Test piece
A-scan
1
Drag the probe onto the plateA back wall echo will appear. The initial pulse is always on screen, probe or no probe.
2
Note the gain while the echo reads 20 % screen height
dB
3
Raise the gain until the echo reads about 80 %, then note the gainUse the + button. Watch the gate box on the screen.
dB
4
Lower the gain until the echo reads about 40 %, then note the gainUse the − button.
dB
What you just measured
| 20 % → 80 % | echo × 4 | |
| 80 % → 40 % | echo ÷ 2 |
Every 6 dB doubles or halves the echo height. Gain does not change the sound in the metal, only how big the echo is drawn on the screen.