Concentration and Condition
The concentration of particles in the suspension is a very important
parameter in the inspection process and must be closely controlled.
The particle concentration is checked after the suspension is
prepared and regularly monitored as part of the quality system
checks. ASTM E-1444-01 requires concentration checks to be performed
every eight hours or at ever shift change.
The standard process used to perform the check requires agitating
the carrier for a minimum of thirty minutes to ensure even particle
distribution. A sample is then taken in a pear-shaped 100 ml centrifuge
tube having a stem graduated to 1.0 ml in 0.05 ml increments for
fluorescent particles, and graduated to 1.5 ml. in 0.1 ml increments
for visible particles. The sample is then demagnetized so that
the particles do not clump together while settling. The sample
must then remain undisturbed for a minimum of 60 minutes for a
petroleum-based carrier or 30 minutes for a water-based carrier,
unless shorter times have been documented to produce results similar
to the longer settling times. The volume of settled particles
is then read. Acceptable ranges are 0.1 to 0.4 ml for fluorescent
particles and 1.2 to 2.4 ml for visible particles. If the particle
concentration is out of the acceptable range, particles or the
carrier must be added to bring the solution back in compliance
with the requirement.
loss is often attributed to "dragout." Dragout occurs
because the solvent easily runs off components and is recaptured
in the holding tank. Particles, on the other hand, tend to adhere
to components, or be trapped in geometric features of the component.
These particles will be "drug out" or lost to the system
and will eventually need to be replaced.
After the particles have settled,
they should be examined for brightness and agglomeration. Fluorescent
particles should be evaluated under ultraviolet light and visible
particles under white light. The brightness of the particles should
be evaluated weekly by comparing the particles in the test solution
to those in an unused reference solution that was saved when the
solution was first prepared. The brightness of the two solutions
should be relatively the same. Additionally, the particles should
appear loose and not lumped together. If the brightness or the
agglomeration of the particles is noticeably different from the
reference solution, the bath should be replaced.