that I can tell that I had moved the valve was that the burner sound would change slightly. I have the same gas valve you do. I welded an 18 inch rod on the strap metal lever that not only made it longer but stuck up higher so I didn't have to crawl around the floor to fiddle with the thing. I really recommend a manifold pressure gauge or a manometer. It make your gas adjustments much more reliable and repeatable. I got a very nice one for $15 off of craigslist.

I'm fantasizing building a heat exchanger that will let me recycle some of the exhaust heat back into the air intake of the burners.

Thanks for your help

Larry



#40 malcolmm

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Posted 21 March 2013 - 09:23 AM

Hello,
I have an Olympic torchbearer gas kiln model 2827G. It's a basic updraft with 4 venturi burners and I fire it manually using cones and a pyrometer with 2 leads to monitor the temperature. I've used it for bisque and oxidation glaze firing-both low and high up to cone 10. The problem has always been the temp difference between the bottom and the top shelves. There is usually around 100 degrees celsius difference (sometimes more) with the top being hotter. I've tried packing the top denser than the bottom with some success. I also tried moving my shelves to act as saggers with the hope it would redirect the flame. That made the bottom even cooler and I aborted that firing until I could rearrange the shelves back to usual configuration. The longer I fire it, the less difference there is but the longer the top shelf is at a mature temperature so it can end up over fired and the bottom under fired. Does anyone else have this problem with this kind of kiln? Any ideas on how to solve this?
Thanks


Hi,
I have been firing a 2827G for the past year with the same disappointing problem. I have tried the diverting flame on two burners, the self 1 1/2 inches from the top, packing more densely with staggered shelves - but to no avail.
I deduced that it was colder at the bottom because more cold air was being drawn in via the rather large burner ports in the bottom of the kiln. Which was also the reason that I couldn't reduce the bottom half of the kiln. What I tried was to take 4" standard insulating bricks and drill a 2" hole near one edge and mitre the edges to fit snugly into the angle so that the hole is centred over the burner. The 4 bricks now support my bottom shelf. The difference in temperature between top and bottom is negligible (the bottom being slightly hotter) and I get reduction throughout the kiln.




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