Outer diameter, outer diameter

It depends on the standards you use; if it’s process piping ASME B36.10/B36.19, then those are the results you’ll obtain.

You mean that different standards lead to different results, is that it?

Yes, you need to check the specific details of the standards and the size list.

The outer diameter of 1/2\" OD tubing for instruments is 12.7 mm, while the outer diameter of 1/2\" galvanized tubing is 21.3 mm.

This is the difference between Pip and Tube: 1. Both are referred to as tubes, but the difference lies in the following: 1) A Pip tube is a circular-section tube manufactured according to relevant standard specifications; its size is indicated by the nominal diameter DN or NPS. Tubes with the same nominal diameter have the same outer diameter, while their wall thickness can vary depending on design requirements. 2) The Tube need not be manufactured in accordance with the above requirements; it can be a tube with a circular cross-section or one with another shape of cross-section (such as rectangular). The specifications of a circular tube are determined by two of the three parameters: outer diameter, inner diameter, and wall thickness. 2. The instrument tubes are usually of the Tube type; for example, a 1/2\" tube has an outer diameter of 1/2\", and converting this value, 1/2 * 25.4 = 12.7. For process pipelines, Pip pipes are used; the pipes have a nominal diameter of NPS 1/2 (DN15). Their outer diameter is not equal to 1/2*25.4=12.7, but rather there is a standard series for such pipes, and this standard series can be found in the relevant specifications. For example, those mentioned by a colleague upstairs such as ASME B36.10/B36.19, as well as the standards we commonly use like SH/T3405 and HG/T20553. 3. Generally, the outer wall of Tube pipes is required to have a higher level of smoothness; they are mostly connected using clamps or buckles, whereas Pip pipes are usually connected by welding or threading.