Frequency Choice and Phase Reference for a Thin-Wall Titanium Tube
A chemical plant has titanium Grade 2 tubes (12.7 mm OD, 0.9 mm wall, conductivity ~2.4 %IACS, relative permeability 1.0). The client wants detection of 10 % wall loss on both ID and OD surfaces. Your technician proposes a 400 kHz absolute bobbin coil with a fill factor of 0.85 and a reference tube with ID and OD EDM notches at 10 %, 20 %, and 40 % of wall. The technician reports phase separation of about 30 degrees between the ID and OD 10 % notches and asks whether the setup is acceptable.
1. Is 30 degrees of phase separation acceptable for 10 % wall-loss detection and sizing? What would you change?
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30 degrees is marginal to inadequate for reliable ID/OD discrimination and depth sizing; a practical target is at least 40-50 degrees. At 400 kHz on 0.9 mm titanium, the standard depth of penetration is roughly 0.5-0.7 mm, so the wall is near the effective depth and phase separation compresses. Lower the frequency to roughly 100-200 kHz to increase penetration and phase separation, and re-verify on the reference tube. Also confirm the fill factor (0.85 is good) and that the coil is matched to the tube ID. The acceptance decision should be based on the demonstrated phase separation on the reference standard, not on the nominal frequency.
2. The client asks whether a single frequency is enough. How do you answer?
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A single frequency can be sufficient if the reference standard demonstrates adequate phase separation and signal-to-noise for the 10 % notch on both surfaces. If not, use a dual-frequency or mix approach: one frequency optimized for OD penetration and one for ID, mixed to suppress noise and lift-off. The Level III should specify the demonstration criteria (phase separation ≥40 degrees, 10 % notch at ≥3:1 SNR) and state that the technique is qualified only when those criteria are met on the reference standard.
3. What documentation must accompany the final report for this inspection?
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The report must include: the probe and frequency used, the fill factor, the reference standard description (material, notch depths, ID/OD), the demonstrated phase separation and SNR, the detection threshold (10 % wall loss), the coverage and any masked regions (tube sheet, supports), and the disposition of every indication. It must also state the limitations: detection is probabilistic, and sizing accuracy depends on the demonstrated phase separation. This lets the client make an informed repair decision.