How to Perform a Non-Destructive Thickness Measurement of a PFA Heater Sheath Using Eddy Current Testing?

Eddy current testing (ECT) provides a non-destructive way to determine the remaining PFA wall thickness on installed heaters without removing them from the tank. The method works because the metal core (Incoloy, titanium or stainless steel) is conductive and the PFA sheath is non-conductive. An eddy current probe on the PFA surface induces currents in the metal core, where the phase lag of the return signal is proportional to the distance between the probe and the metal, i.e. the PFA wall thickness. Calibration against known thickness standards provides an accuracy of ±0.05-0.10 mm for PFA thicknesses in the range of 0.5-3.0 mm. ECT can detect erosion, localised wear or chemical damage causing thinning and so allow predictive replacement before perforation. The process is fast (seconds per point) and can be done by qualified technicians without draining the tanks (probe inserted by a sight port or by a wet-contact technique).

Working principle
An eddy current probe creates an alternating magnetic field (usual frequency 100 kHz–2 MHz). This field generates eddy currents in the metal core. The eddy currents generate a secondary magnetic field that opposes the primary magnetic field. The probe detects the change in impedance. The phase angle difference between the transmitted and received signal depends on the lift-off distance, which is the distance between the probe and the metal. For non-magnetic metals (Incoloy, titanium, stainless steel) the lift-off effect is linear with distance for gaps from 0 to 3 mm. The obtained phase angle can be directly converted to PFA thickness by calibration with PFA shims of known thickness.

The method is based on the use of wet PFA (the liquid is non-conductive and non-magnetic, so it does not interfere). The probe can be used on a wet heating surface when in service however greatest accuracy is obtained on a clean dry surface.

Calibration Process
Obtain a calibration block. A piece of the same metal core (Incoloy, etc.) with a flat surface. Or utilise the chilly end of the heater (thickness is known from manufacture).

Put PFA shims of defined thickness (0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm) between the probe and the metal block.

Record the phase angle for each thickness. Plot phase vs thickness (should be linear).

Check linearity R2 > 0.99 For titanium cores the linear range is 0.5–2.5 mm. For Incoloy, 0.5-3.0 mm.

Make a lookup table or linear equation: Thickness = m x (phase) + b

Measurement Procedure
Step Action
1 Clean PFA surface at measurement location (remove scale, biofilm) - mild wipe is OK.
2. Use a small layer of water or gel couplant (to guarantee acoustic contact - ECT does not need couplant, but regular contact enhances repeatability).
3 Place probe normal to the heater surface. For curved heaters (typically 25 mm OD) use a contoured probe tip or take many readings around the circumference and average.
4 Record the phase angle (3-5 readings).
5 Thickness from calibration curve.
6 Mark measurement points with permanent marker to monitor changes over time.
Limitations and Accuracy
Parameter Performance Comments
Thickness range 0.3–3.0 mmOutside of this range, non-linear
Accuracy ±0.05–0.10 mm Calibration required
Repeatability ±0.03mmOn the same spot
Minimum spot size 5 mm diameter Edge effects on little patches
Metal core effect Need to calibrate each alloy.Incoloy, Ti, SS have differing conductivity
Temperature impact Drift > 60°CWhen it is at room temperature (off and cooled down)
Curvature effect Error up to 0.1 mm for 25 mm outside diameterUse curved probe or average 3 position
Examples of Applications
Service Environment Predicted Thinning RateInspection Frequency Action when thickness <
Clean water <0.05 mm/year Yearly 1.0 (replace)
Abrasive slurry (silica, 2 m/s) 0.2–0.5 mm/year Quarterly 1.2 mm (planned replace)
Acidic chemical (30 % HCl, 80 °C) 0.05–0.15 mm/year (permeation not erosion)Semi-annually 1.0 mm (keep an eye)
High temperature (>150°C) | Creep thinning | Every 6 months | 1.5 mm (risk of rupture)
Ultrasonic cleaning tank 0.1-0.3 mm/year erosion Quarterly 1.0 mm
Field Example
PFA heaters in silica sand service (2.5 m/s, 30% solids) in a mining slurry loop. "We used to replace heaters every 12 months, as a preventive measure. We would find some heaters still thick (1.8 mm) and others nearly perforated (0.4 mm). Measurements of ECT thickness were done every 3 months. After 9 months one heater was 0.7 mm (from 2.0 mm new) others were 1.4-1.6 mm. Replaced thin heating element. The others went on to 18 months. The plant saved $5,000/year in unneeded replacements and averted catastrophic failure from the thin heater .

Equipment Recommendations
Appropriate portable eddy current thickness gauges on metal (PFA-on-metal) include:

Olympus (Evident) 45MG with lift-off calibration

ECP10 Fischer FMP20 Probe

Elcometer N800 (set for non-conductive coating on metal)

Cost: $3000-6000 for a simple probe equipped gauge. Training: 1-2 days The investment pays itself in 1-2 years in decreased unneeded heater replacements and prevented failures.

Conclusion: ECT for Thickness Gauging and Calibration
The residual PFA wall thickness on installed heaters can be determined non-destructively with an accuracy of $\pm 0.05–0.10$ mm by means of eddy current testing. The method is non-destructive to water and scale, takes seconds per spot and requires no tank draining. Calibrate against known thickness standards, compensate for curvature, and do measurements on a quarterly or annual basis. Use ECT, replace heaters based on actual remaining wall thickness, not calendar time. This extends heater life where practical and avoids surprise failures where erosion is severe. For abrasive, corrosive or high temperature usage, ECT is the best value predictive maintenance technology. Measure Thickness Predict Replacement Save Money The probe knows how much PFA is there. Don't assume. Measure.