Tube vs Pipe
The difference between pipe and tube is marginal, they are almost interchangeable. They differ in the way they are sized. The pipe is generally specified by the internal diameter (ID) whereas tube is specified by the outside diameter (OD), but its dimensions may be given as a combination of ID, OD and the wall thickness.
A pipe is used for plumbing purposes and it is measured in IPS (iron pipe size). Copper tube is also used for plumbing purposes and it is measured nominally as it is done on the basis of average diameter.
It is interesting to note that the word ‘pipe’ is used as plural especially when used in the process of agricultural irrigation. There is a marked difference between pipe and tube when it comes to the method of their making.
Piping is done with rigid joints whereas tubing is done with rigid hard tempered joints. Tubing comes in soft tempered rolls as well. It has to be remembered that the sizing is not affected by the temper of copper. It can be rigidly joined or flexibly joined for that matter. The nature of the joint has nothing to do with the size though.
Thickness is another factor that draws a line of difference between tube and pipe. It is for sure that the thickness of a tube and a pipe can vary. The thickness of tube wall can be different from the thickness of the water pipe. The thickness of the pipe wall increases by schedules.
As a matter of fact the thickness of tube comes in four types, namely, the DWV type characterized by thinnest wall, type M characterized by thin wall, type L characterized by thicker wall and type K featured by thickest wall. Thus the DWV type of tube thickness is exactly opposite to the K type of tube thickness. A plumbing expert is supposed to know all these details. It is for sure that the feature namely the thickness of the wall does not in any way affect the piping or tubing size.
The term tube is more widely used in the USA and pipe elsewhere in the world.