Difference between nominal diameters DN65 and DN70

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Many of the practices and names used in our work are followed based on convention, with few people delving into them in depth. It’s also a fun thing to explore when you have time. For example, is there such a specification as DN70 for water pipes? What is the difference from DN65 exactly? Actually, we usually specify DN65 uniformly to avoid misunderstandings; in reality, DN65 and DN70 refer to the same pipe diameter. So where does this practice come from? Why is DN70 labeled as DN65 – what is the reason for that? The following is for reference. What is nominal diameter? Generally speaking, the diameter of a pipe can be divided into outer diameter, inner diameter, and nominal diameter. DN refers to the nominal diameter, which is a standard established artificially for the convenience of design, manufacturing, and maintenance. It is also known as the nominal bore, and it represents the specification of a pipe (or fitting) ； The nominal diameter can be considered a nominal size of a pipe; it is not equal to either the inner diameter or the outer diameter of the pipe. De refers to the outer diameter of the pipe; it is used to denote plastic pipes, but the wall thickness also needs to be specified. d refers to the inner diameter ； Generally, concrete pipes and clay pipes are denoted by d. Φ refers to the outer diameter; generally, the outer diameter of seamless steel pipes is denoted by Φ, but the wall thickness must also be specified. What is the difference between DN65 and DN70? **Product standard \"Welded steel pipes for low-pressure fluid transport\" GB/T3091-2015: As can be seen from the table above, for the standard DN65 welded pipe, the inner diameter is approximately 70 mm.