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Ultrasonic Thickness Gauge Tutorial

Ultrasonic Thickness Gauging Glossary

Accuracy:The agreement between the measured value and the true value of a parameter such as thickness.

Acoustic Impedance:A material property defined as sound velocity multiplied by density. The amount of sound reflection at a boundary between two materials is derived from the ratio of acoustic impedances.

Amplitude:In wave motion, the maximum displacement of material particles. In electronics, the magnitude of a signal, normally expressed as a positive or negative voltage.

Attenuation:The loss in acoustic energy which occurs between any two points in a sound path.

Backwall Echo:The echo received from the side of the test specimen opposite the side to which the transducer is coupled. The timing to this echo corresponds to the thickness of the specimen at that point.

Contact Transducer:A transducer designed to be used in direct contact with the test material.

Couplant:A material, usually a liquid or gel, used between the transducer and test piece to eliminate air and facilitate the coupling of sound energy.

Delay Line:A material (usually a plastic) placed in front of a transducer to create a time delay between the excitation pulse and the echo from the front surface of the test piece.

Delay Line Transducer:A transducer incorporating a delay line.

Dual Element Transducer:A transducer with separate transmitting and receiving elements, commonly used in corrosion measurements.

Excitation Pulse:A brief electrical pulse applied to a piezoelectric element in an ultrasonic transducer, causing it to vibrate and generate sound waves. Also known informally as the "main bang".

Frequency:The number of cycles of vibration experienced by an oscillating body in a designated period of time (normally one second). Electrically, the rate at which a periodic signal such as a sine wave repeats during a designated period of time.

Gain:In an ultrasonic gauge, the increase in signal strength produced by an amplifier, usually expressed as the ratio of output power to input power in decibels.

Immersion Testing:A test method in which sound energy is coupled between the transducer and test piece through a water column or water bath.

Immersion Transducer:A transducer designed to be used for immersion testing.

Interface Echo:The echo reflected from the front surface of a test specimen, seen when using delay line or immersion transducers.

Longitudinal Wave:The wave propagation mode normally used for ultrasonic gauging, characterized by particle motion parallel to the direction of wave travel. Audible sound waves are also longitudinal waves.

Measurement Mode:The method of timing echoes in thickness gauging, commonly differentiated as Modes 1, 2, and 3.

Phase Reversal:An inversion of the positive and negative peaks of a wave.

Range:The interval between the maximum and minimum thicknesses that can be measured in a material with a given transducer and instrument setup.

Resolution:In thickness gauging, the degree to which slightly different thicknesses or time intervals can be distinguished.

Sound Velocity:The speed at which a sound wave travels through a given material.

Sound Wave:A coherent pattern of mechanical vibrations in a solid, liquid, or gaseous medium.

Transducer:A device that transforms one form of energy into another. In ultrasonic testing this normally means converting electrical energy into mechanical energy or vice versa.

Ultrasonic:Referring to sound waves at frequencies above the limit of human hearing, generally defined as 20,000 cycles per second (20 KHz).

Waveform:A graphic presentation of energy levels in a wave train, plotted as amplitude versus time.

Zero Offset:A correction factor representing the difference between a measured time interval and the actual sound transit time in a test specimen, typically accounting for switching delays, cable delays, and wearplate and couplant thickness

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