Wet Fluorescent Method.
When exposed to near ultraviolet light (blacklight), f luorescent magnetic particles emit a highly visible
yellow-green color. Indications produced are easily seen, and the f luorescent particles give much stronger
indications of very small discontinuities than do the non-f luorescent magnetic particles. The differences
between the wet visible method and the wet f luorescent method are comparatively minor regarding
suspension characteristics, maintenance and application, as well as the inspection variables and demagneti-
zation techniques. The following applies only to the wet f luorescent method.
Advantages and Limitations.
Fluorescent particles have one major advantage over the untreated or visible particles. That is their ability
to give off a brilliant glow under blacklight. This brilliant glow serves three principal purposes:
a. In semi- or complete darkness even very minute amounts of the f luorescent particles
are easily seen, having the effect of increasing the apparent sensitivity of the process,
even though magnetically the f luorescent particles are not superior to the uncolored
b. Even on discontinuities large enough to give good visible indications, f luorescent
indications are easier to see and the chance of the inspector missing an indication is
reduced, even when the speed of inspecting parts is increased.
c. Concurrent with the greater visibility of indications formed by f luorescent particles,
the background caused by excessive magnetization is also more severe. Consequently,
greater care must be exercised in selection of the particle concentrations and magneti-
zation levels for the inspection with f luorescent particles.
The f luorescent particle method is faster, more reliable and more sensitive to very fine defects than the
visible colored particle method in most applications. Indications are easier to detect, especially in high
volume testing. In addition, the f luorescent method has all the other advantages possessed by the wet visible
The wet f luorescent method also shares the disadvantages found with the wet visible method. In addition,
there is a requirement for both a source of blacklight, and an inspection area from which the white light can
be excluded. Experience has shown that these added requirements are more than justified by the gains in
reliability and sensitivity.
There is no difference between the f luorescent and non-f luorescent materials as far as the vehicle
requirements. Petroleum distillates must meet the same specifications as listed in Table 3-5, with one
additional requirement. The vehicle itself must not f luoresce strongly.
The particles for this method are magnetically the same as the visible type, but they must carry the
f luorescent dye and the binding material that holds the dye and particle together as a unit. This coating of
the particles could make them less effective in producing indications. However, f luorescent particle
indications require only a small fraction of the particles, as compared to the non-f luorescent type, to be easily
visible. Thus, the overall effect is a significant increase in sensitivity.
Fluorescent particles are supplied primarily as a dry concentrate, incorporating all the ingredients necessary
for use in oil or water, as appropriate.
It is of importance that the bond between the f luorescent dye or pigment and the magnetic particle is able to
resist the vigorous agitation it receives in the pump circulation and the solvent attack from the suspension
f luid. If the dye separates from the magnetic particle, the dye tends to cling to the surfaces of the part,
independent of any magnetic attraction, thus increasing the background against which indications must be
viewed. At the same time the magnetic particles that are held magnetically at indications have lost some or
all of their f luorescing ability, reducing their visibility.