DMTA-10040-01EN, Rev. E, February 2018
Using the Instrument
231
Figure 5
‑
154 ECT principles
The eddy currents interact as follows:
•
Two coils are excited with an electrical current, producing a magnetic field
around them. The magnetic fields penetrate the tube material and generate
opposing alternating currents within the material. These currents are called eddy
currents.
•
Any defects that change the eddy current flow will also change the impedance of
the coils in the probe.
•
The changes in the impedance of the coils are measured and used to detect defects
in the tube.
This application uses a probe adaptor that is configured for differential mode. The
differential configuration subtracts the signals obtained by both coils and, as such, is
ideal for detecting smaller discontinuities such as pitting, but also shorter
discontinuities such as wear under a support (see Figure 5-155 on page 232). The
differential configuration, however, cannot be used to detect longer or gradual
discontinuities for the simple reason that the discontinuity signal is “subtracted”
(diminished) and therefore not useful. To detect longer or gradual discontinuities,
absolute mode must be used.
Magnetic field
Eddy currents
Impedance bridge coils; absolute
and differential
Conductive and nonferromagnetic
tube material
Eddy currents
Содержание nortec 600
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