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A Thermocouple "Short"

Contents: A Thermocouple “Short” Reader Response: Kiln Sitter Maintenance Fire Safety A Kiln Story: Firing Moth Balls

A short in the thermocouple is rare, but you should be aware of it if you own a digital kiln.

The thermocouple is a temperature sensor used on digital kilns and on pyrometers. It is the small rod that protrudes into the firing chamber.

The thermocouple is made of two dissimilar metals joined at the tip. Amazingly, when the thermocouple tip is exposed to heat, it generates its own tiny voltage. The controller or pyrometer converts that voltage to a temperature.

Two wires (called lead wires) connect the thermocouple to the controller or pyrometer. If the lead wires touch at a bare spot where the insulation has worn off or where it has been stripped back too far, they will “short out.”

A short in the thermocouple lead wires cuts off the voltage generated at the thermocouple tip. A partial short can throw the temperature off by as much as several hundred degrees, causing the kiln to over-fire. It can also cause the temperature display to bounce.

A complete short will make the controller display room temperature no matter how hot the interior is. This, too, can over-fire the kiln.

1) When you replace a thermocouple, examine the lead wires for bare spots. Gently arrange the wires inside the switch box to avoid damage.

2) Do not twist the thermocouple inside the firing chamber. In some cases, that can cause it to short out where it goes through the kiln wall.

3) When you replace a thermocouple, do not enlarge the thermocouple hole in the kiln wall. That can expose the lead wires to excessive heat.

READER RESPONSE

KILN SITTER MAINTENANCE Tony Rodriguez, a kiln technician with GSM Enterprises in San Antonio, Texas, comments on a recent Kiln Sitter article:

"Do not blow compressed air into the tube assembly from inside the kiln. This will lodge particles into the pivot area of the sensing rod. Cleaning the porcelain tube with a long Q-tip will not work.

"After every firing, remove and clean the cone supports. Hold them back-to-back up to a light to check them for warpage. Replace if they are bent. If the bend is outward away from the sensing rod, the kiln will underfire. If the bend is inward toward the sensing rod, the kiln will overfire."

FIRE SAFETY Charlie Spitzer wrote, “Kitchen grease fires are better put out with a box of salt and/or a lid. A fire extinguisher tends to blow the burning grease all over the place causing bigger problems.”

Jim Simmons wrote, “Do NOT store the extinguisher right next to the place that is likely to catch fire. If you do get a fire, you want to be able to reach the extinguisher.”

Thanks, Tony, Charlie, and Jim, for your valuable pointers.

A KILN STORY: FIRING MOTHBALLS In a previous kiln pointer Dave Coggins from Queensland, Australia, discussed firing flammable material inside kilns. Louis Katz, a potter in Corpus Christi, Texas, shares a story on a related topic:

“I had been asked to take care of reducing a friend’s kiln to produce lustered surfaces. I had read about the procedure and was in my own naive way comfortable with the idea.

“The procedure was to put mothballs into the kiln a few at a time to put it into reduction as the kiln cooled. When the kiln was below red heat, I followed the instructions. The kiln started to smoke. When the concentration of gas in the kiln reached a critical point, it exploded, pushing the door bricks out a few inches. Fortunately the kiln was small enough that there was not much force.

“The idea of heating naphthalene, a potent poison, as a fuel to reduce a kiln now seems completely foolish (not considering the explosion hazard). I would strongly recommend against it.”

Thanks, Louis. Your experience may benefit some of our readers. Paragon has always warned against using mothballs for reduction inside a kiln.

Yesterday we had one of our first cold mornings of the year. It was still dark as I rode my bicycle to Paragon, enjoying passing through the stillness of a quiet residential street. Burning wood from a fireplace smelled like incense and reminded me that we are beginning the Christmas season. I hope it is a joyous one for you.

With best wishes,

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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