Page 483 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

478 CHAPTER 18 Ancillary Sensors
producing flow interruptions that are measured for fault isolation and quantification. This is the essence of NDT.
Typically, just as a camera from a single angle in a scene will not give the full picture of the events transpiring, a single NDT sensor will not “tell the full story” on the condition of the test piece. A sensor array of varying technologies is usually deployed to test various parameters to fully characterize the item.
Technology in this field is rapidly evolving. This section will highlight some of the more com- mon technologies available for ROV deployment. As this chapter is a high-level overview, the reader is encouraged (and cautioned) to delve deeper into each of these areas. Each of these devices has its peculiar strengths and weaknesses, requiring operator skill in gathering and interpreting the product of the sensor’s measurements.
18.1.1 Magnetic particle inspection
Magnetic particle inspection (MPI) is the traditional method of measuring the magnetic flux leak- age to determine metal surface discontinuity. The theory of operation involves the tracing of the lines of magnetic flux induced into a test subject and then measuring this field for anomalies.
Magnetism comes in three types (Table 18.1):
1. Ferromagnetism: Strongly magnetized metal exhibiting excellent magnetic characteristics
2. Paramagnetism: Materials exhibiting weak magnetic attraction when stimulated with a
magnetic field
3. Diamagnetism: Materials repulsed when stimulated by a magnetic field due to the generation of
an opposing magnetic field in the material
In ferromagnetic metals, the atoms form a lattice structure gathered in groups (termed
“domains”) with individual and isolated magnetic moments. A magnetic field can be induced into a magnetic material in any of three ways:
1. Applying a permanent magnet to the material
2. By passing a current through the material
3. By inductance through a current carrying conductor in close proximity to the material
When demagnetized, the domains have magnetic moments that are randomly distributed. These
domains can be thought of as many small bar magnets arranged at random along the surface of the
Table 18.1 Sample Materials Exhibiting Magnetic Types
Ferromagnetic Paramagnetic Diamagnetic
Iron Nickel Cobalt Steel
Platinum Palladium Most metals Oxygen
Bismuth Antimony
Most nonmetals Concrete