Amplitude — Definition & NDT Use

As a physical principle, Amplitude dictates how energy interacts with the test piece — and that interaction is what an NDT instrument reads out as a signal, image, or measurement. 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. The magnetising current creates a field that runs continuous through the part; at a discontinuity the lines of flux squeeze around the gap and break the surface as a leakage field, where dry powder or wet-suspension particles cluster and outline the flaw to the inspector's eye. Every parameter on the instrument front panel — frequency, gain, range, gate — is ultimately a physical lever on the same underlying interaction, which is why understanding the physics is what turns a button-pusher into a Level II.

Inspectors apply the principle of Amplitude every time they pick a frequency, gain, or probe — even when they are not consciously thinking of the underlying physics.

Operators sometimes treat amplitude as a black-box instrument behaviour rather than a physical lever; the knock-on effect is that they do not realise when a knob change has invalidated the calibration.