When performing a magnetic particle inspection, it is very important
to be able to determine the direction and intensity of the magnetic
field. As discussed previously, the direction of the magnetic
field should be between 45 and 90 degrees to the longest dimension
of the flaw for best detectability. The field intensity must be
high enough to cause an indication to form, but not too high to cause nonrelevant indications to mask relevant indications.
To cause an indication to form, the field strength in the object
must produce a flux leakage field that is strong enough to hold
the magnetic particles in place over a discontinuity. Flux measurement
devices can provide important information about the field strength.
Since it is impractical to measure the actual field strength
within the material, all the devices measure the magnetic field
that is outside of the material. There are a number of different
devices that can be used to detect and measure an external magnetic
field. The two devices commonly used in magnetic particle inspection
are the field indicator and the Hall-effect meter, which is also
called a gauss meter. Pie gauges and shims are devices that
are often used to provide an indication of the field direction
and strength but do not actually yield a quantitative measure.
They will be discussed in a later section.
indicators are small mechanical devices that utilize a soft iron
vane that is deflected by a magnetic field. The X-ray image
below shows the inside working of a field meter looking in from
the side. The vane is attached to a needle that rotates and moves
the pointer for the scale. Field indicators can be adjusted and
calibrated so that quantitative information can be obtained. However,
the measurement range of field indicators is usually small due
to the mechanics of the device. The one shown to the right has
a range from plus 20 gauss to minus 20 gauss. This limited
range makes them best suited for measuring the residual magnetic
field after demagnetization.
A field indicator is shown checking
for residual magnetism in this movie. (194 MB mov)
Hall-Effect (Gauss/Tesla) Meter
Hall-effect meter is an electronic device that provides a digital
readout of the magnetic field strength in gauss or tesla units.
The meters use a very small conductor or semiconductor element
at the tip of the probe. Electric current is passed through the
conductor. In a magnetic field, a force
is exerted on the moving electrons which tends to push them to one side of
the conductor. A buildup of charge at the sides of the conductors
will balance this magnetic influence, producing a measurable voltage
between the two sides of the conductor. The presence of this measurable
transverse voltage is called the Hall-effect after Edwin H. Hall,
who discovered it in 1879.
voltage generated Vh can be related
to the external magnetic field by the following equation.
Vh = I B Rh
Vh is the voltage generated.
I is the applied direct current.
B is the component of the magnetic field that is at a
right angle to the direct current in the Hall element.
Rh is the Hall Coefficient of
the Hall element.
b is the thickness of the Hall element.
Probes are available with either tangential (transverse) or axial
sensing elements. Probes can be purchased in a wide variety of
sizes and configurations and with different measurement ranges.
The probe is placed in the magnetic field such that the magnetic
lines of force intersect the major dimensions of the sensing element
at a right angle. Placement and orientation of the probe is very
important and will be discussed in a later section.