使用双晶探头检测中厚板时，选用探头的焦距以等于钢板厚度的一半为最佳，调整检测灵敏度最好以二倍焦距的大平底为基准，否则要注意近检测面缺陷信号，即使缺陷波高未达到判定条件，而实际缺陷有可能达到或超过控制要求。缺陷边界的测定要注意有效波束宽度问题，避免方法不当，人为地将小缺陷测定成了大缺陷，造成不必要的损失。

The optimally focal distance of the selected probe was the half thickness of the medium thickness plate and the detection sensitivity should be adjusted by flat-bottom which having double times focal distance when inspecting the plate by T-R probe. Otherwise, paying more attention to the echo height of near surface defect, it seemed that the echo height did not reach the requirement, but it had met or exceeded the standard actually. It was also needed to notice the sound beam in order to avoid unnecessary losses by false method when the small flaw was identified as the big one artificially.