A-Scan (Amplitude-Scan) — Definition & NDT Use
A one-dimensional ultrasonic display showing signal amplitude (vertical axis) versus time (horizontal axis). In A-scan presentation, the transmitted pulse appears as an initial spike, internal reflections appear as smaller spikes, and the back-wall echo appears at the far right. The technician measures the distance from the initial pulse to internal reflections to determine flaw depth. A-scan is the most basic UT presentation mode and requires significant operator experience to interpret correctly.
From the inspector's bench, A-Scan is run as a defined sequence: equipment verification on a known reference, scan setup against the procedure, scanning the part, and writing the indications into the report. Procedure writing, inspector qualification, and the reference block establish the chain that lets a remote engineer trust an indication called a kilometre away from the office.
A-Scan is selected when the failure mode the engineer cares about — surface crack, internal void, wall loss, lack of fusion — lines up with what the technique is physically capable of detecting.
- Etymology / Origin
- Coined in WWII RDF/sonar work; the 'A' simply labels the first display style standardised before B and C variants.
- Formula
- Time-of-flight t = 2d/c (pulse-echo); display amplitude A is plotted vs t along the horizontal axis.
- Units
- Amplitude in % FSH (full-screen height) or dB; horizontal axis in microseconds or millimetres of metal path.
- Typical Range
- Range 0–500 mm steel-equivalent in pulse-echo contact UT; gain typically 30–80 dB.
- Measured / Produced By
- Single-channel UT flaw detector (Olympus Epoch, GE USM Go, Sonatest Masterscan).
- Code References
- ASME Section V Article 4 T-462 (calibration); ASTM E317 (instrument performance)
- Worked Example
- A back-wall echo at 13.5 µs in steel (c=5920 m/s) corresponds to a thickness of (13.5e-6 × 5920)/2 = 39.96 mm.
ASME Section V Article 4
Ultrasonic examination methods for welds and components.
ASTM E114 / E164 / E2375
ASTM straight-beam, contact, and wrought-product UT practices.
ISO 16810 / ISO 16811
General principles and sensitivity setting for industrial UT.
The most expensive mistake with A-Scan is treating it as a yes/no test rather than a characterisation — an indication called without a sizing strategy forces a repair where a fitness-for-service review might have left the part in service.
What does "A-Scan" mean in NDT?
A one-dimensional ultrasonic display showing signal amplitude (vertical axis) versus time (horizontal axis). In A-scan presentation, the transmitted pulse appears as an initial spike, internal reflections appear as smaller spikes, and the back-wall echo appears at the far right
Which standards govern the use of A-Scan?
A-Scan is most often referenced under ASME Section V together with the relevant ASTM practice or the matching ISO standard for the method; the contract or purchase order will name the controlling document and edition for any specific job.
What other NDT concepts should I read alongside A-Scan?
The most directly related entries in this glossary are "b scan", "c scan", "ultrasonic testing"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.