外徑表示數學中（zhōng）的直徑。一般（bān）也用字母（mǔ）d來表（biǎo）示。外徑和內徑是相對的. 就比如一根焊管,外圍的那個最（zuì）大（dà）圓的直徑是外徑,管裏麵那個圓的直徑是內徑. 它的外徑=內徑（jìng）+2*焊管的厚度 就（jiù）是兩個同（tóng）心圓,外徑是焊管大圓的直徑,內徑是焊管小圓的直徑（jìng）. 外徑和內（nèi）徑都屬於直徑,隻是分別又有特指.
The outer diameter represents the diameter in mathematics. The letter D is also commonly used. For example, in a welded pipe, the outer diameter of the largest circle is the outer diameter, and the inner diameter of the circle is the inner diameter. Its outer diameter = the inner diameter + 2* is two concentric circles, the outer diameter is the diameter of the larger circle of the welded pipe, and the inner diameter is the diameter of the smaller circle of the welded pipe. 下麵說下直縫焊管的（de）最（zuì）新外徑標（biāo）準參數介紹
Following is the introduction of the latest external diameter standard parameters of longitudinal welded pipe
DN600- ф 630mm
DN15- ф 22mm DN20- ф 27mm
DN25- ф 34mm DN32- ф 42mm
DN40- ф 48mm DN50- ф 60mm
DN65- ф 76 73 mm DN80- ф 89mm
DN100- ф 114mm DN125- ф 140mm
DN150- ф 168mm DN200- ф 219mm
DN250- ф 273mm DN300- ф 324mm
DN350- ф 360mm DN400- ф 406mm
DN450- ф 457mm DN500- ф 508mm
其中DN表示公稱直（zhí）徑： DN是公稱通（tōng）徑，公稱通徑(或叫公稱（chēng）直徑),就是各種管子與管路附件的通用口（kǒu）徑。同一公稱直徑的管子（zǐ）與管路附件均能相互（hù）連接，具有（yǒu）互換性.它不是實際意義上的管道外徑或內徑，雖然其數值跟管道內徑較為（wéi）接近或相等； ф表示外（wài）徑。
Among them, DN denotes nominal diameter: DN is nominal diameter, nominal diameter (or nominal diameter), which is the general diameter of all kinds of pipes and pipeline accessories. Pipes and pipe accessories of the same nominal diameter can be interconnected and interchangeable. It is not the actual outside diameter or inside diameter of the pipeline, although its numerical value is close or equal to the inside diameter of the pipeline; indicates the outside diameter.
