Thermocouple offset vs lowering max temperature

[HenryBurlingame]

I have been dialing in my new L&L kiln (it has been firing a little hot in all zones) and had a question for you all:

I am usually firing custom programs and was wondering if would be better to set a thermocouple offset or just lower the max temperature that I am firing to. I imagine that if I do a thermocouple offset to fire to the correct cone then all of my segments/holds etc. will be offset by the same amount. But do I want this? I think the thermocouple reading is correct with no offset (at least it is at room temp), so why not just set my max temp a little lower rather than do an offset?

I don’t think cone offsets apply to me as I am using custom programs to fire to a certain temp and I am not using the fire-to-a-specific cone feature.

Edited September 12 by HenryBurlingame

[Bill Kielb]

1 hour ago, HenryBurlingame said:

am usually firing custom programs

One question that pops into my mind - do your custom programs follow the final segment speed  in the Orton chart? In order to predictably drop a cone Orton provides three possible speeds in their chart starting in the final 200 degrees of the firing. From there the cone should be relatively spot on. If not the peak temperature or speed can be tweaked in the final segment to fine tune

Clay / Glaze mature because of a fluxed reaction so heatwork (Temperature and Time heating) is cumulative and predominately takes place in the last segment of the firing. So even custom schedules can contain a final segment similar to the chart to get a specific amount of heatwork to drop the cone.

Most folks pick prox. 108 f/ hr  (center column), 270 per hour at top temp is hard for some kilns to achieve.

So I would suggest make the kiln as accurate as possible ( maybe leave it alone) adjust your final temperature in your custom program .

Edited September 12 by Bill Kielb

[Hulk]

Good question!
Well, I like it, on account of new L&L kiln, custom firing profile*, was also running hot, and set the target temp lower to correct, set it a bit lower, and then just a tiny bit lower, voilà!

I'd thought my portable pyrometer would make duplicating the firing profile of my previous kiln easy, given the portable works same as before, which testing verifies.
Now I believe that the difference is the cooling rate. The new kiln has thicker brick - it definitely takes longer to cool (the same amount, e.g. the first 100°F drop).
...and thus, there's more heat work!
Does that make sense? Time and temperature.
I hadn't considered the down ramp after peak. The upward ramp matches my notes. The portable pyrometer appears to "speak" truth, so.

*based on my notebook, where I'm trying to replicate the firing profile of my previous - all manual - kiln.

[HenryBurlingame]

36 minutes ago, Bill Kielb said:

One question that pops into my mind - do your custom programs follow the final segment speed  in the Orton chart? In order to predictably drop a cone Orton provides three possible speeds in their chart starting in the final 200 degrees of the firing. From there the cone should be relatively spot on. If not the peak temperature or speed can be tweaked in the final segment to fine tune

Clay / Glaze mature because of a fluxed reaction so heatwork (Temperature and Time heating) is cumulative and predominately takes place in the last segment of the firing. So even custom schedules can contain a final segment similar to the chart to get a specific amount of heatwork to drop the cone.

Most folks pick prox. 108 f/ hr  (center column), 270 per hour at top temp is hard for some kilns to achieve.

Yup, I am doing 108F for the last 200-250F before I reach max temperature :-).

[Bill Kielb]

10 minutes ago, HenryBurlingame said:

Yup, I am doing 108F for the last 200-250F before I reach max temperature :-).

Unless you can prove it is definitely out of calibration, I would adjust your custom program top temp to tweak. If the cone fire programs consistently fail low or high, then I would consider offsets. This to me likely best represents the accuracy of the kiln and controller combination.

Edited September 12 by Bill Kielb

[HenryBurlingame]

1 hour ago, Hulk said:

Good question!
Well, I like it, on account of new L&L kiln, custom firing profile*, was also running hot, and set the target temp lower to correct, set it a bit lower, and then just a tiny bit lower, voilà!

I'd thought my portable pyrometer would make duplicating the firing profile of my previous kiln easy, given the portable works same as before, which testing verifies.
Now I believe that the difference is the cooling rate. The new kiln has thicker brick - it definitely takes longer to cool (the same amount, e.g. the first 100°F drop).
...and thus, there's more heat work!
Does that make sense? Time and temperature.
I hadn't considered the down ramp after peak. The upward ramp matches my notes. The portable pyrometer appears to "speak" truth, so.

