Gas Turbine Handbook : Principles and Practices

Boroscope Inspection 207
the entry port determines the maximum diameter boroscope probe
that can be used. It is always desirable to use the largest diameter
probe that will fit easily through the opening to obtain the largest,
clearest image and the brightest illumination. Also to be considered,
primarily with rigid boroscopes, is the length of the boroscope probe
and the object distance. The length of the boroscope probe is determined
by the size of the gas turbine and the distance from the viewer
to the object to be viewed. The object distance is the measurement
from the window of the object lens to the surface being viewed. The
location of the area to be inspected relative to the access port dictates
whether the boroscope views directly ahead or at some angle. This is
an important consideration when using a rigid-type boroscope. However,
an articulating, flexible boroscope can accommodate almost any
viewing angle.
A major consideration in the selection of a boroscope is the field
of view (FOV). FOV falls into three categories: narrow, normal, and
wide angle. Where
• Narrow FOV ≡ 10 to 40 degrees
• Normal FOV ≡ 45 degrees
• Wide Angle FOV ≡ 50 to 80 degrees.
Figure 13-2. Courtesy of Olympus America Inc. The flexible boroscope
is used for viewing those hard to get at areas. This instrument
can also be used in most areas viewable with the rigid boroscope.
Flexible boroscopes are available in a range of diameters and working
lengths.