The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves. Whereby the waves are not only reflected at the interfaces but also by internal flaws (material separations, inclusions, discontinuities, etc).
It is said to "Pulse-Echo method". A pulse-echo ultrasonic thickness gauge determines the thickness of a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through the thickness of the material, reflect from the back or inside surface, and be returned to the transducer.
In most applications this time interval is only a few microseconds or less.
The measured two-way transit time is divided by two to account for the down-and-back travel path, and then multiplied by the velocity of sound in the test material.
The result is expressed in the well-known relationship :

S=CT/2 (ideal case) ---------- Formula 1
Where S = sound transit path [mm] of the work piece
C : the velocity [km/s] of sound waves in the material
T : the transit time of flight [μs] being measured by Internal Counter


Figure 1. Block diagram : Pulse Echo Method
Additionally, in actual practice, a Probe Zero is usually subtracted from the measured time interval to account for certain fixed electronic and mechanical delays. In the common case of measurements involving direct contact transducers, the Probe Zero compensates for the transit time of the sound pulse through the transducer's wearplate and the couplant layer, as well as any electronic switching time or cable delays. This Probe Zero is set as part of instrument calibration procedures and is necessary for highest accuracy and linearity.

S=C(T-T0)/2 (real case) ---------- Formula 2
Where T0 : the time of initial pulse [μs], viz. probe zero

In case user already know the sound velocity of the material being measured
You can measure the thickness of work piece by inputting known velocity value directly into Formula 2.

In case user don't know the sound velocity of the material being measured
You must make use of a test piece of the same material as the work piece whose dimensions are known in order to measure accurate thickness. In formula 2, you already knew values as sound transit path, time of flight, time of initial pulse. Therefore, you can calculate the sound velocity of test piece. This data is used to calculate the work piece you want to measure.

If you don't know both velocity and thickness of work piece, you can't measure it.
In this case, however, you can select the similar value as work piece in the sound velocity table for the various materials.


Figure 2. The system architecture of pulse-echo ultrasonic thickness gauge
Figure 2 represents a generalized block diagram of a modern microprocessor-controlled ultrasonic gauge. The pulser, under control of the microprocessor, provides a unidirectional broadband voltage impulse to a heavily damped broadband ultrasonic transducer.
The broadband ultrasonic pulse generated by the transducer is coupled into the test piece, normally with the aid of a liquid coupling medium. Returning echoes are received by the transducer and converted back into electrical pulses, which in turn are fed to the receiver Automatic Gain Control (AGC) amplifier. The microprocessor-based control and timing logic circuits both synchronize the pulser and select the appropriate echo signals to be used for time interval measurement.
If echoes are not detected during a given measurement period, the gauge will shut down to save power until a new measurement cycle is required. If echoes are detected, the timing circuit will precisely measure an interval appropriate for the selected measurement mode, and then repeat this process a number of times to obtain a stable, averaged reading.
The microprocessor then uses this time interval measurement, along with the sound velocity and Probe Zero information stored in the Random Access Memory (RAM), to calculate thickness.
This thickness measurement is then displayed on the Liquid Crystal Display (LCD) and updated
at a selected rate (viz. Pulse Repetition Frequency).
Many modern gauges incorporate an internal data logger and are capable of storing several thousand thickness measurements along with identification codes and setup information in RAM. These stored readings may be recalled to the gauge's display or uploaded to a printer or computer for further analysis.