*based on my notebook, where I'm trying to replicate the firing profile of my previous - all manual - kiln.

I bet the slight overfiring is something like that. My thoughts were that the elements shut off when the kiln reaches top temp, but they are still hot so probably while they (and the rest) of the kiln cools off it is kind of like a little bit of a hold up there. So thermocouple reading is correct but it over fires anyway.
 

Do you happen to have any graphs of exactly how fast your kiln is cooling for you up there once it goes off?

Edited September 12 by HenryBurlingame

[Bill Kielb]

34 minutes ago, HenryBurlingame said:

Do you happen to have any graphs of exactly how fast your kiln is cooling for you up there once it goes off?

I have gas kiln data, several models of updraft and downdraft, so probably not helpful here. I think it’s really good to have though and I think @Hulk or @neilestrick suggested graphing a long cooling cycle which their genesis controllers so they may have for a standard 3” brick kiln. I think my observation has always been how difficult it is to add heatwork after a shutdown, from observation kilns tend to cool at crazy high speeds in the first few hundred degrees if you don’t pump a bunch of energy into them. Drop and hold likely can influence because you are in that 200f - 250f  sweetspot. In general my observed natural cooling has resulted in much slower rates than I intuitively imagined especially at 1000 degree and cooler temperatures.

Edited September 12 by Bill Kielb

[High Bridge Pottery]

There's no "soft hold" when you turn the kiln off. It will drop the fastest from highest temps and all graphs will take this sort of shape once you turn them off, besides the part where I open it before room temp. Dropping 400c in 1 hour is probably in the top of the range but I would guess most big/better insulated pottery kilns still do 250-300c in the first hour. 

Edited September 12 by High Bridge Pottery

[High Bridge Pottery]

I remembered I had some data from kilns at work and the big one cools 100c in the first hour and the small one 220c. Shows how bad mine is at holding heat  Not sure on the size of the big one but it's about 2mx2mx2m ( maybe more 1.2m square after looking at the picture as the tiles are 10cm ). Only dropping from 1040c

 

Big kiln

Edited September 12 by High Bridge Pottery

[Bill Kielb]

2 hours ago, High Bridge Pottery said:

and the big one cools 100c in the first hour and the small one 220c.

Seems pretty well filled, any sense of the r value walls, top, bottom. Seems likely double of the standard home kiln.

[Hulk]

"...have any graphs of exactly how fast your kiln is cooling for you up there once it goes off?"

Several posted already, cool!

Looking at the new kiln's csv file* (seven cubic foot L&L Easy-Fire E23T)
New kiln, segment 5, drop from peak to 2060°F, hold there for forty minutes
Peak temp 2200°F
Rate 9999
End 2060°F

All three segments zero (no firing, no power) for twenty-six minutes, when the top segment fired; two minutes later the bottom began to fire; the middle cooled to target and began to fire fifteen minutes later.
Time to 2060°F:
  Top twenty-six minutes
  Middle forty-one minutes
  Bottom twenty-eight minutes

Old kiln, watching temperature and fiddling with the three high/medium/low switches, peak to 2060°F (one pyrometer, middle section only) was taking as little as twenty minutes, but usually near a half hour.
Id' forgotten how much twiddling was involved.

I'd looked at the first two hundred degrees of drop from peak and tentatively concluded that slower cooling is significant - adds heat work - and may explain why the new kiln appears to achieve way more heat work when the portable pyrometer indicates same peak temp.

How long it takes to cool to open (generally, 200°F or so), so much longer than the old kiln, and with less massy load - I'd thrown away the worst shelves when we moved - I need two half shelves!

*fairly straightforward: Exporting Firing Logs on an L&L Kiln Genesis Control | Hot Kilns
I'll maybe generate graphs to compare several firings. I found what I was looking for just scrolling the file...
 ...I was just curious if there's sufficient data to track element efficiency, err, declining efficiency, per question posed in another thread.
I believe there is, given "everything else" is equal (mass, voltage, current, etc.), the "firing/on percentages" required to reach segment targets on time would seem a useful comparison for that - then, when the kiln can't meet the target on time, when at least one segment is full gas the whole time, compare the times required.
 

