When testing tubing with an encircling coil, the phase relationship of output signals from ID and OD discontinuities is what?

When using an encircling coil to test tubing, it is important to understand the nature of the electromagnetic signal interaction with discontinuities located at the inner diameter (ID) and outer diameter (OD) of the tube. The correct interpretation of the phase relationship between the output signals generated by these discontinuities can be derived from the principles of electromagnetic testing.

In this scenario, the signal from the outer diameter discontinuity typically leads the signal from the inner diameter discontinuity due to the differences in how the electromagnetic field interacts with the two surfaces. The field penetrates the material, and the response from the outer surface is affected by not only the presence of the outer discontinuity but also the characteristics of the tube itself, such as thickness and material properties. As a result, the electromagnetic response from the outer diameter often occurs before that from the inner diameter, establishing a clear phase relationship.

Understanding this phase relationship is crucial for accurately interpreting the signals during testing. This leads to important insights regarding the nature and location of discontinuities present in the material being tested. In contrast, other interpretations, such as signals being in phase or indeterminate, do not accurately reflect the physical principles at play in the electromagnetic interactions involved in encircling coil testing.