Just got my 2ft QualGear extension to replace a 1ft Omnimount extension. Expected this to be a simple switch. Not so: the QualGear extension tube I received is described as "NPT-compatible", but it does not follow NPT standards, in that it is not tapered. NPT standard dictates a 1-in-16 taper on the threaded part (male ends are narrower, female ends are wider). The diameter and the threading pitch do match, so you can start threading the extension pipe into an NPT coupling, but it will block after too few turns, because the internal diameter of the NPT female coupling decreases as the male end goes in deeper. The result: the extension does not fit properly (it's loose in terms of angle, because it's a straight piece jammed inside a cone, so it has some angular freedom) and cannot be fixed into place with the coupling's set screw (because it does not go in deep enough).
Too bad, as the tube seems to be high quality, with a handy cap over the old-fashioned (for large plugs) huge opening common in most such pipes. I'd have been happy to use it, but it cannot be used with the two NPT couplings of my Omnimount holder, so I am going to have to return it.
QualGear QG-PRO-PM-PC-B Pipe Connector for 1.5-Inch npt threaded pipes, Black is used to join two or more columns to extend drop length.