by Marc Ward
Clay Times June 1996 & Jan. 2006
Q: The thermocouples on my burners are continually burning out. Is there something I can do to keep this from happening?
A: Thermocouples are designed to be low maintenance items. I just replaced one on my kiln that was installed ten years ago. A thermocouple works by sending a signal to a safety valve telling that valve that "hey there's a pilot flame burning on me so everything is fine". This signal is a very small voltage current that is produced by the pilot flame and the "difference" between two metals that are welded together. The flame heats up the bulb at the end of the thermocouple while the back end of the bulb stays relatively cool. This "tension" between the front and back of the bulb produces the current. The current is anywhere from 10mV (10 millivolts or 1/100 of a volt) to 35mV (35 millivolts or 3.5/100 of a volt). If the back end of the bulb gets too hot in relation to the end on which the pilot flame is burning, the signal "drops out". In other words, the safety valve is no longer getting the message from the thermocouple and the safety valve shuts the burner down. If the back end of your thermocouple bulb gets constantly overheated, it will fail. With excessive back pressure coming from the burner ports, thermocouples can get burned up quickly.
There are many types of thermocouples, most of which are interchangeable. K16, K15, K19, 17D, 50, ect. are some of the types used on burners. Also, these types come in an industrial or residential grade. The residential type can be picked up at the hardware store and used in a pinch, but you're better off using industrial quality thermocouples such as Johnson Control's "Husky". The industrial type is a tiny bit more resistant to burning out if the rear of the bulb is overheated. The biggest advantage with the industrial thermocouple is it's signal strength or millivolt output. These types of thermocouples have an output range of 25-35mV. Again, whatever type thermocouple you are using, if the back end or "cold junction" gets too hot, you'll have a shutdown.
Another cause of thermocouple problems is an unstable pilot flame. If the flame is "dancing around" on the thermocouple, the difference between front and back is lessened causing the signal to degrade and producing a shutdown. Burners should be protected from wind and positioned properly so that secondary air entering the kiln does not cause the flame to dance. A burner port should be large enough to have the pilot within it's perimeter with room around the pilot. This doesn't mean the burner head and pilot are inside the burner port. All open port systems should have the burner and pilot set back from the outside edge of the kiln anywhere from 1/2" to 1". If your burners are too close or inside the port, the life of the burner will be shortened, the pilot flame will be unstable, and your combustion will be less than ideal. If moving your burner back doesn't solve the problem, then you're getting too much back pressure from the port. You need to open the damper to decrease the kiln pressure. If that doesn't stop the back pressure, you may be forced to enlarge your flue and/or chimney.
So, you don't want to rebuild your kiln and you still want to sock the reduction to it. What now? You can wrap a small amount of ceramic fiber around the back end of the bulb to keep it cooler. Another thing that is commonly overlooked is wire kinks. The wire coming out of the back of the bulb needs to be straight for at least 1/2". Where the wire enters the safety valve also needs to be straight for the same distance. In between these two points, the wire can be bent and twisted as long as it isn't severely crimped.
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There is probably
no item, large or small, that tests the patience of our brave potter.
An item that is small in size. An item that is small in price. An item,
a sometimes wicked, wicked item that can bring the whole show to a
halt. This small probe produces more anxiety than a rectal thermometer
did when you were six years old. Of course, I’m talking about your
thermocouple. This thing causes more heartache and gnashing of teeth
than most things in the clay world. And when this damn thing is making
your life miserable, just remember it is doing its assigned job.
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The job it does is to
detect the presence or lack of flame. It is a simple job done by a
simple soldier. It just does what it is ordered to do with the facts it
has at hand. Matter of fact, it does the job so well, that many potters
don’t want it around. And here is where the mistake is made.
Thermocouples detect a pilot flame and relay that information to a
safety valve. As long as the thermocouple detects the presence of a
flame, it is sending an A-OK message back to the safety valve. When the
thermocouple is no longer sending this message of contented bliss, the
safety valve shuts the gas down. So what gives, you have all this flame
going into the kiln and the thing shuts down. Why/
A thermocouple needs blue flame continuously on the tip of the
thermocouple. Continuously… not 9 seconds out of 10… continuously. And
a blue flame not a yellow flame. If the pilot flame is blowing around
this is enough to cause a shutdown. I’ve written about that before, but
what causes those mystery shutdowns late in the evening? Those ‘pull
your hair out” shutdowns when the kiln has been cruising along fine and
craps out just as cone 9 is starting to bend? Those shutdowns that
cause otherwise cautious people to throw their hands in the air and
tear the safety system off their burners. The answer lies in how these
things actually work.
A thermocouple’s guts are made from two different types of metal.
One metal is solid and the other is a surrounding tube. They are
connected together in a welded end. This welded end is called the “hot
junction”. The surrounding tube, or outer element, is brazed to a
copper or brass tube away from the hot junction. This other connection
point is called the “cold junction”. What make the thermocouple work is
the large temperature difference between the hot junction and the cold
junction. This temperature difference causes electrons to start flowing
back to the safety valve. The more the difference, the more electrons
flow (more voltage). This energy, which is measured in millivolts, is
directly proportional to the temperature difference between the two
junctions. If you don’t have a big enough difference in temperature
between the hot and cold junctions, you won’t get enough current. Low
current means the safety valve is saying, “hey! wait a minute, we don’t
have enough current from our probe, that must mean no pilot flame…we
gotta’ shut it down.” This is why this kind of shut-down only happens
at elevated temperatures. The ambient and radiated temperatures have
increased so much that the relative difference between the cold
junction and the hot junction have substantially narrowed. As you can
see from the illustration, these two junctions are not that far apart.
And once this starts happening, the life of the thermocouple is
limited. This is why you should keep spare thermocouples handy… they
are consumable items. It is easy to blame the burner, the pilot, or the
safety valve for these aggravating, high-temperature shutdowns. After
all they are the things that are ‘doing’ something. But, as with many
things, these shutdowns are a symptom, not the underlying problem.
The real problem may be a burner port that is too small for the
burner (it ain’t one size fits all). Or, it could be the burner is too
close to the port. Or, you are reducing way too much and causing
excessive backpressure and heat at the burner head. It can even be as
mundane as a kiln room that is too small or lacks good air exchange or
a wall that is too close to the kiln. All these things can lead to a
higher than acceptable ambient or radiated temperature around the pilot
burner and thermocouple.
Also, be aware that the voltages that run these things are very
low. The longer the distance between the tip of the thermocouple and
the safety valve, the more resistance to electron flow there is. If you
couple this with high ambient temperatures, you are going to be wailing
at the moon over the unfairness of your life. (read; mystery
shutdowns). Instead of removing or disabling your safety valves, you
may need to provide your thermocouple with the conditions that will
make its life meaningful and your life easier.
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