I had a great conversation about this with Jim Simmons, Production Manager at Evenheat Kilns. He said, “Most manufacturers tell customers that a variance of +/- 20F degrees is an acceptable target to stay within the temperature range.” But in my experience, most of us usually can adjust within about +/- 50F degrees and still be able to accommodate the changes.
He also said, “It’s time to change the thermocouple when the room temperature reading is OVER 50F degrees above or below the true room temps. A thermocouple offset is also an option that your manufacturer can walk you through.” But, in my opinion, it's not a good solution to override a faulty part. Changing out a thermocouple is an easy and less expensive thing you can do to keep your kiln singing along. For most manufacturers, a thermocouple is in the price range of $40-75.
He was pretty specific about this one point, “When thermocouples wear out, you begin to get into an “over fire” situation, which means the temperature of the kiln reads LOWER than the actual firing temperature in the kiln. If you don’t change out your thermocouple soon, it’s just gonna get worse.”
This was actually happening with one of my old kilns. It was 8 years old, and it got quite the workout! The controller was reading 1325F, but the firing results really looked like 1425F. This is a clear sign that it’s time to change out that old thermo! I was lazy about it, even though knew I needed to do it. Like my hero Maya Angelou said, "When you know better, you do better!"
Jim shared this last tidbit... we love Jim, but it’s pretty techie stuff…
“Here’s the technical formula. The air in your kiln, plus the tip of the thermocouple -2 degrees or .4% of temperature, whichever is greater, that is the temperature differential between the tip of the thermocouple to whatever material you have in the kiln. Temps are measured by millivolts on the tip of the thermocouple. Who knows (the engineers know) how that really works for us everyday “Joes,” but you could get another digital pyrometer, drill a hole next to the other thermocouple, and place that in the kiln. One thermo would then prove or disprove the other thermo, as long as the first thermo was calibrated correctly at the factory. There’s really no way you could accurately do this in the studio or shop.”
Well, there you have it. It sounds like we have three options:
- Manually do the adjustment and change your programs accordingly to accommodate the +/- temp factors of your particular kiln.
- Call the manufacturer and have them walk you through a thermocouple offset on your controller.
- Do what I did! Spend the $55 dollars and change out the thermocouple.
Good luck! And may all your kiln temperatures read true!