A thermocouple is an electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures. A thermocouple produces a temperature-dependent voltage as a result of the thermoelectric effect, and this voltage can be interpreted to measure temperature. Thermocouples are a widely used type of temperature sensor.
The most commonly used types of thermocouples in glass equipment are Type K, and Type R/S.
Type K thermocouples are good up to 1350 Celcius, but fare poorly in reduction. Hydrogen formed in reduction atmospheres causes oxidation and reduces the accuracy of the thermocouple and causes them to read low. As some hydrogen is always present in gas fired systems, type K thermocouples are not the best choice. They can be used, but will age quickly, have temperature drift, and need regular replacement.
Wet Dog Glass uses Type K thermocouples in electric ovens. We also use them in gas equipment such as garages that are less temperature critical and at a lower temperature.
Type R/S thermocouples are far more stable than type K and will not age and degrade quickly in reduction. They can be used up to 1600 Celsius. While expensive, the expense is primarily in the metals used in construction, so broken or non-functional thermocouples can be returned to the manufacturer for a steep discount on new thermocouples.
When installing or servicing thermocouples, remember that different types are not interchangeable and if they are wired backwards, the temperature will read backwards.