What type of instrument is used for ultrasound of welds? And what frequency does it operate on?

• A. Through-transmission device, 0.5-2 MHz
• B. Pulse-echo type, 1-5 MHz
• C. Echo-sounder with 10-20 kHz
• D. Magnetic particle tester, 2-3 Hz

Answer: (B)

Ultrasonic inspection of welds is done with a pulse-echo type instrument, where a transducer sends a short ultrasonic pulse into the weld and listens for echoes from flaws, the toe, and the back wall. This setup lets you determine defect location by measuring the travel time of the echoes, and it works well with access from one side as many welds are examined in place. The typical operating frequency for steel welds is about 1 to 5 MHz; higher frequencies give better resolution to detect small flaws but don’t penetrate as deeply due to attenuation, while lower frequencies penetrate more but with less detail. Through-transmission would require access to the opposite side with separate transmitter and receiver, which is less practical for welds in many structures. An echo-sounder at 10–20 kHz is far too low in frequency for reliable weld inspection in steel, and magnetic particle testing is a different, non-ultrasonic method entirely.