Which method is NOT used for applying magnetic fields during MPI?

The method of applying magnetic fields during Magnetic Particle Inspection (MPI) typically involves using tools that create a magnetic field to detect surface and near-surface discontinuities in ferromagnetic materials. Coil-based electromagnets, permanent magnets, and yoke-type magnets are all effective methods employed to generate a magnetic field for this purpose.

Electromagnets and yoke-type magnets directly create the necessary magnetic field to attract magnetic particles, highlighting defects when applied to test materials. Permanent magnets are also used due to their ability to provide a constant magnetic field without the need for power, which can be particularly advantageous in certain inspection scenarios.

Liquid nitrogen, however, does not serve any role in applying magnetic fields during MPI. Instead, it is primarily used in various cooling or cryogenic applications and does not interact with magnetic field generation. Therefore, it is not a method utilized in the context of MPI for the application of magnetic fields. This distinction is crucial for understanding the different techniques used in MPI and their respective roles in the inspection process.