will pick it up and disperse it. After agitating 10 minutes the strength should be checked by the settling test
as in the case of the dry powder concentrate.
As the suspension bath is used for testing, it will undergo changes due to use. Some of these changes are:
Drag-out of magnetic particles, by mechanical and magnetic adherence to parts.
Drag-out of liquid due to the film that adheres to the surface of parts.
Loss of liquid by evaporation.
A gradual accumulation of contaminants: shop dust, dirt from parts not properly cleaned, lint from
wiping rags, and oil from parts that carry a residual film of oil.
Miscellaneous objects and materials which are dropped into the tanks.
Dilution/contamination of the bath from wet test pieces, dripping overhead pipes, and moisture
The magnetic particles are considerably heavier than the vehicle in which they are suspended. When the
agitation system is shut off, the particles rapidly settle out. It is important that all particles be in suspension
before conducting any inspections or concentration tests. When the agitation system has been off for 4 or
more hours, the agitation system SHALL be turned on for at least 30 minutes before conducting an
inspection. This agitation time varies with the downtime due to compacting of the particles from their own
weight. If the machine has been off for 30 to 60 minutes, a 10-minute agitation is usually adequate. If the
unit has been off for a week or more, 60 minutes of agitation plus supplemental stirring may be necessary.
Concentrate should be added when the particle concentration is low. Evaporation or liquid drag-out SHALL
be watched, and volume maintained when the level drops appreciably. Loss of liquid may be either by drag-
out or by evaporation, and corrective measures are different for these two types of loss. To make up for
evaporation loss, only additional oil or water is required. To make up for the drag-out loss, the addition of
bath liquid and particles is required.
It is difficult to know what the cause of volume loss actually is in any given case. For a unit in constant use,
it can be assumed that more than 50% of the loss is due to drag-out. For a unit used only occasionally, loss by
evaporation is likely to be the major cause. Actually the problem is not serious, because with constant use the
accumulation of dirt, scraps, lint, etc. requires the dumping of the tank and a new bath before loss of liquid
becomes serious. Magnetic particle content is of most critical importance and SHALL be carefully watched at
Dirt accumulation in the bath can usually be observed in the settling test for magnetic particles. Dirt, lint,
etc. are usually lighter and settle later. Dirt, lint, etc. are often seen as a second layer on top of the particles
or as a nonf luorescent band or strip in the particle layer. For particle determination, this layer of dirt must
be carefully excluded from the total volume read. When the contamination exceeds 30% of the volume of the
particle layer, formation of proper indications will be impeded, and the bath SHALL be dumped and a new
one made up. This may occur as often as once a week when a unit is in constant use. The layer of dirt and
the vehicle immediately above it SHALL NOT f luoresce. If oil is used as a suspension, it must be considered
a petroleum product and disposition of the bath must conform to all applicable regulations.
Suspension Settling Test.
The difference between milliliters (ml) and cubic centimeters (cc) in this case
is negligible and the two terms are used interchangeably for this paragraph.
The magnetic particle bath SHALL only be agitated in the magnetic particle
machines holding tank when it is necessary to perform a magnetic particle
inspection or meet process control requirements.