The following characteristics affect key test parameters, including required magnetizing current, current on time, when to apply particles and demagnetization requirements.
1. Alloy, shape and magnetic condition of test object.
2. Type of magnetizing current.
3. Direction of magnetic field.
4. Sequence of operations.
5. Value of flux density.
TEST MATERIALS OF MPI
Wet Particles
Particle size distributions for both wet and dry tests are important considerations. For wet tests using true residual techniques, smaller particles may accumulate faster producing an observable indication. For wet tests, larger rounded particles may produce an observable indication faster under continuous magnetization techniques. Colors and backgrounds should be selected to provide optimum contrast when viewed by the technician. Wet particles, including oil based
visible/fluorescent and water based visible/fluorescent, fit various testing requirements and environments.
Dry Particles
For dry tests, very fine particles that move slowly in the air may produce better visible build up for vertical and overhead conditions using yoke magnetization. For dry magnetic particle testing, the use of larger and heavier particles with a definite drop may produce better build up for flat and horizontal prod and yoke techniques.