Which eddy current test coil arrangement uses one area of the test specimen as a reference standard?

The correct choice is the differential coil arrangement. This setup is specifically designed to compare the response of two coil sensors, where one coil measures the condition of the test specimen while the other coil is used as a reference, often at a known, unaffected area of the specimen. This allows for the detection of deviations or anomalies in the material by focusing on the difference in the signals received by the two coils.

By utilizing one part of the specimen as a reference, the differential coil arrangement is adept at isolating changes in the material properties or any defects. This enhances the sensitivity of the eddy current testing by effectively eliminating the noise or background signals, thereby increasing the reliability of the results.

In contrast, the other arrangements do not operate in the same comparative manner. The single absolute coil measures the properties of a single area without a reference, while the double absolute coil involves two similar coils that measure independently rather than comparing one against the other. A de saturation coil is not a common term used in standard eddy current tests and does not pertain to using a reference area for measurement. Thus, the differential coil is uniquely positioned to use a reference standard effectively, distinguishing itself as the correct answer for this question.