What does the indication on the display screen that shows the far boundary of the material being tested signify?

The indication on the display screen that represents the far boundary of the material being tested signifies the back surface reflection. In ultrasonic testing (UT), an ultrasonic pulse is transmitted into the material, and when it encounters boundaries between different media or interfaces, some of the energy is reflected back to the transducer.

The back surface reflection specifically occurs when the ultrasonic sound wave reflects off the back surface of the test material, which is the farthest boundary being examined. This reflection is critical for determining the thickness of the material, as it provides a reference point for time-of-flight measurements based on the speed of sound in the material. Understanding the back surface reflection enables the technician to calculate the thickness and detect any internal flaws present in the material.

Other indications, such as the front surface reflection, relate to the transmission of the pulse at the entry point of the material, while the mid-point would represent something different entirely and is not typically correlated with a reflection indication in standard UT practices.