If you have a digital kiln, you will notice a small rod projecting into the firing chamber. It is the thermocouple, which reads the kiln temperature. Ceramic kilns use two basic types of thermocouples: Type-K or Type-S. Most kilns are equipped with the Type-K, which is less expensive than the Type-S.
The Type-K shows very little wear below 2000F. It is ideal for glass, silver clay, and low-fire ceramics.
Type-S is ideal for porcelain and stoneware. At these high temperatures, Type-S thermocouples show little wear. Some people claim that they last for years of heavy use. Type-S failure is ordinarily due to breakage rather than to wear.
Type-K and Type-S thermocouples are not interchangeable. Some controllers, such as the DTC 100, 600, 800, and 1000 series, and Sentry 3-key accept only Type-K thermocouples. The Sentry 12-key controller accepts either type, but you must select the correct thermocouple in Options. Some brands of controllers require a change in computer chip to accept the Type-S thermocouple.
If you have the wrong thermocouple type wired to the controller, the temperature will be wrong:
Type-K thermocouple wired to a controller that is adjusted for Type-S: The kiln will under-fire.
How far the kiln will under-fire depends on the temperature. The higher the temperature, the greater the error. At 75 deg. F, the controller will think the kiln has reached 100 deg. F. At 500 degrees, the controller will think the kiln is at 2000 deg. F.
Type-S thermocouple wired to a controller that is adjusted for Type-K: The kiln will over-fire. The higher the temperature, the greater the error. At 75 deg. F, the controller will think the kiln has reached 75 deg. F. At 2000 degrees, the controller will think the kiln is at only 500 deg. F.
Type-K and Type-S thermocouples each require a different type of thermocouple wires:
Type-K Yellow wire (+ terminal) Red wire (- terminal) Brown or yellow outer wire insulation
Type-S Black wire (+ terminal) Red wire (- terminal) Green outer wire insulation
If the controller is matched correctly with the thermocouple, you can still get an error in temperature by using the wrong thermocouple wire. However, the error is not too noticeable: 10 – 30 deg. F when the thermocouple wire is only a few feet long. The error becomes greater when the wire length increases, however.