The core method for determining if a magnesium oxide insulation layer is damp is to measure its insulation resistance, combined with a comprehensive assessment of its appearance and operating condition. Under normal circumstances, the insulation resistance of a magnesium oxide insulation layer should be ≥ 100 MΩ. If it is below 10 MΩ, it is considered severely damp.
1. Main Detection Methods
Insulation Resistance Test (Most Direct and Effective)
Use a 2500V megohmmeter to measure the insulation resistance between the heating element and the metal casing under power-off and cooling conditions.
Insulation resistance in new equipment or under dry conditions is typically > 500 MΩ;
≥ 100 MΩ: Well-dryed;
10–100 MΩ: Slight dampness exists, requiring monitoring;
< 10 MΩ: Severely damp, must be addressed.
It is recommended to test monthly. If a rapid drop from 50 MΩ to 2 MΩ is observed, it indicates that dampness is developing.
Power-on Observation Method (Auxiliary Judgment): The following phenomena during equipment operation may indicate moisture ingress: Frequent triggering of leakage protection or grounding fault alarms by the controller; Decreased heating efficiency and slow temperature rise; Normal power supply but no heat output (possibly due to a short circuit causing the heating element to break).
Visual Inspection: Check if the wiring terminals are blackened, rusted, have water stains, or white crystals (magnesium hydroxide); Check if the metal casing has bulges, cracks, or localized expansion (moisture entering causes MgO + H₂O → Mg(OH)₂, resulting in volume expansion); Check if the sealing adhesive has aged and cracked, allowing moisture to penetrate.
2. Common Causes of Moisture Absorption
|
Cause |
Explanation |
|
Exposure to Humid Environments |
Magnesium oxide is a porous material and readily absorbs moisture from the air, especially in environments with humidity > 60%. |
|
Sealing Failure |
Poor sealing at joints and wiring terminals is the main cause of moisture infiltration. |
|
Improper Storage |
Long-term storage without moisture-proofing or failure to pre-dry before use. |
|
Frequent Temperature Cycling |
Alternating hot and cold temperatures easily produce condensation, accelerating internal moisture absorption. |
High-purity magnesium oxide (99.9% or higher) or materials coated with silica can significantly reduce hygroscopicity.
3. Recommendations
|
Situation |
Recommendation |
|
Insulation resistance < 10 MΩ |
Replace the heating element immediately; do not repair on-site. |
|
Insulation resistance 10–100 MΩ |
and continuously decreasing Replacement should be planned to avoid sudden failure. |
|
Only the wiring terminals are damp and the length allows. |
The damp section can be cut off and re-crimped, but complete sealing and waterproofing must be ensured. |
In high-requirement scenarios such as injection molding and pharmaceutical sterilization, insulation resistance < 10 MΩ is considered a failure and must be replaced.
