Kiln Not Firing To Temperature?

[snowluck2345]

My small test fire 120V Cress BT911T kiln is not firing to temperature. I have always noticed it fired slowly, but firing to cone 06 it got there eventually.

 

I recently tried firing to cone 5 and I could not get above 1960 degrees fahrenheit, and going from 1900 to 1960 took 5 hours. I'm controlling the kiln using an SSR, reading temperature with a K-type thermocouple through a MAX31855  with a PID controller running on an Rpi with a webserver hosted.

 

My kiln elements are measuring at 8.8 ohms, rather than 8 ohms nominal. I am getting 90% of full power.

 

Would this be enough to prevent a full firing? This seems strange to me.

 

I am considering replacing my elements, seeing if that fixes my issues, I am not sure though. 

 

Does anyone have experience with these 120V kilns?

[neilestrick]

Many 120 volt kilns are underpowered, and yours is only a cone 6 kiln so it's even more underpowered than a cone 10 version. At a cone 6 rating it really shouldn't be fired higher than cone 3. They need everything working properly in order for them to function correctly. At 8.8 ohms your elements are 10% off from the factory original, which is exactly the amount of wear when replacement is recommended. Assuming everything else it working properly (thermocouple, relays, etc) then it's probably the elements causing the problem.

[snowluck2345]

Cool! I just ordered new elements.I just got a bunch of Cone5/6 glazes, is the main reason to not go to Cone6 is that it will be difficult due to being underpowered? I assume all the same series of kilns have the same bricks and it has 3" insulation. I noticed the smaller models have the same power rating. Here is my temperature profile (I slowed it down a lot due to trouble following a faster one as early as 1700 degrees):

 

I would be suprised if thermocouple isn't working correctly, they are very simple devices and I'm using a digital reading chip. The cone5 cone didn't bend fully. Only 1V is dropped over the SSR. Once I swap out the elements and confirm it is working; I will speed up the higher end area of the profile.

[neilestrick]

Firing to cone 6 will max out that kiln, which means it will fire pretty slowly at the end, which also means you're wasting a lot of electricity. Typically you want a kiln that will fire 3-4 cones higher than what you'll be firing to, so it will have plenty of power to meet your firing schedule. By maxing it out, you're at the mercy of the kiln when it comes to setting the schedule at the high end of the firing. And when the elements start to age even a little bit the kiln won't be able to reach its max. If it was rated to cone 10 and you were firing to cone 6, you could get away with firing on older elements because you're not at its peak.

[snowluck2345]

That makes sense. I will try adding some external rockwool insulation, it looks like I'm loosing about 400w  through the outside at 2000f.

[Min]

That makes sense. I will try adding some external rockwool insulation, it looks like I'm loosing about 400w  through the outside at 2000f.

 

I wouldn't put external insulation on the outside of the metal kiln jacket, the extra heat will cause the jacket to deteriorate really fast. Fiberfrax blanket or boards can be used between the brick and the jacket or on top of the lid. The latter solution is not very safe as every time you move the blanket you are releasing nasty (carcinogenic) fibers into the air. Heat from the kiln causes it to break down also, inside the jacket the fibers are not an issue but loose blanket on top of a lid is.

 

Also, "I would be suprised if thermocouple isn't working correctly, they are very simple devices." Yup they are simple devices but as Neil suggested they wear out and need replacing when they start to drift.

[High Bridge Pottery]

Are you able to share your software? I have also done something similar, just with type R thermocouples.

 

My small test fire 120V Cress BT911T kiln is not firing to temperature. I have always noticed it fired slowly, but firing to cone 06 it got there eventually.

 

I recently tried firing to cone 5 and I could not get above 1960 degrees fahrenheit, and going from 1900 to 1960 took 5 hours. I'm controlling the kiln using an SSR, reading temperature with a K-type thermocouple through a MAX31855  with a PID controller running on an Rpi with a webserver hosted.

 

My kiln elements are measuring at 8.8 ohms, rather than 8 ohms nominal. I am getting 90% of full power.

 

Would this be enough to prevent a full firing? This seems strange to me.

 

I am considering replacing my elements, seeing if that fixes my issues, I am not sure though. 

 

Does anyone have experience with these 120V kilns?

[Steve Vasil]

I don't expect the variance in how your elements read to make that much of a difference. It is more likely that you are losing heat, your supply voltage is low, or the way you're taking temperature is not accurate-you might get a digital thermometer and double-check this. BTW, my wife and I do kiln repair in northwest Georgia for a reasonable fee, if anyone might require our help.

 

Steve Vasil

[perkolator]

I had a problematic little 120v test kiln doing this same thing last year (basically the Skutt equivalent of your Cress).  Everything tested as "functioning" but the resistance on the elements was a tad high - I replaced a few parts but not the elements (since they seemed good to me) but it wasn't until I finally replaced the elements that the kiln finally acted normal and achieved ^6.

[Steve Vasil]

I had a problematic little 120v test kiln doing this same thing last year (basically the Skutt equivalent of your Cress).  Everything tested as "functioning" but the resistance on the elements was a tad high - I replaced a few parts but not the elements (since they seemed good to me) but it wasn't until I finally replaced the elements that the kiln finally acted normal and achieved ^6.

This is good to know; thanks.