KILN THERMOCOUPLES

The Basics

A thermocouple is a heat-measuring sensor composed of two dissimilar metals joined at one end. When the joined end is heated or cooled, it creates a temperature gradient along the length of the thermocouple. That gradient will create a predictable voltage correlated to the temperature of the joined end.

Note: A thermocouple is only a sensor, not a meter. In order to read the temperature, you will need to purchase a compatible analog or digital pyrometer. 

Thermocouples are built into kilns with electronic controllers. They can also be added to a kiln as a backup controller. Thermocouples will age with use and can be damaged (and cause damage) from improper use, so it is important to know a little bit about thermocouples before purchasing.

Types of Thermocouples and Compatibility

There are 10 different types of thermocouple, each distinguished by a letter, which denotes the materials used, temperature range and precision. There are 2 types commonly used by potters: Type-K and Type-S.

 All Type-K thermocouples, regardless of size, shape or brand, are ultimately the same and compatible with the extension wires and pyrometers of the same type. The same goes for Type-S thermocouples; all Type-S thermocouples are compatible with all Type-S accessories.

Are Type-K and Type-S Thermocouples and Accessories Compatible with Each Other?

No, not at all. It cannot be stressed enough how incompatible thermocouples are with unlike accessories. Different thermocouples will produce different voltages for the same temperature and the higher the temperature, the greater the difference will be.

The best case scenario would be to attach a Type-K thermocouple to a Type-S pyrometer, in which the kiln is much hotter than it actually is and cause an under-firing. Doing the opposite and using a Type-S thermocouple with a Type-K pyrometer could potentially damage your kiln and your studio. Also, do not attempt to jerry-rig or MacGuyver together any replacement parts for your thermocouple.

Type-K and Type-S Comparison 

Type	K	S
Price point	>$39.00	>$200.00
Maximum Continuous Temperature	1,100C (2,120F)	1,600C (2,912F)
Absolute Maximum Temperature	1,260C (2,300F)	1,700C (3,029F)
Accuracy	+/- 2.2 C	+/- 1.5 C
+ Terminal Wire*	Yellow	Black
- Terminal Wire*	Red	Red
Outer-Wire Insulation*	Yellow (Brown Extension)	Green
Uses	Low-fire ceramics/glazes, glass fusing	High-fire ceramics/glazes, crystalline glazes
Pros	Less expensive	Greater accuracy, greater temperature range, very long lasting
Cons	Less accurate, lower maximum temperature, less durable	More expensive

*color standard in the U.S. ONLY.

Replacing a Thermocouple

Eventually a thermocouple will get worn-out or damaged and stop giving an accurate reading. If you are experiencing problems with your thermocouple (especially a new thermocouple), there are some things you can check before replacing it.

Thermocouple Troubleshooting

• Is the thermocouple damaged? Is it bent or discolored? Look at the tip of the thermocouple. Is it cracked? If so, it will not give an accurate reading and will need to be replaced.
• Make sure the leads are connected and are connected to the correct terminal. Thermocouples are polarized and will not give an accurate reading if the leads are switched.
• Are you using the right extension wire and pyrometer? See the chart above to make sure the wires are the right color.
• Fire your kiln with a witness cone. Over time, a thermocouple may become off by 5-10 degrees, which may not be significant in basic firing, but if the thermocouple is off by an entire cone then it should be replaced immediately.

If you have determined that it is time to replace your thermocouple, the easiest thing is to buy the exact same one, as recommended by the manufacturer.

Even though all thermocouples of the same type are compatible, they may not be the same size/shape and may not fit in the existing mounting hardware in your kiln. As long as the mounting hardware is undamaged, you will only need to replace the thermocouple.

Click here to browse our selection of thermocouples.