At a construction site, a technician tests a pipeline weld for defects using an ultrasonic phased array instrument. The scanner consists of a frame with magnetic wheels which holds the probe in contact with the pipe by a spring. The wet area is the ultrasonic couplant that allows the sound to pass into the pipe wall. Since this picture was taken (2008), developments to scanners include the ability to run two probes at one time, one probe on each side of the weld, and scanners with a chain or linkage that passes around the whole pipe, ensuring that the probes are guided in a straight line and do not fall off.