1、 Appearance and structural inspection

Observation of wires and protective tubes

Check if there are cracks, deformations, corrosion perforation in the thermocouple protection tube (especially for thermocouples used in high temperature or corrosive environments). If the protection tube is damaged, it may cause oxidation or short circuit of the thermocouple wire.

Peel off the end of the protective tube and observe whether there is any breakage, melting, blackening, or oxidation of the wire: precious metal wires (such as S-type and R-type) may become dull on the surface after oxidation, while base metal wires (such as K-type and J-type) may show rust or embrittlement after oxidation.

Inspection of junction boxes and cables

Open the junction box and check if the terminals are loose, oxidized, or burned, if the insulation layer of the cable is damaged, and if the solder joints are detached (especially for welded thermocouples).

If it is for compensating wire connection, it is necessary to confirm that the wire model matches the thermocouple (such as K-type thermocouple requires K-type compensating wire), to avoid measurement deviation caused by model mismatch.

2、 Electrical performance testing

Continuity testing (multimeter method)

Set the multimeter to the ohm range (Ω), disconnect the thermocouple from the circuit, and measure the resistance values of the two thermocouple wires separately:

If the resistance value is infinite, it indicates that the wire is broken and needs to be replaced;

If the resistance is close to 0 Ω, there may be a wire short circuit (check whether there are metal debris or moisture inside the protective tube).

Thermoelectric potential measurement (millivolt meter method)

Measure the thermoelectric potential of the thermocouple at the cold end (junction box) using a high-precision millivolt meter (or millivolt range of a multimeter):

Referring to the thermocouple calibration table (e.g. K-type thermocouple is approximately 1.000mV at 25 ℃), if the measured value deviates from the theoretical value by more than the allowable error range (e.g. K-type ± 2.5% or ± 2.5mV), the thermocouple wire may age or deteriorate.

Compare the thermoelectric potential of normal thermocouples of the same model. If the difference is significant, it indicates that the measured thermocouple is damaged.

3、 Comparison and verification of actual temperature measurement

Constant temperature environment testing

Insert the thermocouple into a constant temperature source (such as a constant temperature oil bath or muffle furnace) and compare the reading with a standard thermometer (such as a platinum resistance thermometer):

If the temperature difference exceeds the accuracy level range of the thermocouple (such as K-type ± 1.5 ℃ or ± 0.4% t), and after excluding factors such as cold junction compensation and wiring errors, it can be determined that the thermocouple has failed.

Example: In boiling water at 100 ℃, the theoretical thermoelectric potential of a K-type thermocouple is about 4.095mV. If the measured value deviates by more than ± 0.1mV, the performance of the thermocouple may degrade.

Dynamic temperature rise test

Slowly raise the temperature of the thermocouple and observe whether the thermoelectric potential changes linearly with temperature:

If there is no change or jumping fluctuation in thermoelectric potential during heating, it may result in poor internal contact or breakage of the wire;

If the thermoelectric potential cannot return to its initial value after cooling, it indicates irreversible annealing or oxidation damage to the wire.

4、 Judgment methods in special scenarios

Quick assessment in high-temperature environments

In high-temperature scenarios such as kilns and boilers, if the temperature display value suddenly drops to room temperature or is abnormally low, and other instruments display normally, it may be due to the melting of thermocouple wires or the rupture of protective tubes (which can be judged based on the historical trend of furnace temperature).

Anti-interference test

If the measured value fluctuates frequently, the thermocouple can be checked for electromagnetic interference (such as near power cables) or poor grounding (for grounded thermocouples, single point grounding must be ensured). If the reading is still unstable after eliminating interference, the performance of the thermocouple wire may decrease.

summary

To determine the damage of a thermocouple, the three-step method of "appearance inspection → electrical testing → temperature measurement comparison" should be combined, with priority given to using a multimeter and millivolt meter to detect continuity and thermoelectric potential, and then verifying accuracy through a constant temperature source comparison. For critical scenarios such as industrial furnace temperature control and boiler monitoring, it is recommended to calibrate regularly (every 6-12 months) and backup thermocouples of the same model for quick replacement testing. If frequently damaged, it is necessary to check whether the usage environment (such as temperature, atmosphere, vibration) exceeds the tolerance range of the instrument.