In an immersion test of a piece of steel or aluminum, the water distance appears on the display as a fairly wide space between the initial pulse and the front surface reflection because of:

• A. Reduced velocity of sound in water as compared to test specimen
• B. Increased velocity of sound in water as compared to test specimen
• C. Temperature of the water
• D. All of the above

Answer: (A)

In immersion testing, the time between sending the pulse and receiving the reflection from the near surface is governed by how fast sound travels in the medium it’s moving through. The wave spends part of its path in water and part (for reflections from the front surface) essentially only in water again on the way back. Sound travels much slower in water than in steel or aluminum, so the water portion of the path takes more time to cover a given distance. That longer travel time in the water makes the spacing on the display between the initial pulse and the front-surface echo appear larger than you might expect if the medium were faster. The dominant effect here is the slower velocity in water compared to the solid, which is why the water distance shows as a fairly wide gap. Temperature can tweak velocities a bit, but it’s not the primary cause in this scenario.