T.O. 33B-1-1will pick it up and disperse it. After agitating 10 minutes the strength should be checked by the settling testas in the case of the dry powder concentrate.3.5.6.7SuspensionMaintenance.As the suspension bath is used for testing, it will undergo changes due to use. Some of these changes are:a.Drag-out of magnetic particles, by mechanical and magnetic adherence to parts.b.Drag-out of liquid due to the film that adheres to the surface of parts.c.Loss of liquid by evaporation.d.A gradual accumulation of contaminants: shop dust, dirt from parts not properly cleaned, lint fromwiping rags, and oil from parts that carry a residual film of oil.e.Miscellaneous objects and materials which are dropped into the tanks.f.Dilution/contamination of the bath from wet test pieces, dripping overhead pipes, and moisturecondensation.3.5.6.7.1The magnetic particles are considerably heavier than the vehicle in which they are suspended. When theagitation system is shut off, the particles rapidly settle out. It is important that all particles be in suspensionbefore conducting any inspections or concentration tests. When the agitation system has been off for 4 ormore hours, the agitation system SHALL be turned on for at least 30 minutes before conducting aninspection. This agitation time varies with the downtime due to compacting of the particles from their ownweight. If the machine has been off for 30 to 60 minutes, a 10-minute agitation is usually adequate. If theunit 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 SHALLbe 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 forevaporation loss, only additional oil or water is required. To make up for the drag-out loss, the addition ofbath liquid and particles is required.3.5.6.7.2It 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 byevaporation is likely to be the major cause. Actually the problem is not serious, because with constant use theaccumulation of dirt, scraps, lint, etc. requires the dumping of the tank and a new bath before loss of liquidbecomes serious. Magnetic particle content is of most critical importance and SHALL be carefully watched atall times.3.5.6.7.3Dirt 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 particlesor as a nonf luorescent band or strip in the particle layer. For particle determination, this layer of dirt mustbe carefully excluded from the total volume read. When the contamination exceeds 30% of the volume of theparticle layer, formation of proper indications will be impeded, and the bath SHALL be dumped and a newone made up. This may occur as often as once a week when a unit is in constant use. The layer of dirt andthe vehicle immediately above it SHALL NOT f luoresce. If oil is used as a suspension, it must be considereda petroleum product and disposition of the bath must conform to all applicable regulations.3.5.6.8SuspensionSettlingTest.NOTEThe difference between milliliters (ml) and cubic centimeters (cc) in this caseis negligible and the two terms are used interchangeably for this paragraph.The magnetic particle bath SHALL only be agitated in the magnetic particlemachine’s holding tank when it is necessary to perform a magnetic particleinspection or meet process control requirements.Change 23-69
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