The actual temperature of the tempering furnace under vacuum is inconsistent with the measured temperature

The actual temperature of the tempering furnace under vacuum is inconsistent with the measured temperature, which may be caused by thermocouple failure, temperature control system error, vacuum environment influence, furnace structure problems, or operational factors. The following is a specific analysis of the reasons and corresponding solutions:

1. Thermocouple related issues
Thermocouple model mismatch
Reason: If the thermocouple type (such as N type, B type, S type) is not compatible with the temperature controller, or if the temperature measurement range exceeds the rated value of the thermocouple (such as using low-temperature thermocouples in high-temperature environments), it will cause measurement errors.
resolvent:
Confirm that the thermocouple model matches the temperature controller (if an S-type thermocouple requires an S-type temperature controller).
Check if the thermocouple graduation number is consistent with the equipment requirements (e.g. if the equipment requires N type but mistakenly uses B type).
Improper installation position of thermocouple
Reason: The thermocouple is not inserted into the workpiece or the center of the heating zone, or there is poor contact (such as looseness or oxidation), which causes the measured value to deviate from the actual temperature.
resolvent:
Insert the thermocouple probe into the center of the workpiece stack or a uniform location in the heating zone.
Tighten the thermocouple wiring terminals and remove the oxide layer (such as sanding with sandpaper).
Damaged or aged thermocouple
Reason: The thermocouple wire is broken, the insulation layer is damaged, or the performance decreases after long-term use, resulting in signal distortion.
resolvent:
Use a multimeter to check the resistance value of the thermocouple (normally close to 0 Ω), and replace it if there is an open circuit.
Regularly replace thermocouples (it is recommended to replace them every 1-2 years).

2. Temperature control system error
Temperature controller parameter setting error
Reason: PID parameters (proportional, integral, derivative) were not optimized, or temperature upper and lower limits and alarm values were set improperly, resulting in unstable temperature control.
resolvent:
Re debug PID parameters (can be adjusted through self-tuning function or manually).
Check whether the temperature setting value is consistent with the process requirements.
Temperature controller malfunction
Reason: The internal circuit of the temperature controller is damaged, the sensor interface is loose, or there is a software bug, resulting in the displayed value not matching the actual value.
resolvent:
Check if the wiring of the temperature controller is secure, restart the device and observe if it is restored.
Contact the manufacturer for repair or replacement of the temperature controller.
Solid state relay malfunction
Reason: The solid-state relay (SSR) is damaged or has poor contact, resulting in unstable heating power output.
resolvent:
Use a multimeter to check if the input/output terminals of the SSR are conductive (normally should be connected or disconnected with the temperature controller signal).
Replace the damaged SSR.

3. Impact of vacuum environment
Insufficient vacuum degree leads to differences in heat exchange
Reason: When the vacuum degree is low, the heat exchange between the workpiece and the furnace wall is enhanced, which may cause the actual temperature of the workpiece to be lower than the measured value (especially for thin-walled parts).
resolvent:
Check the operating status of the vacuum pump unit (such as mechanical pump oil level, diffusion pump heating power).
Check the sealing of the furnace body (use a helium mass spectrometer leak detector to locate the leakage point).
Radiation heat transfer error
Reason: Thermal radiation is dominant in a vacuum environment. If the reflectivity of the furnace wall is low or the workpieces are placed densely, it may lead to uneven temperature field.
resolvent:
Optimize the placement of workpieces (maintain spacing and avoid obstruction).
Apply a high reflectivity coating (such as alumina) on the inner wall of the furnace.

4. Furnace structure issues
Insufficient power of heating element
Reason: Heating strips (such as molybdenum strips) may age, break, or have uneven power distribution, resulting in low local temperatures.
resolvent:
Check the resistance value of the heating element (which should be close to the nominal value), and replace the damaged element.
Adjust the power distribution of the heating zone (such as adding an auxiliary heating zone).
Decreased furnace insulation performance
Reason: The insulation material (such as ceramic fiber) is damaged or aged, causing heat loss and resulting in measured values higher than the actual temperature.
resolvent:
Repair or replace the lining insulation material.
Check if the sealing strip of the furnace door is intact and replace it if necessary.

5. Operational factors
Excessive loading capacity
Reason: Too many or densely stacked workpieces hinder thermal cycling, resulting in uneven temperature distribution.
resolvent:
Reduce the single loading capacity (recommended not to exceed 70% of the furnace volume).
Use tooling racks to disperse and place workpieces.
Process parameter setting error
Reason: Excessive heating rate, insufficient insulation time, or improper cooling method can cause temperature fluctuations.
resolvent:
Strictly follow the process curve to set parameters (such as heating rate ≤ 5 ℃/min).
Increase the insulation phase time (to ensure sufficient organizational transformation).

6. Comprehensive investigation steps
Calibrate thermocouple: Use a standard temperature source (such as a dry body furnace) to calibrate the thermocouple and confirm the measurement accuracy.
Check the temperature control system: observe whether the displayed value of the temperature controller is consistent with the actual temperature curve, and adjust the PID parameters.
Detecting vacuum degree: Measure the pressure inside the furnace with a vacuum gauge to ensure that it meets the process requirements (such as ≤ 10 ⁻ ² Pa).
Check the furnace body: visually inspect whether the heating elements, insulation materials, and sealing strips are intact.
Simplified testing: Run the furnace empty, observe whether the temperature is stable, and eliminate the influence of workpieces.