When the medium, whether dry or liquid, is applied to the specimen while the magnetizing current is flowing, the procedure is known as The Continuous Method.
If the medium is applied after the
magnetizing current is shut off, the procedure is known as The Residual Method.
In the dry method, the powder is applied by sprinkling or dusting the specimen.
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| dry method |
In the wet method, the medium is mixed with a liquid (kerosene or oil) to make a bath, which is then applied to the surface of the specimen.
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| wet method |
1.High Permeability
2.Low Retentivity
Magnetic
particles containing these characteristics will give maximum response in a
leakage field but will not remain magnetized when the field is removed. The
size of the particles used in the dry method varies but they will usually all
pass through a 100-mesh screen.
The shape of the magnetic particle should be spherical and must have a high degree of the magnetic mobility and still have substantial attractive power.
Round smooth particles offer good mobility but have low attractive power. Long, slender, jagged particles have excellent attractive power but do not have the mobility to move in leakage field.
In the Wet Method, Magnetic Oxides of iron are used because they are extremely fine and have a lower Permeability than the metallic Dry particle.
The particle size must be small in the wet method to permit the particles to remain in the suspension in the liquid.
Mobility is important because when the particles are brought into a leakage field they must be able to move to form a pattern or indication.
- In the dry method, mobility is assisted by dusting or blowing the particles over the surface of the specimen.
- Mobility can also be assisted by vibrating the specimen after the particles the have been dusted on the surface.
- Alternating current also assists mobility, since the alternating field causes the particles to “dance”.
- In the wet method mobility is greatly assisted because the particles are suspended in a liquid bath.
Visibility is
important in magnetic particle testing and a good light source is essential. Magnetic
particles are usually available in grey, red and black, and the choice of color
is determined by the best contrast with the specimen surface.
Fluorescent particles are commonly used in the wet method to aid visibility, but this requires the use of a black light. Light.
The wet suspensions (bath) used in the
wet method consist of a liquid in which the particles are suspended.
The particles
used are obtainable in a highly concentrated from and may be either fluorescent
or non-fluorescent. To achieve the required test sensitivity, the degree of
particle concentration in the bath must be correct.
Agitation must be constant while the bath is in used to maintain the particles in suspension. Agitation is usually accomplished by electrically driven pumps. The bath should be checked daily due to evaporation and loss of particles that are removed from the bath by the specimen.
1. Agitate the suspension thoroughly to assure particle distribution.
2. Fill 100 cc sample from the delivery hose into the centrifuge tube.
3. Demagnetize, if necessary (when clumping occurs in the tube).
4. Allow settling for 30 minutes.
5. Take reading and record log.
6. Adjust bath, either by adding particles or vehicles, if necessary.
The reading
should be between 1.5 and 2.cc for a non-fluorescent bath and between 0.2 and cc
for a fluorescent bath as shown above. (these data are guidelines and will vary
from one specification to another.)
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