When conducting an eddy current test using a differential comparison coil arrangement that compares an external reference standard with the test specimen, the system should be nulled or balanced:

When using a differential comparison coil arrangement in eddy current testing, it is essential to balance the system properly to ensure accurate measurements. Nulling or balancing the system with both the reference standard and an acceptable test specimen in the respective coils allows the operator to establish a baseline response from the standard against which the test specimen can be compared.

This balanced setup ensures that any differences in signal induced by the test specimen, such as variations in conductivity or geometry, can be accurately identified. By having an acceptable test specimen alongside the reference standard, the operator can discern normal variations and establish limits for what constitutes acceptance criteria. Thus, this approach enables effective differentiation between the response of a known good specimen and any anomalies that may be present in the test specimen.

Using only the reference standard or mixing it with an unacceptable test specimen would not provide a reliable assessment of performance or characteristics since these arrangements would not adequately reflect the conditions present during an actual test. Therefore, balancing with an acceptable test specimen allows for both the calibration of the system and a precise comparison for detecting faults or anomalies in the material being tested.