T.O. 33B-1-1 3-59 Table 3-5.  Requirements for Magnetic Particle Wet Method Oil Vehicle (DOD-F-87395) Requirement Specification/ Test Minimum Maximum Standard Flash Point, C(F) 94(200) — ASTM D 93 Odor — None DOD-F-87395 ASTM Color — 1.0 ASTM D 1500 Background Fluorescence Less than the standard DOD-F-87395 Viscosity Centistokes — 3.0 ASTM D 445 Particulate Matter, mg/L — 0.5 ASTM D 2276 Total Acid Number, mg KOH/L — 0.015 ASTM D 3242 b.      Magnetic particles SHALL comply with ASTM E 1444 and the specific aerospace material specification (AMS).  (see Table 3-6) Table 3-6.  Procurement Data for Magnetic Particles per ASTM E 1444 Type of Particles (Specification Title) Specification Magnetic Particle Inspection Material, Dry Method AMS 3040 Magnetic Particles, Wet Method, Oil Vehicle AMS 3041 Magnetic Particles, Wet Method, Dry Powder AMS 3042 Magnetic Particles, Wet Method, Oil Vehicle Aerosol Canned AMS 3043 Magnetic Particles, Fluorescent, Wet Method, Dry Powder AMS 3044 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle AMS 3045 Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Aerosol Canned AMS 3046 3.5.4 Current / Particle Application Techniques. The  part  may  be  magnetized  first  and  particles  applied  after  the  magnetizing  force  has  been  turned  off  (the  residual method); or the part may be covered with particles while the magnetizing force is still present (the continuous method). With parts having high retentivity, a combination of these methods is sometimes used.  The choice between the residual and the continuous method is a relatively easy one. 3.5.4.1 Residual Method. 3.5.4.1.1 Description and Use. In  the  residual  method,  parts  are  magnetized  and  the  magnetic  particles  are  then  applied.    This  method  can  be  used only  on  parts  having  sufficient  retentivity.    The  magnetic  field  they  retain  must  be  sufficiently  strong  to  produce leakage  fields  at  discontinuities,  which  in  turn  will  produce  readable  indications.    The  method  in  general  is  reliable only  for  the  detection  of  surface  discontinuities.    Since  hard  materials  which  have  high  retentivity  are  usually  low  in permeability,  higher  than  usual  magnetizing  currents  may  be  necessary  to  obtain  a  sufficiently  high  level  of  residual magnetism.  The difference in the behavior between hard steels and soft steels is usually not very serious, if only surface discontinuities are sought. 3.5.4.1.2 Dry Versus Wet. Either the dry or the wet method for particle application can be used in the residual method.  With the wet method, the magnetized parts may be immersed in an agitated bath of suspended magnetic particles, or they may be flooded with bath by a spray.  In these circumstances a favorable factor occurs that affects the strength of indications.  This factor is the  time  of  immersion  of  the  part  in  the  bath.    By  leaving  the  magnetized  part  in  the  bath  or  under  the  spray  for  a considerable  time,  the  leakage  fields  have  time  to  attract  and  hold  a  maximum  number  of  particles  even  at  fine