T.O. 33B-1-13-653.5.6 WetVisibleParticles.CAUTIONThe wet visible method SHALL NOT be used on aerospace vehicles or aerospacevehicle parts without specific approval of the appropriate engineering authority forthe individual inspection requirements.3.5.6.1 General.Wet method magnetic particles are fundamentally similar to each other, once they are dispersed in the suspendingliquid. In past years, the most common form of the material concentrate was a paste. Today, however, the pastes havebeen almost exclusively reformulated and produced as drypowder concentrates. These powders incorporate the neededmaterials for dispersion, wetting, corrosioninhibition, etc. The powders are much easier to use, as they need merely tobe measured out and added directly to the agitated bath. The agitation system of the modern magnetic particle unitswill pick up the powder and quickly disperse it in the bath.3.5.6.2 AdvantagesandLimitations.As is true of every process, the wet method has both good points as well as less favorable characteristics. The moreimportant good points of the wet method, which constitute the reason for its extensive use, as well as the less attractivecharacteristics are tabulated as follows:a. It is the more sensitive method for very fine surface cracks.b. It is the more sensitive method for very shallow and fine surface cracks. It quickly and thoroughlycovers all surfaces of irregularly shaped parts, large or small, with magnetic particles.c. It is the faster and more thorough method for testing large numbers of small parts. The magneticparticles have excellent mobility in liquid suspension.d. It is easy to measure and control the concentration of particles in the bath, which makes for uniformityand accurate reproducibility of results.e. It is easy to recover and re-use the bath.f. It is well adapted to the short, timed shot technique of magnetization for the continuous method. It isreadily adaptable to automatic unit operation.g. It is not usually capable of finding smaller defects lying wholly below the surface if more than a fewthousandths of an inch deep.h. It is messy to work with, especially when used for the expendable technique, and in field testing. Arecirculating system is required to keep the particles in suspension.i. It sometimes presents a post-inspection cleaning problem to remove magnetic particles clinging to thesurface.3.5.6.3 ParticleSelectionbyVisibilityandContrast.The need to meet a variety of conditions for successful magnetic particle testing has resulted in the development ofdifferent materials to obtain this result. The most commonly used materials, black and red, oil and water suspendibleare listed below with the special characteristics of each.
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