What is Ultrasound?
We are surrounded by invisible sound waves. In media, such as water or air, they are mechanical vibrations. Ultrasound is a sound with a frequency greater than the limit of human hearing (generally accepted as greater than 20kHz). Ultrasonic testing uses sound frequencies typically in the range of 500kHz to 20MHz.
Ultrasonic thickness gauges work by generating a sound pulse in a transducer probe. The time taken for the sound to travel through the material being tested and back is accurately measured.
Sound waves reflect from the boundaries between dissimilar materials, and the transducer not only creates the sound pulse, but also listens for the echo. The time between the emitted sound pulse and the resultant back wall echo can be measured using the following formula:
Distance = Speed x Time
Where:
Distance = thickness of substrate x 2
Speed = speed of sound in the material being measured
Time = time to travel to and from the back wall
The pulse is the burst of sound energy, like the bang of a drum. The echo is the same sound reflected back.
The sound waves are coupled into the test material using a couplant gel and travel through it until they encounter a back wall or another boundary.
The reflections then travel back to the transducer which converts the sound energy back into electrical energy. Effectively, the gauge listens for the echo from the opposite side.
Usually, the time interval is only a few millionths of a second. The gauge is pre-programmed with the speed of sound in the test material from which it can then calculate thickness.
S = V x t ÷ 2
Where:
S = substrate thickness
V = the velocity of sound in the substrate material
t = the measured round trip time (pulse to echo)
Note that the velocity of sound in the test material is an essential part of this calculation. Different materials transmit sound waves at different velocities, typically faster in hard materials and slower in soft materials.
It is always necessary to calibrate an ultrasonic thickness gauge to the speed of sound in the material being measured. The accuracy of the measurements can only be as good as this calibration. This is normally done with a reference standard whose thickness is precisely known.
Sound velocity decreases with temperature, therefore the reference standard should be at the same temperature as the piece to be measured.
Couplant: A water-based gel which is placed on the substrate, forming a layer and eliminating air between the transducer and substrate. This enables the sound to transmit into the substrate. High frequency sound will dissipate rapidly in air. Hence the term “the sound is coupled into the substrate”.