For us in the educational, studio, and hobby art sectors the Thermocouple and the Circuit Relay (see next month’s Tech Tip) have become common wearing parts that we need to better understand.  Neither of these parts are particularly complicated, and understanding a bit about how they work can really help us out when trying to keep a kiln in proper working order.  For this month’s Tech Tip, I am going to focus on the Thermocouple.

A kiln’s thermocouple can best be described as the eyes and ears of the kiln’s control system.  Much like we use our senses of sight, hearing, etc, to get information about the world around us, the kiln’s computer (or pyrometer), uses a thermocouple to get information about what is going on inside the kiln chamber.

How a thermocouple works (the EASY version):

The kinds of thermocouples we typically see in an electric kiln tend to work on the same principle.  Two different metals are connected together at one end (the hot end), and then connected to the kiln’s computer from the other end.  It looks something like this:

Screen Shot 2014-01-16 at 3.05.05 PMWhen the thermocouple is heated up, it generates its own microvolt current.  As the temperature in the kiln increases, the amount of voltage generated by the thermocouple also changes.  The computer or pyrometer interprets this change in voltage as a temperature.

Simple, huh.  So why does that matter?

As a thermocouple gets older, the metal it is made of oxidizes and can slough off.  This means that thermocouples will eventually wear in such a way that they are no longer accurate.  Kiln controllers can account for some of this variance, but there is a point where the thermocouple simply needs to be replaced.

Corroded Thermocouple.  Note one wire is thinner than the other now.

Corroded Thermocouple. Note one wire is thinner than the other now.

 Okay, so when do I need to replace a thermocouple?

There is no hard rule to this, as there are many other factors in the kiln environment that can affect thermocouple lifespan.   In my experience, I generally tell people that their thermocouple(s) should be changed about every 100 firings, or as often as they change their elements (whichever is the shorter number).  Of course, if the thermocouple is broken, then it should be replaced as well.


How do I know if my thermocouple is broken?

If your thermocouple is broken, your kiln’s computer display will let you know with an error message.  Often, it will also lock out any firing until the thermocouple is changed.  If the computer can’t tell what the temperature in the kiln is, it won’t know when to shut off.  The most common error message for a thermocouple is simply the word FAIL, though the error code may be different for your kiln or meter.  Your kiln manual will let you know for sure.


Are there different types of thermocouples?

Yep, and you have to make sure you are getting the correct one for your kiln or pyrometer.  The most common ones used in electric kilns are Type K and Type S (though there are a few others as well).  The vast majority of you have type K.  If you just don’t know, you can always give me a call, or you can call the manufacturer to find out.  As always, make sure you have the Model, Serial Number, voltage, and phase of your kiln when you call so we can get the info for YOUR kiln.

Can I change my own thermocouple?

ABSOLUTELY!!!  A thermocouple is one of the easiest replaced parts in an electric kiln.  The specific steps will vary depending on your kiln model.  You can call Bracker’s Good Earth Clays to order the thermocouple for your kiln.  I am happy to send you a video link on how to change it, or to talk you through it over the phone.  I am also happy to do the repair as well, whatever works out best for you.

A replacement thermocouple is one of the mainstay spare-parts that every kiln owner should have in their studio.  They are not very expensive, relatively easy to change, have no shelf-life, and every one will need to be changed eventually.  Having one, and knowing how to change it, will help keep your kiln running smoothly while minimizing down-time due to a broken thermocouple.  If you haven’t looked at yours recently, take a look at it today.  You might just avoid a problem in the future.

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