The issue you raised regarding the repair of an open-circuit temperature sensor wire is indeed critical to the stability of your temperature measurement system and overall production safety. I understand the dilemma you face when a critical sensor fails-the urge to quickly restore operations, coupled with the worry that the repair might compromise accuracy. A prudent approach is essential to prevent a "minor fix" from escalating into a "major overhaul."
The method for repairing an open-circuit temperature sensor wire depends on the location of the break and the type of sensing element involved. The core principle is to "avoid compromising the integrity of the sensing element itself." External lead wires can be repaired via soldering and properly insulated; however, if the internal platinum or copper sensing wire itself is broken, replacement of the unit is generally the preferred option. Platinum resistance thermometers constructed with a mica skeleton can often be repaired with careful handling, whereas those built on ceramic or glass skeletons are typically unrecoverable once damaged.
I. Diagnosing the Break Location and Repair Feasibility
Methods for Locating the Break:
Remove the sensing element from its protective sheath and measure its resistance using a multimeter set to the R×1 range.
Resistance reading is ∞ (infinity): This indicates the presence of an open circuit.
Further Troubleshooting: If the resistance reading at the terminal block appears normal but there is no continuity in the external wiring, the break lies in the external compensation leads. If the resistance reading at the element's own terminals is ∞, the break lies within the internal lead wires or the sensing wire itself.
Repair Feasibility Based on Structural Type:
Mica-Skeleton Platinum RTDs: Repairable, as their structure allows for disassembly.
Ceramic/Glass-Skeleton Platinum RTDs: Not repairable; they must be replaced immediately upon an open-circuit failure.
Copper RTDs: Repairable if the enamel-coated wire has only partially detached; however, if the wire shows signs of severe corrosion, the unit should be scrapped.
II. Repair Procedures (Applicable to Repairable Types)
1. Repairing an Open External Lead Wire (Recommended as a Priority)
Applicable Scenario: The break occurs between the terminal block and the external connecting cable.
Procedure:Splice the wires using conductors of the exact same material and gauge (use a soldering iron for copper wires; use low-voltage arc welding for silver wires).
Insulate the splice joint using double-layer heat-shrink tubing: first, apply a smaller tube to seal the immediate joint, then apply a larger tube to cover the entire spliced area.
Ensure that the resistance values of all three lead wires are identical to prevent the introduction of measurement errors.
2. Repair of Open Internal Lead Wires (Mica Framework Only)
Procedure:
Open the terminal box and carefully remove the temperature-sensing element.
Rotate the clamping plate and the mica sheet by 90° to expose the break point in the platinum wire.
Use a 6–10V arc welder to repair the break point (the use of a soldering iron is strictly prohibited to prevent heat conduction from causing the platinum wire to anneal).
After welding, secure the repaired area using a mica sheet or high-temperature-resistant adhesive to prevent re-fracture caused by vibration.
3. Repair of Open Copper RTD Circuits
Repair Method:
Unwind the outer layer of wiring back to the break point, then use a soldering iron to join the wires.
Rewind the wires neatly using the double-winding method, and apply insulating varnish to the soldered joint.
Immerse the entire element in insulating varnish and then bake it dry to restore its insulation properties.
III. Key Inspection Items After Repair
Regardless of the repair method used, the following verifications must be performed:
0°C Resistance Value (R₀) Verification: Calibrate using an ice-water mixture or a standard resistance box to ensure compliance with Pt100 (100Ω) or Cu50 (50Ω) standards.
Insulation Resistance Test: Use a 500V megohmmeter to measure the insulation resistance between the lead wires and the protective tube; the reading should be ≥100 MΩ.
Stability Test: Observe the element at a constant temperature of 100°C for 30 minutes; the temperature drift should remain within the allowable error range.
Safety Note: Repairs involving the manipulation of internal components are recommended to be performed by professional instrumentation technicians. After repair, the device must undergo re-verification before being used in high-precision applications.
