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Magnetic Bias – Eddy Current with superimposed DC field magnetization.

Magnetic Bias is an electromagnetic form of inspection based on the Eddy Current principal with superimposed DC field magnetization to alter the permeability of the tube wall. It is a highly efficient and effective screening tool to sample large quantities of tubes rapidly and prioritize inspection time. Typically used on ferromagnetic tubing in air coolers, heat exchangers and boilers.

A magnetic bias probe is constructed similar to a conventional eddy current bobbin probe, however due to the high relative permeability of ferromagnetic material the penetration depth of the eddy currents are limited. The addition of a electromagnet is used to generate a variable magnetic field which allows the relative permeability to be altered based on the specimen under examination. This can be seen in the image below:

                                                 

Although we are able to alter the value of the relative permeability, the value is still higher than 1, as the magnetization field strength is not sufficient to saturate the tube wall, consequently the depth of penetration of the eddy current field does not increase significantly as compared to the non-magnetized state. However, in the case of a reduction of the tube wall in the presence of a localized defect, the compression of the magnetic field lines occurs, therefore increasing the field strength at this particular area. This increase in field strength further alters the permeability of the material and this change can be detected by the eddy current field, even though it has little depth of penetration.

The amplitude of the signal response can be evaluated, however the response is directly related to the defect volume. Due to this limitation to the technique, the sought after defect type, shape and volume should be known prior to inspection.

Advantages:

Quick Screening tool.

Slightly less cleaning required in comparison with IRIS. Requires a 90% fill factor to inspect.

Can detect internal and external defects.

Limitations:

Gradual wastage/wall loss cannot be detected.

Low accuracy for sizing defects.

Defect size, shape and volume should be known prior to Inspection.

Small volume defects can go undetected during inspections for larger volume defects.

Cannot distinguish between internal and external defects.

Requires secondary method to provide accurate data.