thermocouple lag, and an older L&L kiln

[GEP]

I have two L&L e23t kilns, one from 2003 and one from 2013. Apparently the kiln went through a design change in 2004, so my two kilns are not exactly the same.

My older kiln has a feature called Thermocouple Lag, which comes factory set at 25°.  It means that if one zone falls behind by 25°, the controller will put on the brakes and try to even things out before continuing the program. 

My newer kiln does not mention Thermocouple Lag in the manual, nor does it have the feature when cycling through the “other” options on the controller. 

I think this means my newer kiln does not allow any zone to fall behind in the first place. When using the exact same program as the older kiln, it does take longer to fire, and results in slightly hotter firings (based on witness cones), and I think that explains why. But someone pls correct me if that’s wrong.

None of this mattered to me before, but recently I figured out that one of my claybodies has changed, resulting in pinholes in one of my glazes. The solution to the pinholes is to lower my glaze firing by 10°. However, if I fire any lower than that, another one of my glazes, a semi-matte, starts to look a little dry. So now my tolerance for glaze firing temp variation has gotten a lot tighter. 

In my older kiln, my middle zone generally fires 10° hotter than the top and bottom. This is based on both what the controller says, and the witness cones, I have tried changing my kiln load to put more mass in the middle. But I can’t go any further with that, because I don’t have any more flat pots to fire. Need to have room for mugs and vases too! 

Has anyone tried to change the Thermocouple Lag on an older L&L kiln down to maybe 3° or even 0°? If so, how did it effect the kiln’s performance? Or is this a bad idea for the kiln?

[neilestrick]

@GEP The design change in 2004 was a change in the element holders, that allowed them to use a larger diameter element coil. However there have been changes to the controllers since 2003, so you're correct that they aren't the same version.

I believe in the newer controller the Lag function is being accomplished by the PID and Averaging functions, which turn on the middle section to help the other sections, and holds temp when the hottest section hits the set point.

There is some info about LAG here, more here in the Averaging section.

For the most accurate information on lag and changes to the programming, I would contact Bartlett Instruments directly. They'll know what the best solution is, because many of these settings work together and changing one could mess things up.

[GEP]

Thanks for the links, @neilestrick. So it looks like L&L does not recommend changing the lag. And if you really want to, don’t go below 10. This doesn’t improve my situation, since I can already get it to fire within a 10° variation.

I guess I’ll have to learn how to live with it. The pinhole problem is only on certain pots, so I can try to keep those out of the hotter middle zone. Or maybe I can figure out another way to glaze them. 

[neilestrick]

2 minutes ago, GEP said:

Thanks for the links, @neilestrick. So it looks like L&L does not recommend changing the lag. And if you really want to, don’t go below 10. This doesn’t improve my situation, since I can already get it to fire within a 10° variation.

I guess I’ll have to learn how to live with it. The pinhole problem is only on certain pots, so I can try to keep those out of the hotter middle zone. Or maybe I can figure out another way to glaze them. 

Maybe tweak the semi-matte so it will melt better at the lower temp?

Have you tried a thermocouple offset to fix the middle firing hotter?

[GEP]

54 minutes ago, neilestrick said:

Maybe tweak the semi-matte so it will melt better at the lower temp?

Have you tried a thermocouple offset to fix the middle firing hotter?

Both good ideas, thanks! I have a big show in 2.5 weeks, so I need to figure this all out pronto. 

Meanwhile, I was doing a bisque firing in the same kiln today, and a relay failed! My other kiln had a relay fail recently too. I am cursed!!

[neilestrick]

@GEP When it rains it pours..

At least relays are an easy fix!

[GEP]

Just wanted to report a happy ending to this story. I ended up not changing the Thermocouple Lag. The solution was change the way I stack the kiln, and to adjust Thermocouple Offsets when needed. In the past, I never used witness cones except during the first firing after replacing elements/TCs, and when I was troubleshooting a problem. From now on, I will use witness cones every 10th firing or so, so I can keep up with the drift of aging TCs.

These measures are probably overkill for most potters. It's necessary for me now, because I need to keep my firing temps in a fairly narrow range. 

Here is my new shelf-stacking strategy for a 7 cubic ft kiln, in case there are others out there who need to make their glaze firings more even from top to bottom. By having all of the shelves in the middle zone, my newer kiln's controller can keep all three zones exactly even. My older kiln will keep them within 4 degrees of each other. Sometimes, the witness cones will tell a slightly different story than the controller, and when that consistently happens, the answer is to adjust the TC offset. 

 

[neilestrick]

Yep. Looser at the top and bottom, tighter in the middle. On a Genesis controller, the display shows the temp of all three thermocouples, as well as the percentage of time that each section of the kiln is on. Often the top and bottom will be at 90%, and the middle will be at 50%. By altering the stacking you make the middle take on more of the work load.