Radiographic Examination – Radiograph Quality

The acceptability of a radiograph for the detection of flaws is determined with a device, referred to as a “penetrameter,” that is placed upon the surface of the workpiece when the exposure is made. Penetrameters are relatively thin pieces of material with radiation attenuation characteristics similar to the workpiece that contain holes with diameters of 1, 2, and 4 times the thickness, as illustrated in Figure 700-24.

Penetrameter for Evaluating Quality of Radiograph

The quality level required for a radiograph is designated by a two-part expression X-YT, where X is the maximum thickness permitted for the penetrameter as a percentage of the thickness of the workpiece, Y is the diameter of the hole as a multiple of the thickness, and T is the thickness. A quality level of 2 to 2T is adequate for most applications of RT for the in-service inspection of pressure vessels, and is consistent with the requirements of ASME Code, Section V.

A radiograph is considered to be acceptable for the quality specified if the entire outline of the penetrameter is visible, the density of the penetrameter is within the required range of 1.8 to 4.0, and the hole is discernable. However, the acceptability of the radiograph for flaw detection is limited to areas that have densities within 15% under and 30% over the density of the hole in the penetrameter. These limits may not encompass the entire range of densities in a radiograph, and interpretation of areas with densities outside these limits is of questionable validity. It is important to recognize that the workpiece must have an essentially uniform thickness for the background density of the radiograph to be within these limits.

Note that the penetrameter functions only to determine that a radiograph has acceptable quality. The penetrameter does not serve as a calibration standard. Therefore, it should not be used to estimate flaw sizes, and should not be used to establish acceptance limits for flaws based upon relative densities in the radiograph.

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