Electromagnetic Testing Chapter 3- Electromagnetic Testing

3.1.2 Limiting Frequency f g of Encircling Coils
Encircling coils are used more frequently than surface-mounted coils. With
encircling coils, the degree of filling has a similar effect to clearance with
surface-mounted coils. The degree of filling is the ratio of the test material
cross-sectional area to the coil cross-sectional area. Figure 3.7 shows the
effect of degree of filling on the impedance plane of the encircling coil. For
tubes, the limiting frequency (point where ohmic losses of the material are the
greatest) can be calculated precisely from Eq. (3.2):
f g = 5056/(σ∙ d i ∙ w∙ μ r ) (3.2)
Where:
f g = limiting frequency
σ = conductivity
d i = inner diameter
w = wall thickness
μ r (rel) = relative permeability
‘The degree of filling is the ratio
of the test material crosssectional
area to the coil crosssectional
area.” – How this fill
factor reflected in the f g equation?
Charlie Chong/ Fion Zhang