So, What Is This (M) and What Does It Mean??

Strangely enough we get this question frequently, concerning our post emulsified fluorescent penetrants. These are listed as follows:

  • Level 1 sensitivity – FP-94
  • Level 2 sensitivity – FP-93A(M)
  • Level 3 sensitivity – FP-95A(M)
  • Level 4 sensitivity – FP-97A(M)

This nomenclature resulted from a massive change in the qualification requirements in the early 1980’s. The story goes like this: As we have described in previous issues of the PENETRANT PROFESSOR, measuring the sensitivity of fluorescent penetrants was a challenge, and initially used thermally cracked aluminum specimens to make comparisons. This was grossly unsatisfactory, yet was used to classify the penetrants according to their sensitivity. In the early 1970’s there were four groups for fluorescent
pentrants, as follows:

  • Group IV – water washable fluorescent.
  • Group V – post-emulsifiable fluorescent.
  • Group VI – high sensitivity fluorescent post-emulsifiable.
  • Group VII – high sensitivity kits

Since the measurement method was so crude, it was not possible for a penetrant user to compare the penetrants made by different manufacturers. Consequently, when a customer published a bid request for a Group VI penetrant (for example), there was no way to know whether the various manufacturers were offering the same sensitivity, despite
the fact that they were listed on the QPL as being equivalent. This situation was untenable, and led to the refinement of the qualification procedure, by which the sensitivity
was accurately measured using low cycle fatigue cracked bars and a photofluorometer. When this happened, the U. S. Air Force required that all penetrants had to be re qualified
and classified according to the system that is now used, which has five sensitivity levels, ranging from 1/2 to level 4. So what has all of this history to do with the (M)?

The Met-L-Chek Group VI penetrant was identified as FP-95 and was slightly modified to lower its sensitivity to meet the new level 3 sensitivity requirements. We wished to retain the same designation to eliminate any confusion on the part of our customers. However, the Air Force objected because of the difference in the two materials and asked that we add the (M) to the identification. So this was done for levels 2, 3, an 4. Since there had been no previous level 1 penetrant, it was named FP-94 and did not need the (M). That is how it all happened.

We bring this up because the Air Force Materials Laboratories, is currently updating their procedure for qualification of penetrants to AMS-2644, and all qualified manufacturers
materials will have to go through this renewal process. In this revision each manufacturing location will have to qualify the materials made in that facility. This may well result in some modifications by the different suppliers and could result in nomenclature changes. Just another step in the direction of reliability in penetrant inspection.