Edited September 13 by Hulk
idk, forgot - something, an' a comma

[HenryBurlingame]

5 hours ago, High Bridge Pottery said:

There's no "soft hold" when you turn the kiln off. It will drop the fastest from highest temps and all graphs will take this sort of shape once you turn them off, besides the part where I open it before room temp. Dropping 400c in 1 hour is probably in the top of the range but I would guess most big/better insulated pottery kilns still do 250-300c in the first hour. 

Awesome, thanks for posting, this is exactly what I was looking for!

 

45 minutes ago, Hulk said:

"...have any graphs of exactly how fast your kiln is cooling for you up there once it goes off?"

Several posted already, cool!

Looking at the new kiln's csv file* (seven cubic foot L&L Easy-Fire E23T)
New kiln, segment 5, drop from peak to 2060°F, hold there for forty minutes
Peak temp 2200°F
Rate 9999
End 2060°F

All three segments zero (no firing, no power) for twenty-six minutes, when the top segment fired; two minutes later the bottom began to fire; the middle cooled to target and began to fire fifteen minutes later.
Time to 2060°F:
  Top twenty-six minutes
  Middle forty-one minutes
  Bottom twenty-eight minutes

Old kiln, watching temperature and fiddling with the three high/medium/low switches, peak to 2060°F (one pyrometer, middle section only) was taking as little as twenty minutes, but usually near a half hour.
Id' forgotten how much twiddling was involved.

I'd looked at the first two hundred degrees of drop from peak and tentatively concluded that slower cooling is significant - adds heat work - and may explain why the new kiln appears to achieve way more heat work when the portable pyrometer indicates same peak temp.

How long it takes to cool to open (generally, 200°F or so), so much longer than the old kiln, and with less massy load - I'd thrown away the worst shelves when we moved - I need two half shelves!

*fairly straightforward: Exporting Firing Logs on an L&L Kiln Genesis Control | Hot Kilns
I'll maybe generate graphs to compare several firings. I found what I was looking for just scrolling the file...
 ...I was just curious if there's sufficient data to track element efficiency, err, declining efficiency, per question posed in another thread.
I believe there is, given "everything else" is equal (mass, voltage, current, etc.) the "firing/on percentages" required to reach segment targets on time would seem a useful comparison for that - then, when the kiln can't meet the target on time, when at least one segment is full gas the whole time, compare the times required.
 

 

Thanks Hulk! So it looks like the middle of your kiln (basically same as mine, I have the eq2327-3) is cooling off at about 200F/hour when it first goes off.  Wonder if there would be any benefit to setting the the cooling rate to 200 instead of 9999 so if would keep all the zones even as it cooled... "crash cool" segment of the program would also be consistent between firings independent of kiln load if you set the cooling rate to match the slowest zone of your most fully loaded kiln too...

Edited September 13 by HenryBurlingame

[Bill Kielb]

44 minutes ago, Hulk said:

Top twenty-six minutes
  Middle forty-one minutes
  Bottom twenty-eight minutes

Interesting, if we graphed this with straight cooldown and no energy added, I am thinking it would be 400 - 500 F degrees per hour (first hour) with a reasonable portion exiting top and bottom which is a bit less than @High Bridge Pottery 400c but maybe actually somewhat consistent because we are adding a bunch of energy around the 1/2 hour mark. These curves seem to reaffirm that at quartz inversion, natural cooling rates likely slower than one would think. Good to know I think.

[Hulk]

"Wonder if there would be any benefit to setting the cooling rate..."

Aye Henry, this is what I was thinking as well*!
Now that you mention it, I'd guessed that the peak temperature would have to be lowered to offset the additional heat work that the controlled cool would impart, hence, some study and trials would likely be necessary to get all three segments cooling equally and hit the desired heat work.
I'm still thinking that cool down speed from peak is as significant as the ramp up rate to peak. I don't know that to be a fact, but that's what it looks like!

*...however, was thinking of starting a new thread on that, for your original question is lower max or edit the offset.

