Remote Field Eddy Current Frequency Planning
Why Frequency Matters
Remote field eddy current testing helps inspect conductive tubes from inside. It is useful when the wall is thick, magnetic, or coated. The exciter coil sends an alternating magnetic field through the tube wall. The field travels along the outside region. It then returns through the wall to the receiver coil. Because the signal crosses the wall twice, frequency choice matters.
Penetration and Signal Strength
A low frequency penetrates deeper. It gives stronger remote field coupling. It also lowers phase change for small defects. A high frequency gives more wall interaction. It may improve sensitivity to metal loss. Yet it can reduce signal strength quickly. The best setting balances penetration, attenuation, phase, spacing, and instrument range.
Skin Depth Basis
This calculator starts with skin depth. Skin depth is the depth where the field falls to about thirty seven percent. It depends on frequency, conductivity, and magnetic permeability. Higher conductivity lowers skin depth. Higher relative permeability also lowers skin depth. Thick ferromagnetic tubes therefore often need lower frequencies.
Target Wall Ratio
The target wall ratio is a practical tuning value. It compares wall thickness with skin depth. A ratio near one gives deep penetration. A larger ratio raises phase shift and attenuation. For remote field work, the chosen ratio should be tested against calibration standards. Real probes, fill factor, support plates, and defects change the final response.
Probe Spacing
Probe spacing also affects the remote field condition. Many tube inspections place receiver coils about two to three tube diameters from the exciter. This distance helps the direct field decay before measurement. The calculator estimates that spacing from the outside diameter and a selected multiplier.
Field Verification
Use the output as a planning guide. Start with material data from the tube specification. Enter measured wall thickness when possible. Compare the recommended frequency with an available instrument setting. Then verify the response using reference tubes with known flaws. Adjust frequency until signal level, phase separation, and noise are acceptable.
Practical Limits
The attenuation value is only an estimate. It uses a simple exponential model for double wall travel. It does not replace calibration. It helps compare candidate settings before field trials. The phase estimate gives another useful reference. A larger phase shift can help sizing, but it may also make signals harder to interpret. Record settings after every adjustment.