Poor insulation in reduced-diameter thermocouples can cause temperature signal drift, jumps, or PID control oscillations. In severe cases, it can lead to false alarms in the temperature measurement system, temperature control failure, or even equipment shutdown.
1. Typical Symptoms
|
Symptom |
Specific Phenomenon |
Possible Consequences |
|
Temperature Reading Drift or Jumps |
Irregular fluctuations in displayed values (e.g., ±10℃ or more), especially under stable operating conditions |
Leads to uncontrolled process temperature, affecting product quality |
|
Frequent PID Control Oscillations |
Repeated start-stop of the temperature control system, abnormal heating/cooling actions |
Shortens equipment lifespan, increases energy consumption |
|
Insulation Resistance Alarm |
Control system displays "grounding fault" or "insulation too low" |
Triggers safety protection, automatic shutdown |
|
Large Deviation from Other Measuring Points |
Differences in readings between multiple thermocouples within the same system exceeding 5℃ |
Difficulty in determining the true temperature distribution |
Note: Poor insulation often manifests as an "intermittent fault," which worsens under high temperature or humidity changes.
2. Root Causes and Influencing Mechanisms
MgO Moisture Absorption: The conductivity of the MgO insulation layer increases after absorbing moisture, leading to leakage between the electrode and the protective tube.
Microcrack Formation: Residual stress from the necking process or mechanical impact causes cracks in the insulation layer, forming conductive paths.
High-Temperature Aging: The performance of the insulation material degrades during long-term operation, and the insulation resistance gradually decreases.
Even if the appearance is intact, internal insulation deterioration can still cause signal distortion, requiring confirmation through professional testing.
3. On-Site Rapid Identification Methods
Preliminary Multimeter Test:
Use a 500V megohmmeter to measure the insulation resistance between the electrode and the protective tube.
<10 MΩ indicates poor insulation.
High-end applications (such as hot runners) require ≥1000 MΩ.
Dynamic Observation Method:
Observe the temperature curve for sudden changes or lags during the heating process.
Combined with the control system log, check for any accompanying grounding alarm records.
Recommendation: Conduct insulation tests regularly, especially in humid environments or highly corrosive conditions.
