Thermocouple A has two posts next to it. The posts are 3/4” away from Thermocouple B. The test was done in a Paragon Caldera kiln.
A Thermocouple Experiment
Recent Q&As: Replacing damaged firebricks; firing wet ceramic glaze
A Thermocouple Experiment
All brands of digital kilns have a thermocouple, which senses the temperature. The thermocouple is a small rod that extends into the kiln, usually from a wall. Some kilns have three thermocouples.
In a recent Kiln Pointer I wrote that objects such as shelves and posts should be kept at least 1/2” away from the thermocouple tip to avoid throwing off the accuracy of the temperature readings. Last week I experimented to find out how much a nearby post can affect the accuracy.
I installed two 14-gauge, exposed tip test thermocouples in the back wall of a small Caldera digital kiln, 1 1/8” away from each other side by side. The kiln’s Sentry Xpress took temperature readings from the main thermocouple on the opposite wall. Two Sentry 12-key controllers took readings from the two test thermocouples. I ran two test firings to 1700F at a full rate, with a 30 minute hold for each test.
I fired the kiln empty the first time to find out how much the test thermocouples differed in their readings. While heating to 1500F, Thermocouple A was 3 degrees F hotter than Thermocouple B. During cooling at 810F, Thermocouple A was 9 degrees hotter than Thermocouple B. I factored in those discrepancies to come up with accurate comparisons in the second firing. (Normal variations exist between two sets of thermocouples and controllers.)
For the second firing, I placed two posts (1” wide x 6” long and 1” wide x 5” long) near Thermocouple A. One post was 1/8” away. I kept the posts 3/4” away from Thermocouple B and ran the same firing schedule as in Test #1.
THE RESULTS FOR TEST #2
Readings for Thermocouple A, with the posts nearby:
310F, 10F cooler than Thermocouple B
1130F, 8F hotter than Thermocouple B
1330F, 6 degrees hotter
1500, 16 degrees hotter
1300F, 8 degrees hotter
800F, 5 degrees hotter
Conclusion: At 300F as the kiln heats up, a nearby thermal mass lowers the temperature reading of the thermocouple. Starting at around 1100F as the kiln continues to fire, the nearby thermal mass raises the temperature reading for the rest of the firing. Please feel free to send comments about the test firings and how you interpret the results.
Q. We are having an old electric kiln repaired. We have an estimate for changing out the elements, and for an additional $84 to $126 they will replace broken bricks. Two of the bricks will no longer hold the element. Will replacing the bricks give better insulation and maybe pay for itself in electricity savings?
A. If the only brick damage is missing element grooves, then replacing the bricks will not lower the cost of electricity. This is because the grooves add little to the wall insulation.
Q. Is it okay to fire ceramic glazes while they are still wet?
A. The glaze should be dry before you begin the firing. Otherwise the glaze can crawl. This is because firing wet glaze causes the glaze to dry too quickly on the ware. (Crawling means the glaze has moved, leaving a bare spot on the clay.)
“Twenty years from now you will be more disappointed by the things that you didn’t do than by the ones you did do, so throw off the bowlines, sail away from safe harbor, catch the trade winds in your sails. Explore, Dream, Discover.” -- H. Jackson Brown's mother
During my second test firing last Friday, the row of red temperature displays blinked a few feet from my desk. Music and the voice of a DJ blared from the grand opening of Fuel City next door. Plastic pennants fluttered in the sun. Bright color flashed into my office when an orange 18-wheeler truck rumbled past the window.
With best wishes,
Arnold Howard Paragon Industries, L.P. – Better Designed Kilns 2011 South Town East Blvd., Mesquite, Texas 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 / email@example.com / www.paragonweb.com / www.facebook.com/paragonkilns
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