PORTABLE MPI TECHNIQUES
PORTABLE MAGNETIC PARTICLE FIELD INSPECTION TECHNIQUES.
Portable induced field inspection equipment is generally referred to as either a probe or a yoke. These
terms are synonymous and differ only due to manufacturers nomenclature. This category of
inspection equipment is described here in conjunction with the techniques for their use and
Magnetic yokes are small and easily portable. They are easy to use and are adequate when testing
small castings or machine parts for surface cracks and for weld inspection. They induce a strong
magnetic field into that portion of a part that lies between the poles or legs of the yoke. The induced
field f lows from one leg of the yoke to the other regardless of the style or leg configuration. Yokes or
probes are available with either fixed or articulated legs as shown in Figure 3-29.
Portable Induced Field Inspection Equipment
Either dry powder or wet magnetic particles may be used in conjunction with a yoke for the detection
of discontinuities. Yokes are available for operation from a 115 volt, 60-hertz AC outlet, or from a 12
volt DC battery. A permanent magnet yoke is also available, permitting inspections to be performed
without the use of electric current.
Capabilities and Limitations.
The units are designed for simplicity, ease of handling, and operation by one person. They may be
used on machine-finished surfaces, as well as castings and weldments fabricated in a variety of
configurations. The units induce a strong magnetic field at the surface of the part being inspected.
Since no current is f lowing through the part being subjected to inspection is impossible to overheat or
burn the part. The f lexibility of a yoke with articulating legs is greatly increased permitting
inspections to be performed on parts of varied configurations.
Yokes or probes are limited to the detection of surface and near surface discontinuities only. They
should not be used for deep-seated, subsurface discontinuities due to the limited penetration of the
induced magnetic field. Because of their size they cannot be used with a 100% duty cycle. Rather,
they are limited essentially to spot-checking and occasional sample testing rather than continuous
production testing. Under optimum operating conditions, the fixed leg yoke has a limited inspection
area governed by the distance between and immediately surrounding the legs. The moveable or
articulated leg yoke can inspect either a larger area (legs apart) or detect finer discontinuities by
concentrating the magnetic field in a smaller area (legs closer together).