Gain (Amplification) — Definition & NDT Use
The electronic amplification applied to ultrasonic signals to increase their amplitude for better visibility on the display. Gain is typically expressed in decibels and adjusted to bring the back-wall echo to a reference level (80% or 6dB below full screen height). Proper gain setting is critical to avoid missing small flaws (insufficient gain) or false indications (excessive gain). Different materials and thicknesses require different gain settings for consistent inspection results.
From the inspector's bench, Gain 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. Gain is set in decibels referenced to a known reflector — a side-drilled hole, flat-bottom hole, or notch on a reference block — so two operators on two instruments can produce comparable amplitudes from the same indication. 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.
Gain 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
- Engineering use of 'gain' for amplifier output predates electronics (mechanical advantage); standardised as dB gain in radio (1930s).
- Formula
- Gain_dB = 20 × log10(A_out / A_in); reference gain set against a calibration reflector.
- Units
- dB; instrument gain range typically 0–110 dB in 0.1 or 0.5 dB steps.
- Typical Range
- Reference gain 30–60 dB on most steel inspections; scanning gain set +6 dB above reference.
- Measured / Produced By
- Read from the flaw-detector gain knob; verified by linearity test against a step-amplitude reflector.
- Code References
- ASME Section V T-461.2 (gain linearity ±5%); ASTM E317 (vertical linearity)
- Worked Example
- Reference reflector set to 80% FSH at 42 dB; scanning gain = 42 + 6 = 48 dB; an indication at 80% FSH at scanning gain reads −6 dB to reference.
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 Gain 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 "Gain" mean in NDT?
The electronic amplification applied to ultrasonic signals to increase their amplitude for better visibility on the display. Gain is typically expressed in decibels and adjusted to bring the back-wall echo to a reference level (80% or 6dB below full screen height)
Which standards govern the use of Gain?
Gain 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 Gain?
The most directly related entries in this glossary are "decibel", "amplitude", "sensitivity"; reading those together gives you the surrounding vocabulary used in inspection reports and procedures.