One would think that as simple in concept that penetrant inspection is, the number of questions that could be posted would be finite. That may be true, but finite is just a number that does not reach infinity, as we are finding, after half a century of answering questions. There seems to always be something different that folks want to know about. So this is what we received, via email:
“When performing multiple types of PT inspections on the same part, is it preferable or recommended to performing them in any particular order. For instance, group 1 then group III then group IV.”
This took a bit of thought and a trip back into time. Many, many years ago, MIL-I-25135 was in its infancy and described the available penetrants in terms of groups. Group one was solvent removable visible penetrant, group 2 was post emulsifiable visible penetrant, group 3 was water washable visible penetrant, and group 4 was water washable fluorescent penetrant. These classifications persisted through revision C of MIL-I-25135, and were altered to the present classification system in the D revision. When did this occur? It happened in 1984, 22 years ago. So, apparently, this inquiry concerned inspections that might be conducted in accordance with the old MILSPEC.
Answering a question like this poses some significant problems. The group classification no longer exists, and while it is possible to relate groups one, two, and three to the present classification of visible penetrants, one cannot relate group 4 to them. Group 4 had no sensitivity assigned to it, and one could only guess that it might be a sensitivity level 1/2 or one, at best. But in a larger sense, one must wonder what the objective was of making a test that used these three groups. That is something that we may never know. Our advice was twofold. First, if this type of inspection was to be performed, the fluorescent penetrant should be used first, followed by the visible penetrant inspections. Second, we suggested that a review of the latest documents, AMS-2644 and ASTM E-1417 might be in order, so that modern penetrant systems would be used in accordance with the latest specifications. But one must wonder if the inquiry revolved around a laboratory that still had ancient cans of the old “Group” penetrants on hand. One would hope not.