Determination of Vehicle Fluorescence.
The settling test for particle concentration can be used to also judge vehicle f luorescence and is readily
performed at a stationary unit. It is not as accurate as the laboratory test but is reasonably quantitative and
reproducible. It can be easily standardized with the material in use, and is quite satisfactory as a daily guide
for the inspector. The following procedure SHALL be used in performing the vehicle f luorescence test after
the steps in the settling test have been completed (see paragraph 184.108.40.206).
a. Illuminate the suspension in the centrifuge tube with black light in a darkened area.
Only the particle layer should f luoresce. Dirt, lint, etc. will usually settle more slowly
than the particles and may be seen as a nonf luorescent band or strip toward the top of
the particle layer. For particle concentration determination, this layer of dirt must be
carefully excluded from the total volume read. Dirt accumulation that exceeds 30% of
the total volume of the particle layer can impede the formation of indications,
requiring replacement of the bath.
b. Fluorescence in the liquid may indicate bath breakdown (f luorescent pigmentation
being stripped from the magnetic particles or fine magnetic particles remaining
suspended in the vehicle). If the vehicle f luoresces excessively, place the centrifuge
tube in its stand (which must not be ferromagnetic) with a horseshoe magnet in
contact with the centrifuge tube (see Figure 3-34) and let sit on a vibration free surface
for 1 hour for oil baths and 30 minutes for water baths. Illuminate the vehicle in the
centrifuge tube with black light in a darkened area. If the vehicles f luorescence is
reduced or eliminated the cause of the f luorescence is fine magnetic particles
remaining suspended. If the level of f luorescence remains at the same level, the
f luorescent pigmentation has been stripped from the magnetic particles.
c. If it is determined that the cause of the vehicles f luorescence is stripping of the
pigmentation, the suspension SHALL be replaced if it is serious enough to interfere
with the results of the system effectiveness check using the Ketos ring.
d. If it is determined that the cause of the excessive suspension f luorescence is fine
magnetic particles remaining in the vehicle, and they are of a volume to interfere with
the results of the system effectiveness check using the Ketos ring an effort may be
made to remove them from the holding tanks magnetic particle bath. This can be
done with magnets. The magnetic particle bath in the magnetic particle machines
holding tank should be allowed to rest (not agitated) for forty (40) minutes. Place the
magnets in the magnetic particle bath, taking care not to place them so deep that they
will attract the particles that have settled out of suspension. The length of time or
number of times that the magnets will have to be cleaned of particles and submerged
is dependent upon the seriousness of the problem. The bath SHALL be able to pass
the system effectiveness check, after the removal of as many suspended particles as
possible or be replaced.