Distance-Gain-Size (DGS)
DGS is an ultrasonic sizing technique that compares the echo amplitude from an unknown reflector to a family of curves representing equivalent flat-bottom-hole sizes at varying distances, calculated from probe characteristics.
Definition
The Distance-Gain-Size (DGS) method, also called AVG, uses probe-specific curves derived from acoustic theory rather than reference reflectors in a block. Each curve represents the echo amplitude from a flat-bottom hole of a specific equivalent diameter as a function of distance.
Technical Context
DGS provides an 'equivalent reflector size' for any indication, useful for sizing volumetric flaws. It is widely used in European weld inspection per ISO 11666 and in forging inspection.
What it represents
A calibration and sizing approach relating echo amplitude, sound path distance and equivalent reflector size through a family of curves derived for a given probe. It allows flaw response to be expressed as an equivalent disc-shaped reflector diameter without needing a block containing that exact reflector at that exact depth.
How it is derived
Probe-specific: the curves depend on element diameter, frequency and beam characteristics, so a DGS diagram belongs to a probe type, not to ultrasonics generally. Attenuation and transfer correction must be applied or equivalent sizes are systematically wrong.
Where it is defined
EN/ISO 16811 addresses sensitivity setting and range calibration including DGS; ISO 17640 permits it as an evaluation technique.
How it is misread
Quoting equivalent reflector size as flaw size in a report without saying so. It is a comparison to an idealised disc; a real planar flaw of the same equivalent size is typically larger and more significant.
Where Distance-Gain-Size fits in an inspection programme
A term is only useful when it connects to a decision. Distance-Gain-Size 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
- Distance-Amplitude Correction (DAC) — DAC is an ultrasonic calibration curve plotted on the A-scan display that shows expected echo amplitude from a reference reflector at varying depths, used to evaluate flaw size while compensating for beam spread and attenuation.
- Flat-Bottom Hole (FBH) — A flat-bottom hole is a reference reflector drilled into a calibration block to a precise depth with a flat bottom perpendicular to the beam axis, used as a known disc-shaped target for normal-beam sizing.
- Ultrasonic Testing (UT) — Ultrasonic Testing (UT) is an NDT method that uses high-frequency sound waves (typically 0.5–25 MHz) to detect internal flaws and measure thickness in metals, plastics, and composites by analyzing reflected echoes from a transducer.
More data terms
A-Scan · B-Scan · C-Scan · D-Scan · S-Scan · L-Scan · Time-Corrected Gain · Gating · Decibel · Indication
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.