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Eddy current inspection - Coggle Diagram

- - - - do not respond to gradual changes that would generate unacceptable levels of noise in absolute coils,
    - - noise levels are further reduced
    - - one coil to drive the ET and another to receive the test signals
    - - same coil to drive the ET and receive signals
  - - - Slotted calibration blocks
        
        spark eroded slots
        
        detect cracks
      - Tube standards
        
        Manufactured tube
      - Step wedges
        
        wall thinning in a thin metal plate
- - - - circulate in a plane perpendicular to the magnetic flux
    - - flow parallel with that surface
  - - - encircled by their own magnetic fields
      - direction to oppose the field from the coil
        
        choking effect on the coil current
        
        reflected in the coil's impedance
  - - - few Hertz to several mega Hertz.
      - depth of penetration
        
        skin effect
        
        frequency increases so the depth of penetration decreases
      - δ=660/√(f × σ × μ)
        
        δ is the standard depth of penetration in mm
        
        f is the frequency in Hertz
        
        σ is the conductivity in %IACS
        
        μ is the relative permeability.
    - - affected by:
        
        Thermal processing history.
        
        Working.
        
        Internal stresses.
        
        Temperature.
        
        Chemical composition.