Figure 6-31. Penetrameter Information.
The penetrameter has lead numbers permanently attached to indicate the material thickness on which the penetrameter
is to be used. In Figure 6-31, the ID number indicates the penetrameter is for use on a 0.750-inch test object. The
thickness (T) of the penetrameter is normally made to be 2 percent of the test object thickness. Therefore the
penetrameter with an ID of 6.0 inches would be 0.120 inch thick. Except in some special instances, plaque
penetrameters less than 0.005 inch in thickness are not available. Therefore, in normal operation, the 0.005-inch
penetrameter is used on test objects whose thickness may be 0.25 inch or less.
The penetrameter material thickness is added to the thickness of the test object. This increase in thickness causes more
radiation being absorbed, and the penetrameter outline is seen on the final image as a less dense area. This change in
film density due to the additional radiation absorption is a measure of the image contrast. The human eye is normally
used as a detector in reading radiographic images, and the eye responds to differences in the quantity of light being
transmitted through the film due to the density differences. It is usually assumed that under practical industrial film
inspection conditions the human eye is capable of just detecting density differences of LD = 0.02 which corresponds to
a light transmission difference of 4.72 percent. Since density differences of 0.02 are considered just barely discernible,
good practice is to strive for a density difference of 0.08 to assure good visualization of discontinuities.