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Temperature Overshoot


Terry, customer service rep, looking into the Paragon SC-2 kiln.

CONTENTS:

Temperature Overshoot

Recent Q&A: When to replace a thermocouple

Pointer: Mixing greenware and glazed ware in the same firing

TEMPERATURE OVERSHOOT

A car traveling 30 miles per hour does not stop immediately when you apply the brakes. It has to first slow down before stopping. The same applies to kilns, both digital and manual.

Yesterday I dried shelves in a small jewelry kiln. I programmed a full rate to 200 degrees F with a 45-minute hold. But after the kiln reached 200 degrees and the element shut off, the kiln kept on climbing to 332 degrees before settling down to 200 in the 45-minute hold.

I have also seen temperature overshoot in a small switch-operated kiln. I was heat-treating a piece of high carbon steel using a pyrometer to monitor the temperature. I discovered that if the temperature climbed too fast, it overshot the target, which was 1750 degrees F. I had to turn the switch to a lower setting before reaching 1750.

Temperature overshoot is normal. It is more pronounced in small kilns especially at lower temperatures. The faster the firing rate, the greater the temperature overshoot. In larger kilns, it may not even be noticeable.

At any rate, temperature overshoot rarely affects the ware. This is because the ware is exposed to the higher temperature for such a brief period.

To avoid temperature overshoot, fire at a slower rate. If you need to fire at a fast rate, then add an extra segment in a Ramp-Hold program to slow the firing. The segment with the slower rate should begin approximately 40 - 100° F below the target temperature.

I did a test firing today in a small ceramic fiber kiln to demonstrate how rate affects temperature overshoot:

TEST #1

Segment 1, FULL rate to 200 degrees F, no hold

Temperature overshoot: 132 degrees F.

TEST #2

Segment 1, 300 degree rate to 200 degrees F, no hold

Temperature overshoot: 6 degrees F.

Suppose you are firing at FULL rate to 1600 degrees F. If your kiln is overshooting past 1600, then program an extra segment:

Segment 1, FULL rate to 1500 degrees F, no hold Segment 2, 300 degree rate to 1600 degrees F

By firing to 1600 degrees in two segments instead of one, you can avoid overshoot while still reaching 1600 rapidly.

RECENT Q&A

Q. How often should the K-Type thermocouple be replaced: after a certain length of time, or after a certain amount of temperature drift?

A. Instead of replacing the thermocouple by a time schedule or by how much it drifts, I suggest replacing it when it fails. Or replace it if the digital temperature readout fluctuates wildly when you move the thermocouple tip back and forth. That indicates that the thermocouple is about to fail.

POINTER FROM TONY RODRIGUEZ OF SAN ANTONIO

Do not fire glazed ware and greenware in the same cone 04 kiln load. This is because not all glazes are fired to cone 04--only specialty glazes such are bronze (metallics). There must be a 2 cone difference between bisque and glaze firing for good body fit. Otherwise the glaze can separate from the bisque and peel off.

Thanks, Tony, for the pointer.

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An x-ray technician told me recently about the stress of her job. She is left shaken when x-rays reveal breast cancer in her patients. On some days, that happens to more than one patient.

When the technician gets home, she sits in front of her torch and makes glass beads. For an hour or more she is lost in the world of glass. When she emerges from that world, the stress is forgotten.

As Thanksgiving approaches, I thought of that technician. I enjoy being with glass, pottery, ceramics, and enameling artists like her, because they are happy making things. Thanks for reading these Kiln Pointers and for including us in your life.

With best wishes from all of us at Paragon,

Arnold Howard Paragon Industries, L.P. – Better Designed Kilns 2011 South Town East Blvd. Mesquite, TX 75149-1122 Voice: 972-288-7557 & 800-876-4328 / Fax: 972-222-0646 ahoward@paragonweb.com / www.paragonweb.com



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