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.
Definition
Fill factor (η) = (coil diameter / tube ID or OD)². High fill factor improves sensitivity but reduces clearance for tube variation. Standard heat-exchanger tubing ET uses fill factors of 0.75–0.85.
The principle
In tube eddy current testing, the ratio of probe diameter to tube inside diameter (squared, for the flux linkage). It sets how strongly the coil couples to the tube wall.
How it shows up in practice
Low fill factor means weak coupling and lost sensitivity; too tight risks the probe jamming in dents or deposits. Probes are chosen per tube batch, and heavily fouled bundles may need cleaning before the intended fill factor is achievable at all.
Where the codes address it
ASME Section V Article 8 tubing appendices state fill-factor expectations for bobbin examination.
Where practitioners get caught
Reporting bundle condition from a run where deposits forced an undersized probe. The reduced fill factor quietly degrades sensitivity, and the report should say so rather than presenting the run as standard.
Where Fill Factor fits in an inspection programme
A term is only useful when it connects to a decision. Fill Factor 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
- 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.
- 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.
- Eddy Current Testing (ET) — Eddy Current Testing (ET) is an electromagnetic NDT method that induces circulating currents in conductive materials using an AC coil; changes in coil impedance reveal surface and near-surface defects, conductivity variations, and coating thickness.
Further reading
heat exchanger tube inspection methods procedures
More physics terms
Back-Wall Echo · Beam Spread · Near-Field · Far-Field · Dead Zone · Shear Wave · Longitudinal Wave · Surface Wave · Lamb Wave · Mode Conversion
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.