There's some discussion here, where Neil points out that the cooling rate would have to be slow enough to be achievable, and hence II, I'm thinking, perhaps adjustment of peak target to account for the heat work.
Interesting Cooling Cycle Info - Equipment Use and Repair - Ceramic Arts Daily Community

...and this, help with graphing:
Anyone with a Bartlett Genesis - Equipment Use and Repair - Ceramic Arts Daily Community

 

 

[HenryBurlingame]

1 hour ago, Hulk said:

*...however, was thinking of starting a new thread on that, for your original question is lower max or edit the offset.

Yes, that would work. I think the consensus is I should just be lowering the max temp and not messing with offset unless I really need to 🙂. Thanks for the thread links!

[High Bridge Pottery]

16 hours ago, Bill Kielb said:

Seems pretty well filled, any sense of the r value walls, top, bottom. Seems likely double of the standard home kiln.

No idea on an r value but I just measured the walls and they are all 15cm/6inch thick

Edited September 13 by High Bridge Pottery
Got inch symbol wrong so wrote inch xD

[Hulk]

 "...straight cooldown and no energy added...400 - 500 F degrees per hour (first hour) "

Wanted to come back to this.
My kiln lost 160°F in twenty-six minutes (top segment), all elements off.
From there, at the same cooling rate (the rate would have to be less - all the graphs I've seen flatten out), over 300°F/hour.
Smaller kilns, less massy kilns (thinner/lighter brick) should cool faster, bigger heavier kilns, slower?

[PeterH]

Just for the record a cone 6 controlled cooling cycle for glaze development is given in
https://ceramicartsnetwork.org/daily/article/Super-Cool-Slow-Cooling-in-an-Electric-Kiln
Natural cooling down to 1900 F(the poster seemed to get 500F/hr but programmed 9999F/hr)
125-175F/hr down to 1450F (tweak based on glaze results)
Natural cooling down to room temperature
 

Edited September 13 by PeterH

[Bill Kielb]

1 hour ago, Hulk said:

Wanted to come back to this.
My kiln lost 160°F in twenty-six minutes (top segment), all elements off.
From there, at the same cooling rate (the rate would have to be less - all the graphs I've seen flatten out), over 300°F/hour.
Smaller kilns, less massy kilns (thinner/lighter brick) should cool faster, bigger heavier kilns, slower?

So 323 per hour max. Definitely in the ballpark of 400 but better than many I think. If we could get folks to share deeper data for different kilns,  might help someone some day.

[Hulk]

Much slower.

It took 24 minutes for the top segment to drop 156°F*
The middle only lost 91 degrees over the 24 minutes

The top and bottom cool quicker at first, then the curves flatten significantly.
Meanwhile, if we're going by the laggard - the middle, even if the rate of cooling remains constant, that's just over 225°F/hour.
My guess would be, if the kiln were allowed to continue to cool without any power, the middle's cooling rate would flatten out

First, 12 minute, and 24 minute temperatures, top, middle and bottom segments:

			12 mins	24 mins
#1	#24	#28	Difference	Difference
2198	2104	2042	94	156	Top
2198	2153	2107	45	91	Middle
2198	2126	2074	72	124	Bottom

 

*Not 26, my bad.
Then the controller started firing the top to hold at 2060°F - the top segment had dipped below the target there.
A short time later, the bottom began to fire.

I'm still thinking the rate of cooling after peak contributes to heat work, where slower is more, faster less, just like rate climbing to that peak.
If I find something to corroborate, I'll start another thread.

[neilestrick]

Quite often the kiln will run hot at cone 5/6 but not at low fire temps. If you do a thermocouple offset then your bisque firing will be affected as well, and it may not need it.

The temp starts to drop almost immediately when the elements shut off. There is not enough run-on to matter. If you program a slow cooling rate from the peak temp it will over-fire. In my kilns a slow cool of 175/hr from the peak will impart at least a 1/2 cone of heatwork. Best to drop about 100 degrees at full ramp then begin the slow cooling. If you're cooling for glaze effects, you can drop down to 1900F before beginning the cooling cycle. I have 3 kilns of different sizes (10cu/ft, 4 cu/ft, 1 cu/ft) so I do a quick drop of 100F, hold 30 minutes for pinholes to smooth out, then drop 175/hr to 1500F. This gives me the same results from all 3 kilns since they cool at dramatically different rates. Without it the baby would cool down to room temp in 6 hours, whereas the big on would take 3-4 times that long. The big on can barely cool 175/hr at the low end of the cycle if it's full.