The nominal diameter used to be an integer numerical number for the inside diameter of a pipeline or a fitting in mm.
Example: A DN 100 pipe had an inside diameter of (about) 100 mm.
Since a large number of pipes, especially steel and plastic pipes with different wall thicknesses, have been produced in the meantime, this information should be more precise.DIN EN 805 therefore recommends using either the combination DN/OD or DN/ID. These designations are also used, for example, in DIN 11853-1, -2, -3 and DIN 11864-1, -2, -3.
Average inner diameter of the pipe.
EN ISO 6708 defines DN as follows: "An alphanumeric description of the size of components in a piping system used for reference purposes. It comprises the letters DN followed by a dimensionless integer indirectly related to the physical size of the bore or outside diameter of the connections, expressed in millimetres.
The standards mentioned can be obtained from www.beuth.de.
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