What are the common faults of Teflon-sheathed corrosion-resistant platinum resistance thermometers

Teflon (PTFE)-sheathed corrosion-resistant platinum resistance thermometers are widely used in corrosive environments such as chemical, pharmaceutical, and food industries. Their faults mostly stem from sheath integrity failure, insulation degradation, and mechanical stress damage, rather than the platinum core itself.

 

I. Mechanical Damage and Penetration Failure of the Sheath

Sheath Cracks or Wear: Under vibration, installation stress, or high-pressure flushing conditions, the PTFE sheath is prone to micro-cracks or scratches, allowing corrosive media (such as acids, alkalis, and organic solvents) to penetrate the interior, directly contacting the platinum wire or leads, initiating electrochemical corrosion, and causing abnormally high or drifting resistance values.

Sealing Failure: Aging, loose, or thermal expansion and contraction of the gasket at threaded or flanged connections can cause the gap to widen, allowing moisture or corrosive vapors to intrude, resulting in oxidation of the terminals or internal short circuits, manifesting as output signal jumps, zeroing, or infinity.

High-Temperature Softening: Prolonged operation above 250℃ (exceeding the upper limit of PTFE stability) will cause the sheath to soften and deform, losing its mechanical protection capabilities, accelerating the aging of internal components, and even leading to probe bending and breakage.

Typical Scenarios: High-pressure water jet impact during chemical reactor cleaning and repeated steam circulation in pharmaceutical sterilizers are high-risk conditions for sheath penetration.

 

II. Insulation Resistance Decrease and Electrical Faults

Insulation Deterioration: Although PTFE has excellent insulation properties, under long-term thermal cycling, humid, or polluted environments, its surface may absorb moisture or oil, causing the insulation resistance to drop from >100MΩ to <1MΩ, leading to signal interference or measurement errors.

Lead Wire Oxidation: If the lead wire connectors of three-wire or four-wire platinum resistance thermometers are not silver- or gold-plated, they are prone to oxidation in high-temperature and high-humidity environments, increasing contact resistance, causing three-wire compensation failure, and resulting in higher temperature readings.

Grounding Fault: Damage to the sheath can cause the metal probe to become accidentally conductive with the equipment casing, forming a grounding loop. This short-circuits the measurement signal to ground potential, resulting in abnormally low instrument readings or no readings.

Standard Basis: IEC 60751:2022 clearly requires that the insulation resistance of corrosion-resistant platinum resistance thermometers at 25℃ should be ≥100MΩ; otherwise, it is considered unqualified.

 

III. Internal Component Damage and Structural Failure

Platinum Wire Breakage: Although the PTFE sheath has good flexibility, excessive bending during installation (radius of curvature <50mm) or continuous vibration can easily cause fatigue fracture of the extremely fine platinum wire (Φ0.03–0.07mm), resulting in infinite resistance.

Solder Joint Desoldering: The solder joints between the platinum wire and the leads may loosen under repeated thermal stress, especially in systems with frequent start-stop cycles, leading to poor contact and unstable temperature readings.

Moisture in the Filler: If the magnesium oxide powder filling the sheath becomes damp, it will reduce insulation performance and accelerate metal corrosion. This is common in sensors stored for extended periods in unsealed packaging.

 

IV. Fault Diagnosis and Prevention Recommendations

Table: Fault Phenomenon	Possible Causes	Quick Diagnosis Method
High Temperature Reading	Lead oxidation, increased contact resistance	Measure the resistance between leads with a multimeter and compare with the nominal value (e.g., 100Ω for Pt100 at 0℃)
Low Temperature Reading or Zero	Sheath damage, short circuit, grounding	Measure the probe's insulation resistance to ground; <1MΩ is abnormal
Signal Jumps, Glitches	Vibration causing poor contact, electromagnetic interference	Check the tightness of the terminals and whether the shielding wire is grounded at a single point
Infinite Resistance	Broken platinum wire, open circuit	Directly measure the two ends of the platinum resistance with a multimeter; the normal value should be 100–138Ω (0–100℃)

Precautions:

Avoid sharp bends during installation; bend radius ≥ 50mm; Regularly check the sheath integrity using a 12kV spark leak detector; Use PFA (perfluoroalkoxy) instead of PTFE, which has a temperature resistance up to 260℃ and superior impermeability; In corrosive environments, prioritize products with a double-sheath structure (stainless steel + PTFE).