How to determine the cause of unstable thermocouple output signal?

How to determine the cause of unstable thermocouple output signal?
Thermocouple output signal instability may be caused by a variety of reasons, which can be judged from the following aspects:
Thermocouple itself
Thermocouple damage
Observe the appearance: Check whether the thermocouple thermocouple has obvious cracks, fractures, deformation or corrosion. If the thermocouple is damaged, it will cause poor contact or change the thermoelectric characteristics, making the output signal unstable.
Check the resistance value: Use a multimeter to measure the resistance value of the thermocouple and compare it with the specifications of the thermocouple. If the resistance value is abnormal, it means that there may be a problem with the thermocouple.
Loose terminals
Visual inspection: Check whether the terminals of the thermocouple and the compensation wire, measuring instrument and other connecting parts are firm. Loose terminals will cause unstable contact resistance, which will cause output signal fluctuations.
Gently shake the circuit: When the power is on, gently shake the thermocouple terminals and connecting circuits to observe whether the output signal changes significantly. If the signal fluctuates, it means that there is a problem with poor contact.
Compensation wire related factors
Model mismatch
Check the model: Check whether the model of the compensation wire is consistent with the model of the thermocouple. Different types of thermocouples need to use corresponding compensation wires, otherwise it will lead to inaccurate thermoelectric potential transmission and unstable output signal.
Damaged line
Check the appearance: Carefully check whether the outer skin of the compensation wire is damaged, aged, cracked, etc., and whether the internal wire is broken or short-circuited. If the compensation wire is damaged, it will affect the stability of signal transmission.
Perform insulation test: Use an insulation resistance tester to measure the insulation resistance of the compensation wire. If the insulation resistance value does not meet the requirements, it means that there is an insulation problem with the wire, which may cause signal leakage or interference, making the output signal unstable.
Measuring environmental factors
Drastic temperature changes
Monitor ambient temperature: Use a thermometer to measure the ambient temperature around the thermocouple and observe whether the temperature fluctuates greatly. If the ambient temperature changes too quickly or exceeds the normal operating range of the thermocouple, the thermoelectric potential of the thermocouple will change significantly, resulting in unstable output signal.
Electromagnetic interference exists
Observe peripheral equipment: Check whether there are large motors, transformers, inverters and other equipment that generate strong electromagnetic fields near the thermocouple. These devices may cause electromagnetic interference to the output signal of the thermocouple, resulting in unstable signal.
Use an electromagnetic interference tester: If electromagnetic interference is suspected, an electromagnetic interference tester can be used to detect the electromagnetic environment on site. If a strong electromagnetic interference signal is detected and the interference frequency is close to the frequency of the thermocouple output signal, then electromagnetic interference is likely to be the cause of the unstable signal.
Measuring instrument factors
Instrument failure
Check the instrument display: Observe whether the display screen of the measuring instrument has abnormal display, such as garbled characters, flickering, numerical jumps, etc. If the instrument display is abnormal, it may be that the instrument itself is faulty, resulting in unstable measurement and display of the thermocouple output signal.
Calibrate: Send the measuring instrument to a professional metrology agency for calibration to check whether the measurement accuracy of the instrument meets the requirements. If the instrument has errors or the calibration is unqualified, it will affect the accurate measurement of the thermocouple signal and make the output signal appear unstable.
Input impedance mismatch
Check the instrument manual: Check the input impedance parameters of the measuring instrument and compare them with the output impedance of the thermocouple. If the two do not match, it will cause reflection or attenuation during signal transmission, making the signal received by the measuring instrument unstable, which in turn affects the stability of the output signal.