The ratio of the specific diameter of a probe to the minimum discontinuity of interest should be:

The correct understanding regarding the ratio of the specific diameter of a probe to the minimum discontinuity of interest is that it should be less than 2. This criteria is significant because it ensures effective detection capability of the probe for the intended discontinuities.

When the specific diameter of the probe is smaller than twice the size of the minimum discontinuity, the probe is sufficiently sensitive to pick up the signals caused by these discontinuities. A smaller ratio enhances the electromagnetic coupling between the probe and the test material, allowing for better detection and characterization of flaws.

In contrast, if the ratio were greater than 2, the probe might not adequately interact with smaller discontinuities, leading to undetected flaws or misinterpretation of the data collected during testing. Thus, maintaining this ratio is crucial for ensuring that Eddy Current Testing is effective and reliable.