These penetrameters were formerly the subject of MIL-STD-453. In common with the cost cutting measures of many governments, the US military has delegated most standardisation activities to a civilian/commercial body. So, MIL-STD-453 has now become ASTM E-1742. The penetrameter design and numbering system remain the same.
The penetrameters have three holes: 1x, 2x and 4x the plaque thickness, a material identification (e.g. FE) and the penetrameter number in lead. The overall length is slightly greater than the ASTM E 1025 types.
The penetrameter number is 50x the penetrameter thickness and indicates, in inches, the specimen thickness for which it is intended.
Penetrameters (Image Quality Indicators / IQIs) to ASTM E-1742 (MIL-STD)