I am working on developing a penetrant inspection for the detection of fatigue cracks in aluminum. I am faced with the decision of selecting the right sensitivity penetrant for the job. I am wondering what the NDT community's experience has been with the advantages of inspecting with a level 4 penetrant as compared to a level 3 penetrant. I know that a sensitivity level 4 penetrant is considered "ultra" sensitive, but I guess I'm wondering how much better "ultra high" is than "high". Maybe this can not be quantified, but if it can it would be nice to know the answer. In this case, the penetrant will be limited to Method A, due to equipment availability. Any information will be greatly appreciated.