T.O. 33B-1-16-64Figure 6-31. Penetrameter Information.6.7.3.1.1The penetrameter has lead numbers permanently attached to indicate the material thickness on which the penetrameteris to be used. In Figure 6-31, the ID number indicates the penetrameter is for use on a 0.750-inch test object. Thethickness (T) of the penetrameter is normally made to be 2 percent of the test object thickness. Therefore thepenetrameter with an ID of 6.0 inches would be 0.120 inch thick. Except in some special instances, plaquepenetrameters less than 0.005 inch in thickness are not available. Therefore, in normal operation, the 0.005-inchpenetrameter is used on test objects whose thickness may be 0.25 inch or less.6.7.3.2 ContrastSensitivity.The penetrameter material thickness is added to the thickness of the test object. This increase in thickness causes moreradiation being absorbed, and the penetrameter outline is seen on the final image as a less dense area. This change infilm density due to the additional radiation absorption is a measure of the image contrast. The human eye is normallyused as a detector in reading radiographic images, and the eye responds to differences in the quantity of light beingtransmitted through the film due to the density differences. It is usually assumed that under practical industrial filminspection conditions the human eye is capable of just detecting density differences of LD = 0.02 which corresponds toa 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.
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