Tester for flaws, cracks or corrosion in ferromagnetic materials e.g. in pipelines - comprises two ultrasonic transducers positioned between pole pieces of premagnetising permanent magnet units for stray flux measurement.

Abstract
The tester incorporates two stray flux magnetisation units (3,4) in the form of U-shaped permanent magnets with axially aligned pole pieces (5,5',6,6') joined by a yoke (7,8). One unit (3) has an electromagnetic ultrasonic transducer (9) radiating at right angles to the static magnetic field. Another transducer (11) radiates parallel to the static field from a position between the neighbouring opposite poles of the two units. The function of these is solely to premagnetise the material under investigation and to measure the stray flux. USE - In pipelines (2) or similar structures. Sufficient signal/noise ratio is achieved by suppression of Barkhausen noise when transducers are moved.