How to Determine if a Platinum Resistance Temperature Detector (PTD) with a Junction Box Mounted on a Movable Flange is Faulty

To determine if a PTD with a junction box mounted on a movable flange is faulty, it is necessary to focus on identifying the failure modes specific to the movable structure, based on general PTD testing methods. The core difference lies in the fact that movable flanges have a thermal expansion compensation mechanism; therefore, faults not only originate from electrical and sealing issues, but are also closely related to fatigue, contamination, and stress transmission caused by mechanical displacement.

 

I. Abnormal Electrical Performance: Basic Diagnostic Items (Consistent with Fixed Flanges)

Sudden Drop in Insulation Resistance: Use a 500V megohmmeter to measure the insulation resistance between the terminal block and the protective conduit:

≥100MΩ: Normal

<10MΩ: Warning, indicating junction box seal failure or internal moisture ingress

<1MΩ: Failure, immediate replacement required. Due to frequent displacement, the sealing ring of a movable flange wears down, making it more susceptible to moisture intrusion; this failure rate is higher than that of a fixed flange.

Resistance value deviates from the standard curve.

Measure the resistance of Pt100 at 20℃:

Standard value: 107.79Ω ±0.1Ω (Grade A)

Deviance > ±0.5Ω: Indicates platinum wire contamination, aging, or internal short circuit.

Resistance value jump: May be due to oxidation or poor contact of the moving connector contact.

Three-wire wiring verification.

Measure the resistance of the three leads:

R12 ≈ R13 ≈ 0.3–0.5Ω (lead resistance)

R23 ≈ 0Ω (should conduct)

If R12 ≠ R13 or R23 is open: Metal fatigue fracture of the lead occurred in the moving section.

 

II. Specific faults of moving structures: Key identification points

Table: Fault type, Typical manifestations, Detection method

Wear of moving connector seal ring: Liquid seepage and corrosion at the bottom of the junction box, accompanied by a continuous decrease in insulation resistance. Visually inspect the seal ring for hardening and cracking; measure the wear depth of the sealing surface after disassembly.

Polypotentiometer/Contact contamination: Temperature reading drift and irregular jumps, not caused by temperature changes: Open the junction box and check if the copper contacts are contaminated with black conductive carbon powder or lubricating grease (see [link]).

Lead wire fatigue fracture at the moving section: Displays "OL", multimeter shows continuity, but the protective tube has no cracks. Check the lead wire at the bend of the moving joint for metal streaks, cracks, or discoloration; observe with a magnifying glass if necessary.

Compensation mechanism jamming: Slow temperature response, higher readings after thermal cycling, and increased deviation from sensors in the same area. Manually push the moving flange joint gently; it should move freely without obstruction. If jammed, it indicates deformation or fouling of the internal guide structure.

Unique diagnostic rule: If the insulation resistance is normal and the resistance value is accurate, but the temperature reading is abnormal, prioritize checking the flexibility of the moving mechanism and the cleanliness of the contacts.

 

III. Comprehensive Judgment Process (Five-Step Method Available)

Visual Inspection: Check for liquid leakage in the junction box, aging of the sealing ring, and looseness of the moving joint.

Insulation Test: Measure the insulation resistance with a megohmmeter (<10MΩ indicates a warning).

Resistance Measurement: Measure the resistance at 20℃ using a bridge circuit to confirm if it is within the A/B tolerance level.

Moving Mechanism Test: Gently push the flange to confirm smooth displacement; check for fatigue marks at the bends in the lead wires.

Signal Verification: Compare with other sensors operating under the same conditions to eliminate system errors; if only this point is abnormal, it indicates a sensor failure.

Core Conclusion: Over 60% of failures in moving flange type platinum resistance thermometers stem from mechanical deterioration of the moving structure, not the temperature sensing element itself.

Prevention Key: Check the flexibility of the moving joint every 3 months, replace the sealing ring annually, and leave a ≥200mm redundant arc segment in the lead wires to avoid rigid constraints.