Detecting Medium

The detecting media should normally be in accordance with BS EN ISO 9934-2 Detection Media.
When inks are used for MT the test may be referred to as a wet method; when powders are used the test may be referred to as a dry method.

The basic requirements for magnetic particles are as follows:

1. Fine grains to reduce the gravitational effect. However, if the grains are too fine, this could coagulate* the particles. (“coagulate” – to group together).
2. Elongated shape for easier polarization. Spherical particles are also needed to ensure dispersal over the surface.
3. High permeability for magnetisation in weak flux leakage fields.
4. Low retentivity if particles are to be removed from the test surface after the test.
5. High contrast against the background of the test surface.

BS EN ISO 9934-2 the particle size for inks is between 1.5µm and 40 µm (no more than 10% of particles may exceed this), whereas powders are generally > 40µm.

The main considerations when producing and selecting detection media are:

1. Permeability
2. Particle size
3. Flammability