What causes a thermocouple sensor to fail?

Oxidation

Oxidation is a major issue for bare thermocouple wire since it produces considerable changes in the composition of the wires. This is due to the formation of oxide scales on the surface of the thermoelements. Oxidation can be prevented by using a mineral insulated cable, the outer sheath of a thermocouple will protect the wires inside. Thermocouples nowadays are mostly protected with MgO and an outer sheath, providing a positive effect on reducing the contributions of oxidation. The impact on drift of oxidation has dramatically reduced due to the introduction of mineral insulated thermocouple cables. The outer sheath also improves the operational lifetime of thermocouples.

Contamination of conductors / metal fatigue

The thermocouple conductors of an MI cable can be contaminated after being exposed to demanding operating environments for a longer period of time. This will result in metal fatigue and changes in composition. Constant changes in temperatures can result in thermocouples to weaken over time and thus cause drifts.

Insulation resistance / thermal shunting

To overcome thermal shunting, Okazaki has created a recipe for the MGO of the mineral insulated cable to prevent the shunt error from happening. Please consult one of our colleagues if you require more information or if you would like to receive a quotation for this improved cable.
The electrical resistance of all insulating materials used in sheathed thermocouples (such as MgO powder) decreases exponentially with the increase of temperatures. An example is when the insulation resistance decreases at temperatures above 1000°C, it could cause errors in thermocouple thermometry. This phenomenon causes the creation of a second junction in the thermocouple.

This virtual junction will indicate a temperature that is closer to the temperature of the hottest part of the thermocouple, and not that of the measuring junction. This will cause the electromotive force in the sensor to be disrupted, resulting in volatile readings. This is called thermal shunting, or the shunt effect.

Short range ordering

When a type K thermocouple is exposed to a temperature range of approx. 250 to 550℃, the thermo-electromotive force gradually increases at the exposed part, and when the insertion length is changed, a temperature higher than actual temperature is indicated higher by several ℃; however, the thermocouple returns to the original status when the temperature becomes approx. 650℃ or higher. This temperature range slightly differs depending on references. This phenomenon appears when the insertion depth in the actual use differs from the insertion depth under calibration. If the insertion length is not changed, a large change in the measurement value does not appear. For more information about this phenomenon, please visit https://www.okazaki-mfg.com/en/Tech_info/faq_06.html.