a Penetrant Inspection Improves the Detectability of Flaws
The advantage that a liquid penetrant inspection (LPI) offers
over an unaided visual inspection is that it makes defects easier
to see for the inspector. There are basically two ways that a
penetrant inspection process makes flaws more easily seen. First,
LPI produces a flaw indication that is much larger and easier
for the eye to detect than the flaw itself. Many flaws are so
small or narrow that they are undetectable by the unaided eye.
Due to the physical features of the eye, there is a threshold
below which objects cannot be resolved. This threshold of visual
acuity is around 0.003 inch for a person with 20/20 vision.
second way that LPI improves the detectability of a flaw is that
it produces a flaw indication with a high level of contrast between
the indication and the background also helping to make the
indication more easily seen. When a visible dye penetrant inspection
is performed, the penetrant materials are formulated using a bright
red dye that provides for a high level of contrast between the
white developer. In other words, the developer serves as a high contrast background as well as a blotter to pull
the trapped penetrant from the flaw. When a fluorescent penetrant
inspection is performed, the penetrant materials are formulated
to glow brightly and to give off light at a wavelength that the
eye is most sensitive to under dim lighting conditions.
Additional information on the human eye can be found by following
the links below.