Yes, thermocouple contacts must be recalibrated after repair. Regardless of whether the repair method involves welding, annealing, or cleaning, it will affect the thermocouple's thermoelectric characteristics, potentially causing deviations between the output thermoelectric potential and the standard calibration table. Therefore, recalibration is necessary to verify measurement accuracy and correct errors.
I. Why is recalibration necessary after repair
Thermoelectric characteristics may change. The contact is the core part of the thermocouple that generates the thermoelectric potential. During the repair process, such as re-welding or high-temperature annealing, the grain structure, composition distribution, or surface condition of the metal wire may change, causing the thermoelectric potential-temperature relationship to deviate from the original calibration curve.
The quality of the solder joint directly affects the accuracy of temperature measurement. Uneven welding, the presence of pores, or impurities can introduce additional contact potential, causing measurement drift. Only through calibration can it be confirmed that the solder joint meets performance requirements.
Repair falls under the "re-entry into service" condition. According to industry standards, thermocouples that have been re-welded, shortened and reconnected, or have undergone major repairs must be treated as "new" sensors for verification and calibration; otherwise, measurement traceability and reliability cannot be guaranteed.
II. Specific Calibration Requirements and Methods
Preparation before calibration:
Clean the contact surfaces to remove oxides or residual flux.
Check insulation resistance (should be >100MΩ).
Ensure cold junction compensation is functioning correctly.
Recommended calibration method:
Comparative calibration (preferred): Place the repaired thermocouple and a standard thermocouple in the same constant temperature environment (e.g., a tube furnace or constant temperature bath), compare their output values, and calculate the deviation.
Two-Point Simplified Calibration:
Calibrate the zero point in an ice-water mixture (0℃).
Calibrate the range point in boiling water (100℃) or an oil bath.
Adjust instrument parameters to compensate for deviations.
Calibration Point Selection Recommendations:
Select at least two temperature points (e.g., 0℃ and 100℃).
For high-temperature applications, add additional high-temperature points (e.g., 500℃, 800℃).
Hold each point at a constant temperature for 15-30 minutes, taking the average of multiple measurements to reduce errors.
III. Risks of Non-Calibration
|
Risk Type |
Consequence |
|
Measurement Deviation |
Causes malfunction of the temperature control system, affecting product quality. |
|
System Alarm |
Triggers shutdown due to abnormal readings, causing production interruption. |
|
Safety Hazard |
May cause overheating or leakage accidents in boilers or gas equipment. |
|
Data Distortion |
Affects process records and quality traceability, failing to meet ISO and other system requirements. |
IV. Special Circumstances:
Temporary Emergency Use: For non-critical temperature measurement points where accuracy requirements are not high, a simple verification can be performed first (e.g., holding the instrument to heat it and observing voltage changes), but formal calibration should still be arranged as soon as possible. For use in safety interlock systems, such as gas stove flameout protection and pressure vessel over-temperature alarms, after repair, the device must be verified and qualified by a metrology institution before it can be used. Skipping the calibration process is strictly prohibited.
