What are the common causes that lead to the damage of the thermocouple probe in a furnace temperature tester?

I. High-Temperature-Related Damage (Core Cause)

1. Over-Temperature Operation: Thermocouples have a fixed temperature measurement range (e.g., the conventional range of K-type thermocouples is -200~1300℃). If the furnace temperature is continuously close to or exceeds the upper limit, it will cause the thermoelectric materials inside the probe (such as nickel-chromium - nickel-silicon) to undergo grain coarsening and oxidative failure, and may even lead to melting and fracture, directly losing the temperature measurement function.
2. Thermal Shock from Sudden Temperature Changes: Rapid heating/cooling of the furnace (e.g., directly placing the probe into an 800℃ furnace from room temperature) results in inconsistent thermal expansion coefficients between the probe's metal sheath and internal core wire, generating thermal stress. Repeated shocks will cause the core wire to break or the sheath to crack.

II. Physical Damage

1. Mechanical Impact/Bending: Improper operation during installation or disassembly (e.g., pulling or bending the probe cable forcefully), or collisions caused by falling workpieces inside the furnace or equipment transportation, can lead to deformation or fracture of the probe sheath, resulting in short circuits or open circuits of the internal core wire.
2. Wear and Aging: Long-term friction with workpieces and furnace walls inside the furnace, or repeated thermal expansion and contraction under high-temperature environments, causes the probe sheath to thin and break, thereby damaging the internal thermoelectric elements.

III. Chemical Corrosion and Contamination

1. Corrosion by Furnace Media: The presence of oxidizing gases (e.g., O₂ at high temperatures), reducing gases (e.g., H₂, CO), or corrosive gases (e.g., SO₂, Cl₂) inside the furnace will oxidize and corrode the probe sheath (such as stainless steel sheaths) and core wires, leading to the failure of thermoelectric properties.
2. Contaminant Adhesion: Dust, oil stains, molten metal splatters, etc., inside the furnace adhere to the probe surface and form hard coatings at high temperatures. This not only affects the temperature measurement response speed but may also penetrate into the sheath, corroding the core wire.

IV. Improper Installation and Use

1. Incorrect Installation Position: If the probe is close to the furnace wall, heating elements, or workpieces, local overheating will occur; if it is not inserted into the effective temperature measurement area of the furnace, it will be affected by the external environment and operate under unstable conditions for a long time, accelerating damage.
2. Improper Wiring: Loosening or oxidation at the wiring connections increases contact resistance, generating false temperature measurement signals. At the same time, abnormal current heating may indirectly damage the probe's terminal connections.
3. Lack of Regular Maintenance: Failure to clean the probe surface or check the wiring status for a long time leads to the accumulation of contaminants and poor contact, which gradually aggravates probe damage.

V. Quality and Aging Issues

1. Product Quality Defects: Inferior thermocouples have impure core wire materials, uneven sheath thickness, or poor welding processes, making them prone to premature fracture and failure even under normal operating conditions.
2. Natural Aging: Thermocouples have a service life (usually thousands to tens of thousands of hours). After long-term use at high temperatures, the thermoelectromotive force of the thermoelectric materials will gradually attenuate, eventually reaching the damage threshold.