Bobbin Coil
A bobbin coil is a differential or absolute eddy current probe that travels axially inside a tube, inspecting the entire tube circumference simultaneously for wall loss, pitting, and through-wall defects.
Definition
Bobbin probes are the standard tool for heat-exchanger tube inspection per ASME Section V Article 8 and EPRI guidelines. They detect circumferential and volumetric defects efficiently but are insensitive to axial cracks and to defect circumferential location.
What it is
A circumferential eddy current coil that fills the tube bore, sensing the average condition of the full circumference at each axial position as it is pulled through.
Correct use
Fast and effective for volumetric loss — general wall thinning and pitting — in non-ferromagnetic tubing, which is why it is the first pass in heat-exchanger inspection. Circumferentially oriented cracks are poorly detected because they barely disturb the circumferential current path, so array or rotating probes follow up.
Governing standards
ASME Section V Article 8 and its tubing appendices; ASTM E243 for seamless copper and copper-alloy tube examination.
The usual mistake
Concluding a bundle is sound from a bobbin pass alone. The technique averages around the circumference, so a short circumferential crack can sit inside an acceptable-looking trace.
Where Bobbin Coil fits in an inspection programme
A term is only useful when it connects to a decision. Bobbin Coil appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
- Encircling Coil — An encircling coil is an eddy current probe configuration in which a circular coil surrounds the outside of a tubular or wire product, inspecting the full circumference simultaneously as the product passes through.
- Rotating Probe — A rotating probe (also called RPC or motorized rotating pancake coil) is an eddy current probe with one or more pancake coils mounted on a rotating head, used inside tubes to detect circumferentially-localized defects that bobbin probes cannot characterize.
- Fill Factor — Fill factor is the ratio of the cross-sectional area of an encircling or bobbin coil to that of the tube it inspects, governing inspection sensitivity and the strength of geometric (e.g., support-plate) signals.
- Internal Rotary Inspection System (IRIS) — IRIS is an ultrasonic tube-inspection technique in which a water-driven rotating mirror inside the tube directs an ultrasonic beam radially outward, producing a high-resolution image of wall thickness and ID/OD pitting.
Further reading
heat exchanger tube inspection methods procedures
More equipment terms
Calibration Block · Transducer · Dual-Element Probe · Angle-Beam Probe · Normal-Beam Probe · Couplant · Reference Reflector · Side-Drilled Hole · Flat-Bottom Hole · Notch Reference
Where this comes up in practice
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Atlantis NDT provides NDT training and certification against ASNT SNT-TC-1A and ISO 9712, ASNT Level III consulting for written practices and procedure approval, inspection management software that holds qualification, calibration and procedure-revision evidence in recoverable form, and an asset integrity platform that binds inspection results to the asset they describe. Browse the full NDT glossary or ask a Level III directly.