From Eq. (2), the percentage of signal reflected from the surface to the sensor through air is 99.94 %. As the
mismatch impedance between two materials is high, the percentage of signal reflected will be high. Whereas
through transmission mode, the energy received after transmitted is too low, which is 0.06 % (1-Pr) due to the air
gap between the sensor and pipe surface that reduced the energy transferred. Therefore, the reflection mode is
chosen as the ultrasonic sensing mode since the percentage of energy received is high and fit with the ultrasonic
sensing configuration.