Bobbin Probes, Array Probes, and Rotating Probes: Choosing the Right Tool for Tubing Inspections

As eddy current testing (ECT) continues to evolve, especially in sectors like power generation, chemical processing, and aerospace, array probes are gaining momentum. Yet the familiar bobbin and rotating probes remain critical parts of any complete inspection strategy.

Understanding the similarities, differences, advantages, and limitations of each technique is crucial for ensuring optimal detection, sizing, and characterization of flaws.

Here’s a breakdown of these three essential ECT techniques for tubing inspections:

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Bobbin Coil Testing: The Workhorse of Tubing Inspection

Overview:

Bobbin probes contain a pair of coils wound circumferentially around the probe body and provide an averaged condition of the entire tube circumference at each axial probe position. Operated typically in differential and absolute configurations, bobbin probes detect flaws by monitoring impedance changes as they travel through the tube bore.  Unlike rotating probes, bobbin coils do not spin — they are inserted and retracted axially while data is recorded during retraction.

Similarities to Others:

• Like array probes, bobbin probes offer rapid, volumetric coverage of the tube.

Pros:

• Speed: Fastest data acquisition of the three methods.

• Simplicity: Easy to deploy, operate, and relatively easy to analyze.

• Baseline Data: Excellent for degradation trending and establishing inspection baselines.

• Established Standards: Well-documented techniques in ASME, EPRI, and other industry guidelines.

Cons:

• Directional Sensitivity: Limited ability to detect small circumferential cracks.

• Resolution Limits: Inadequate for detailed sizing or complex flaw characterization.

• Signal Distortion: Discontinuities near complex geometries, like tube support plates and dents, can complicate interpretation.

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Rotating Probe Coil (RPC) Testing: The Precision Specialist

Overview:

Rotating probes use one or more surface-riding coils mounted on a rotating head. As the probe moves axially through the tube, the coil spins helically along the inner surface, capturing high-resolution data.

Similarities to Others:

• Like bobbin and array probes, rotating probes detect changes in conductivity and permeability.

• Like array probes, they offer high spatial resolution — achieved mechanically through rotation rather than electronically.

Pros:

• Crack Sensitivity: Superior detection of tight cracks (e.g., IGA, SCC) in any orientation.

• Detailed Characterization: Exceptional for determining flaw orientation with 360-degree resolution,  crack sizing, and flaw profiling

• Complementary Use: Ideal for confirming and characterizing indications found during bobbin or array inspections.

Cons:

• Slow Acquisition: Data collection is significantly slower compared to bobbin or array probes.

• Higher Cost: Greater equipment cost and more maintenance due to mechanical complexity.

• Fragility: Susceptible to damage from poor snorkel runs, debris, bore obstructions, or even EDM notches in calibration standards.

• Mechanical Challenges: Managing the mechanical rotation across tube geometries can introduce risks of probe wear or failure.

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Array Probe Testing: The Emerging Contender

Overview:

Array probes employ multiple sensing coils arranged circumferentially or in a grid, electronically switching between coil pairs to simulate rotation or multidirectional scanning — without any mechanical rotation or indexing.

Similarities to Others:

• Like rotating probes, arrays offer directional flaw sensitivity with 360° coverage.

• Like bobbin probes, arrays enable rapid, full-length scans — though typically not as fast as bobbin inspections.

Pros:

• Faster Than RPC: Acquisition speed significantly outpaces rotating probe exams.

• Good Directional Sensitivity: Capable of detecting both axial and circumferential flaws.

• Enhanced Resolution: Having 360-degree resolution, provides better flaw disposition capabilities than bobbin, usually reducing the number of required rotating coil exams.

• Flexibility: Newer flexible array designs can adapt to moderate tube bending and complex contours.

Cons:

• Cost: Higher upfront equipment and probe costs (and longer probe-order lead time) compared to bobbin.

• Data Complexity: Analysis demands advanced training due to more complex signal combinations.

• Standardization Lag: Industry standards and calibration practices are still catching up compared to bobbin and RPC methods.

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Which Should You Choose?

The right choice depends on your inspection goals, budget, timeline, and the criticality of flaw detection:

Inspection Objective

Recommended Method

Fast, cost-effective full-length screening:

Bobbin Coil

High-sensitivity detection of tight cracks in any orientation:

Rotating Probe Coil

Sampling bulk locations that require more information than the bobbin coil can provide:

Array Probe

Best flaw resolution, shape characterization, and flaw profiling:

Rotating Probe Coil

Baseline trending and routine inspections:

Bobbin Coil

Critical inspections requiring layered analysis:

Combination: Bobbin + Array + RPC

Note: Bobbin and Array coils are often delivered on the same probe body, which reduces the need for additonal exams.

Final Thought

Choosing the right eddy current probe isn’t about finding the “best” tool — it’s about selecting the right tool for the specific inspection challenge.

Whether performing a fast baseline survey, a high-resolution crack detection, or a hybrid inspection using arrays, understanding the strengths and limitations of each method is the key to a successful inspection program.

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