Regular Maintenance Schedule
Routine Calibration: According to JJG 229-2010 "Verification Regulations for Industrial Platinum and Copper Resistance Thermometers," industrial-grade flanged platinum resistance thermometers must undergo mandatory calibration every 12 months to ensure accuracy remains within the A-class ±(0.15 + 0.002|t|)℃ range.
Preventive Inspection: In high-corrosion, high-vibration, and continuous operation conditions (such as chemical reactors and nuclear power circuits), it is recommended to perform non-destructive inspections of the appearance, insulation, and grounding every 6 months.
Gasket Replacement: Flange sealing gaskets (graphite spiral wound gaskets, PTFE coated gaskets) are recommended to be replaced every 1-2 years or during each major overhaul. Even without signs of leakage, replacement should be performed to prevent the risk of micro-leakage due to aging.
Hygienic Applications: In pharmaceutical CIP/SIP systems, probes should be visually cleaned after each sterilization cycle. Seals and surface polishing status should be inspected quarterly to prevent protein residue or biofilm formation.
Core Maintenance Operating Procedures
Power Off and Isolation: Turn off the power to the relevant control system, isolate the process medium, and depressurize to zero to ensure safe operation.
Flange Disassembly: Loosen the bolts in a diagonal sequence to avoid flange deformation; remove the old gasket. The use of metal scrapers to clean the sealing surface is strictly prohibited; use a lint-free cloth dampened with anhydrous ethanol for gentle wiping.
Probe Cleaning:
Use a soft brush or lens tissue moistened with 95% anhydrous ethanol to gently wipe the outer surface of the protective tube to remove oil, salt spray, or crystallization.
Direct rinsing or high-pressure air blowing is prohibited to prevent moisture from entering the junction box.
Hygienic probes require the use of cleaning agents that comply with USP Class VI and require residue testing.
Junction Box Inspection:
Disassemble the junction box and check for terminal oxidation and loose wires.
Clean the terminal surface with an anhydrous ethanol cotton swab. Terminals or wires with severe oxidation need to be replaced.
Check if the sealing ring is hardened or cracked; it must be replaced with original factory fluororubber or silicone material.
Insulation and grounding test:
Use a 500V DC megohmmeter to measure the insulation resistance between the lead wire and the protective tube; the acceptable standard is ≥100 MΩ.
Use a low-resistance tester to measure the resistance between the flange and the grounding terminal; the acceptable standard is ≤0.03 Ω.
If the insulation resistance <10 MΩ, it indicates water ingress or deterioration of the insulation material, and the probe must be replaced.
Reinstallation and tightening:
Install a new gasket, ensuring it is centered and wrinkle-free.
Apply torque using the staged tightening method (10%→20%→60%→100%), with a recommended torque of 25–35 N·m for DN25 flanges.
Perform an additional tightening after heating and cooling to compensate for the loss of preload due to thermal expansion and contraction.
Common Fault Diagnosis and Troubleshooting
|
Fault Phenomenon |
Possible Cause |
Troubleshooting Measures |
|
Temperature reading drift |
Platinum wire oxidation, mechanical stress, moisture ingress |
Attempt to restore by annealing (300℃ for 2 hours); if ineffective, replace the probe |
|
Signal interruption/no output |
Lead wire breakage, water ingress into the junction box, shielding layer grounding failure |
Check cable continuity; disassemble the junction box, dry it, and replace the seals; re-ground at a single point |
|
Unstable/jumping readings |
Loose terminals, electromagnetic interference, decreased insulation |
Tighten terminals; check if the shielding layer is grounded at a single point; retest the insulation resistance |
|
Flange micro-leakage |
Gasket aging, flange deformation, insufficient bolt preload |
Replace the gasket; check the flange flatness; retighten according to specifications |
Calibration and Verification
Calibration equipment: Use a smart dry-block calibrator (such as ConST660) or a constant-temperature oil bath, together with a high-precision standard platinum resistance thermometer for multi-point calibration (0℃, 100℃, 200℃, 300℃).
Calibration Procedure:
Place the probe into the heat block, ensuring an insertion depth of ≥100mm;
Set up the calibration task, automatically collect resistance values and calculate temperature errors;
Output a calibration report, record correction values, and archive for future reference.
Calibration Cycle: Annual calibration is mandatory. If the environment is harsh or the accuracy is outside the tolerance, the cycle should be shortened to 6 months.
Spare Parts Replacement and Compliance
Probe Replacement: Can be replaced online without shutting down the system; recalibration is mandatory after replacement and the probe cannot be used directly.
Recommended Spare Parts:
Gaskets: Graphite spiral wound gaskets (PN1.6–PN40), PTFE coated gaskets (sanitary grade)
Sealing Rings: Fluororubber O-rings (resistant to 150℃)
Junction Box: IP67 explosion-proof cast aluminum box (Ex d IIC T4)
Compliance Basis:
Pharmaceuticals: Complies with GMP Annex 1 and 3A standards
Nuclear Power: Complies with IEEE 323 and ASME NQA-1
Chemical Industry: Complies with ISA-18.2 preventive maintenance framework
The maintenance of fixed flange-type platinum resistance probes is essentially a continuous safeguarding of industrial safety and data credibility. Every cleaning, every tightening, and every calibration is building the last line of defense for process stability, personnel safety, and compliance audits.
