What are the common faults of metal tube probe insertion junction box type platinum resistance thermometers

Metal tube probe insertion junction box type platinum resistance thermometers are widely used in industrial temperature measurement. Their common faults mainly focus on four aspects: damage to the sensing element, deterioration of insulation performance, poor wiring connections, and external environmental corrosion. These faults can lead to inaccurate temperature measurements, signal fluctuations, and even system shutdowns, affecting production stability. The following is a systemic fault analysis:

I. Platinum Resistance Element Body Faults

Platinum Wire Breakage: The diameter of the platinum wire is typically only 0.03–0.07 mm. If the probe is excessively bent during installation (radius of curvature < 50 mm) or subjected to severe vibration, fatigue fracture can easily occur due to concentrated mechanical stress, manifesting as infinite resistance and the instrument display exceeding the range.

Resistance Value Drift or Aging: Long-term operation in high-temperature environments (>500℃) or frequent thermal cycling may cause slight changes in the crystal structure of the platinum wire, leading to deviations in the resistance-temperature characteristics from the standard curve and measurement errors. Poor manufacturing processes can also accelerate the aging process.

Internal Short Circuit: If the magnesium oxide insulating powder filling becomes damp or contains metal shavings, it may conduct at high temperatures, causing short circuits between platinum wires or leads, resulting in low resistance or even zero resistance.

II. Insulation Performance Deterioration:

Sheath Leakage or Moisture: If the metal protective tube and junction box are not properly sealed, moisture, steam, or corrosive liquids may penetrate, causing the insulation resistance to drop from the normal >100MΩ to <1MΩ, leading to signal interference or grounding faults.

Insulation Material Aging: Prolonged operation at high temperatures (>250℃) will cause the PTFE insulation layer to soften and carbonize, losing its insulating ability; in strong oxidizing media, the material may also undergo chemical degradation.

Standard Requirements: IEC 60751 stipulates that the insulation resistance to ground of a platinum resistance thermometer at room temperature should be ≥100MΩ; otherwise, it is considered unqualified.

III. Junction Box and Connection Faults

Loose or Oxidized Terminals:During long-term operation, terminal blocks can easily loosen due to thermal expansion and contraction or vibration. Oxidation on the copper wire surface forms a high-resistivity layer, increasing contact resistance and causing three-wire compensation failure, resulting in higher temperature readings.

Accumulated Metal Shavings or Dust: Poor sealing of the junction box allows dust or metal shavings to enter, potentially causing localized short circuits or leakage, resulting in signal fluctuations or zeroing.

Waterproof and Dustproof Failure: Aging of the sealing ring, loose threads, or damage to the junction box lowers the IP protection rating, allowing rainwater or steam to directly corrode the internal circuitry.

IV. External Environment and Installation Factors

Electromagnetic Interference (EMI): When laying long-distance signal lines near inverters and motors, the lack of shielded twisted-pair cable or insufficient grounding of the shielding layer can easily introduce interference, causing temperature data fluctuations.

Insufficient Insertion Depth: If the probe is immersed in the measured medium to a depth less than 10 times the diameter of the protective tube (usually <100mm), it will lead to insufficient heat conduction, resulting in delayed or low measurement values.

Mechanical Damage: Forced impacts, bending during installation, or collisions during transportation may flatten the metal tube, deform the internal structure, and indirectly damage the platinum wire.

V. Quick Fault Identification and Handling Suggestions

Key Prevention Measures: Regularly conduct resistance and insulation tests, standardize installation procedures, use junction boxes with IP65 or higher ratings, and ensure correct shielding and grounding.