Disparity in temp readings when firing

[oldlady]

My L&L 23T has 3 thermocouples, 1 at the top, 2 in the middle and 3 at the bottom.    over the years i have noticed a disparity in the temp readings during firing.   last night i watched as my cone 6, heavily loaded firing finished and wrote down the numbers for someone else to see and make any recommendation.   normally the temp shown on the readout is from the middle, or #2.

with 6 minutes of an 11 minute hold at the end of the firing, the temps read     1    2151

                                                                                                                                                                             2   2188

                                                                                                                                                                              3  2165

these remained constant until the 11 minute hold was up and the kiln shut down.     i missed the actual end of the firing but previous firings went to 2199 and ran about 14 hours for a packed kilnload.

this firing was 13.57 hours so the time was right.    i expect to find a perfect firing on most pieces when i open it tomorrow. 

my question is about the possibility of the kiln showing an error code because of the disparity in temps.    i know it did this when i first got the kiln in about 2002 or so but it has not done anything like that since then.  can the disparity predict a problem with elements or thermocouples or relays if it gets to some particular point?    what should i learn from this?

i do not keep a log of firing data so i cannot answer specific questions about the previous firings.  and no, i do not use large witness cones because most of my posts are too short to accomodate  the height of a cone.    most of my work is less than 3/4 inches high so i use 1 1/2 inch posts between shelves.   except for the spaces where the thermocouples are located.

[neilestrick]

The kiln allows for a certain amount of difference between the thermocouples. It's not uncommon for the middle to run a little bit hot, because it is affected by the top and bottom rings. The difference would be less if your kiln wasn't packed throughout with very short shelves, because then you could pack it with taller pieces at the bottom and top, which would mean less mass to heat up in those areas. A cover shelf at the top would help to hold heat in at the top.

I wouldn't worry about it, though, since it's been working for 17 years. If you really need it to be closer, contact Bartlett Controls (they built the controller) and they may be able to recommend some changes to the settings in the controller that will help. There is a least one setting that has an affect on how the rings interact with each other in evening out temperature.

[oldlady]

thank you, neil,   when i panicked years ago, i talked to the L&L tech who lives in Bend,  Oregon.   he talked me through several situations and i think he might have done that setting so i did not see an error code again.

so, all is well until something actually fails and there is no warning except firings slowing down excessively.