A damaged thermocouple contact will directly lead to inaccurate temperature measurement or signal interruption. Common symptoms include abnormal instrument readings, slow response, or system alarms. These can be identified through three aspects: appearance, electrical signals, and temperature measurement performance.
I. Symptoms at the Instrument and System Level
Abnormal Temperature Display
The displayed value deviates significantly from the actual temperature, such as displaying room temperature in a high-temperature environment, or displaying negative values.
Readings fluctuate frequently or are unstable, with significant fluctuations even after eliminating electromagnetic interference.
System Alarms or Protection Actions
The temperature control system triggers alarms such as "sensor failure" or "wire breakage."
In safety equipment such as gas stoves, a damaged thermocouple can cause the ignition to extinguish immediately after releasing the valve, as the solenoid valve cannot maintain power supply.
Control Failure
The temperature control loop becomes unstable, causing heating/cooling equipment to run continuously or fail to start, affecting the process.
II. Physical and Electrical Testing Results
Visible Damage
Contacts or protective tubes are blackened, oxidized, deformed, or perforated, especially noticeable in high-temperature or corrosive environments.
Thermocouple wires are broken, melted, or embrittled; the surface of noble metal type (S/R type) is dull, and base metal type (K/J type) shows rust.
Abnormal Resistance Measurement
Using a multimeter to measure resistance:
Infinite resistance (OL) → Thermocouple wire is open-circuited.
Resistance close to 0Ω → Possible short circuit (e.g., metal shavings or moisture inside the protective tube).
Normal thermocouple resistance is generally no greater than 50Ω (depending on length and model).
Thermoelectric Potential Response Failure
When measuring the cold junction voltage using the millivolt range, the voltage shows no change or minimal change when the hot junction is heated.
In an ice-water mixture (0℃), the output should be close to 0mV. In boiling water (100℃), the K-type should output approximately 4.095mV. Excessive deviation indicates performance degradation.
III. Performance Related to Operation and Environment
Slow Response
The signal response is slow when the temperature changes, failing to track the actual temperature rise/fall trend.
Poor Repeatability
Inconsistent output values when measuring the same temperature point multiple times indicate that the contact material has aged or partially deteriorated.
Environmental Factors Exacerbate Faults
Faults are more likely to occur in humid, vibrating, or strong electromagnetic field environments. For example, water ingress into the junction box can cause a short circuit, or vibration can cause a loose connection.
