7/30/2019 MP Key Points

1/2

The earths field is .03 milli tesla(0.3 Gauss)

Lodestone Magnetite an iron oxide leading Stone

Curie Point 650 to 870 C

The strength of the magnetic field is the controlling factor of sensitivity in MPT.

Magnetic attraction will only ocur where an external pole exists.

No leakage field, no magnetic attraction, no indications.

When electricity passes through conductive materials, a magnetic field is produced.

Principles of electricity

Electrons produce magnetism.

Protons identifies the element with an atomic number.

Atoms that make up the elements have up tosix shells.

Outer shell is valence shelland the electrons on this shell are valence electrons.

Valence shell gives the element its chemical and electrical properties.

f a valence electron gains sufficient energy from some outside force, it can break away fromthe parent atom and called afree electron.

Balancing act

Electron flow

Current Flowa) Conventional Flow (+) >>>>>> (-)

b) Electron Flow (-) >>>>>> (+)

Current The flow of electrons through a conductorVoltage (Electromotive Force) a measure of pressure, forcing electrons to move around acircuit.Resistance The opposition encountered by a current.

Limiting Factors to Current Flowa) Available Force (Voltage)

b) Resistance by the conductor

Free electrons in conductors move around n a random manner. When we apply a voltage, therandom motion converts to a net drift from negative to positive poles.

7/30/2019 MP Key Points

2/2

Current is directly proportional to the applied voltage and inversely proportional to theresistance of circuit.

DC Subsurface discontinuitiesHalf wave Rectified AC Maximum sensitivity for sub-surface defects.

Full wave Rectified AC Not used much for MPIFull wave Rectified 3-phase AC Used to test large equipment

Magnetizing Force (H) Ampere/meter -Magnetic Flux () Weber (Maxwell) - The total number of lines established in a magnetcircuit.

Magnetic Flux Density (B) Tesla(Gauss) The number of lines per unit area.Permeability () Henrys/meter - B/H - The relative ease with which a magnetic flux isestablished in a given material.

Permeability Reluctance

Swing Field Multidirectional magnetic field circular and longitudinal

Note that residual magnetism and retentivity are the same when the material has been

magnetized to the saturation point.

More turns or higher current in the coil = stronger magnetic field

ASTM 1444

Head Shot 300 800 amp DC / inch diameter.12 32 amps DC / mm diameter

Preparation

Surface Conditions Permeability with a permanent magnet

Surface roughness

Precleaning

The viscosity of the suspension should not exceed 5.0 centitokes.

The oils with a flash point over 93.3C are recommended.

High Fire Point greater than 121C

When oil is used as a vehicle, the suggested colour value is plus 25 saybolt.(Lower values)