Internal probes must be used when the external surface is not accessible, e.g. when testing pipes that are buried or supported by brackets. Success has been achieved in measuring thickness variations in ferromagnetic metal pipes with the remote field technique. See Sec.5.5 Remote Field Sensing . Unfortunately, the effects of electrical conductivity a cannot be phased out and it is important to verify that any variations of a over the region of interest are at a sufficiently low level. At the lower values of normalized frequency, where there is detectable penetration, the variations of probe impedance are more sensitive to changes in electrical conductivity.
Calibration is made with samples having known thicknesses.
Depending on the required degree of penetration, measurements can be made using a single-coil probe or a transformer probe, preferably reflection type. The method has also been used successfully for measuring thicknesses of very thin protective coatings of ferromagnetic metals, e.g. chromium and nickel, on non-ferromagnetic metal bases. These, in combination with suitably high test frequencies, can provide very high sensitivity to thin coating layers and minimize effects related to property or thickness variations in the underlying base metal. The purpose here is to confine the magnetizing field, and the resulting eddy current distribution, to the thin coating layer and to minimize the field within the base metals. It is also highly desirable to provide sensitivity to small discontinuities and defects in the coating layer, if these are significant to the test.