What should be the ratio of the specific diameter of a probe to the minimum discontinuity of interest?

The correct ratio is less than 2. This guideline ensures that the probe's diameter is adequately small relative to the discontinuity size being inspected. A smaller probe increases the ability to detect smaller discontinuities effectively because it can provide better resolution and sensitivity.

When the probe's diameter is significantly larger than the discontinuity of interest, the chances of missing smaller defects increase, as the probe may not interact adequately with the critical areas of the material. Keeping the ratio under 2 allows for sufficient overlap between the detection capability of the probe and the size of the discontinuities being evaluated, leading to more reliable inspection results.

It's essential in ultrasonic testing and other forms of non-destructive testing to maintain this ratio to ensure the effectiveness of the inspection process.