10.2 Equipment & Techniques

Transducers and Search Units

The transducer (search unit or probe) is where electrical energy becomes sound and returning sound becomes voltage. It relies on the piezoelectric effect: a crystal changes shape when a voltage is applied and generates a voltage when it is mechanically stressed. Common active elements include PZT (lead zirconate titanate), natural quartz (rugged, low efficiency), lithium sulfate (good receiver, easily damaged by heat), and barium titanate. Behind the crystal a backing (damping) block shortens the pulse to improve near-surface resolution, and a wear plate or wedge protects the face and sets the beam angle. A pulser fires the element; the same or a companion element receives echoes that the instrument amplifies, digitizes, and displays.

Couplant, Contact, and Immersion

Because a thin air film reflects almost all the energy, a couplant must bridge the transducer and the surface.

• Contact testing applies couplant (water, gel, glycerin, oil, cellulose paste) by hand and slides the probe on the surface. It is portable and fast but coupling varies with pressure, tilt, and surface roughness.
• Immersion testing submerges the part and probe in water so a water path replaces the couplant. Coupling stays uniform, the probe never touches the part, and the beam angle is set by tilting the probe, ideal for automated tanks, bubblers, and squirter systems on production lines.

Pulse-Echo vs Through-Transmission

Two basic energy-path arrangements exist:

Pulse-echo is the workhorse: a single probe emits a pulse and listens for echoes, so it locates flaws in depth and needs only one accessible surface. Through-transmission places a transmitter on one face and a receiver on the opposite face; a discontinuity reduces the received signal, flagging a flaw but not its depth. A trade-off of single-element pulse-echo is the near-surface dead zone, the initial pulse and transducer ring-down mask echoes from very shallow flaws just under the entry surface.

Straight Beam vs Angle Beam

A straight-beam (normal) probe sends a longitudinal wave perpendicular to the surface, excellent for thickness gauging and for laminations that lie parallel to the surface, since they present a large reflecting face to the beam. An angle-beam probe mounts the element on a plastic wedge to launch a refracted shear wave (commonly 45°, 60°, or 70°) into the part. Angle beams reach the fusion faces and planar reflectors of welds that a straight beam would strike edge-on and miss. A planar crack oriented nearly parallel to the beam gives a weak echo, so procedures scan from several angles and directions to catch unfavorably oriented flaws.

Dual-Element and Delay-Line Probes

Some inspections use a dual-element (transmit-receive, or TR) probe with separate sending and receiving crystals angled slightly toward each other in one housing. Splitting the two functions eliminates the near-surface dead zone, so TR probes read thin walls and near-surface corrosion a single-element probe cannot resolve. A delay-line probe adds a short plastic column ahead of the crystal to move the initial pulse off the display, sharpening near-surface resolution and protecting the element on hot parts. Couplant selection matters too: thin oil or water suits smooth surfaces, heavier gel or paste clings to vertical and rough surfaces, and the couplant used to calibrate should match the one used to inspect so the sensitivity setting transfers correctly.

A-Scan, B-Scan, and C-Scan Displays

UT data are presented in three classic formats:

• A-scan plots amplitude (vertical) versus time or depth (horizontal) for one beam position. Each reflector appears as a peak; the operator judges depth from horizontal position and size from peak height. It is the fundamental display for manual UT.
• B-scan builds a cross-sectional side view, plotting depth against probe travel so flaws appear at their through-thickness location, useful for profiling remaining wall and corrosion.
• C-scan produces a plan (top-down) map, showing the projected position and extent of reflectors across the surface like an X-ray-style image, common in immersion and automated inspection.

Phased Array and TOFD (Survey Level)

Two advanced techniques appear at Basic-exam breadth. Phased array UT (PAUT) uses a probe of many small elements pulsed with tiny time delays so the composite beam can be electronically steered and focused without moving the probe, producing sector (S-scan) images and covering complex geometry quickly from one position. Time-of-Flight Diffraction (TOFD) uses a fixed pair of angled probes, one transmitting and one receiving, and measures the diffracted signals from the tips of a flaw rather than the mirror-like reflection. Because it times tip diffraction, TOFD sizes through-wall height accurately and is relatively insensitive to flaw orientation, making it valued for weld flaw sizing and in-service monitoring.

Common Mistakes to Avoid

• Do not confuse the displays: A-scan is amplitude versus depth, B-scan is a side cross-section, and C-scan is a top-down plan view.
• Through-transmission gives no flaw depth, only a signal drop, and it needs aligned access to both faces.
• Phased array steers and focuses the beam electronically from a fixed position, whereas TOFD sizes through-wall height from tip-diffracted signals rather than echo amplitude.