Technological advancements in distance gain sizing result in greater inspection accuracy and reliability.
For more than 50 years, Distance Gain Size (DGS) inspections have been performed for the code compliant flaw detection and sizing of welds across oil and gas, power generation and general fabrication applications. They are also used to inspect tubes, pipes, pressure vessels, containers and pumps. Introduced by Krautkrämer in the late 1950s, the DGS method originally employed straight-beam probes with circular transducers (or crystals), resulting in a rotationally symmetric sound field. The technique, which quickly became a widely accepted inspection method across the globe and found entry into a variety of industry specific norms and codes, records flaw echo amplitude on either a probe-specific printed diagram—a series of curves that represent various disc size reflectors to sound path and gain—or a generic diagram applicable for straight- and angle-beam ultrasonic probes.
Most commonly used for analyzing welds and work pieces, the DGS method relies on an ultrasonic inspector to manually fit a display flaw indication to the DGS curve diagram to determine the equivalent reflector size of the flaw. In the mid-60s, the method expanded to the use of angle-beam probes with rectangular transducers. However, due to the shape of the transducers, non-symmetrical sound beams were produced, which are even more distorted through the refraction at the interface between probe and specimen. At the time, analog ultrasonic testing instruments with small, non-linear screens were most often used. These rudimentary technologies did not enable operators to read incremental gains, such as a tenth of a dB, and distances as small as one-hundredth of a millimeter—giving rise to a host of measurement deviations.