Originally Posted by
WizardOfBoz
The Turnbull circumference tape is nice. I'm familiar with the (very expensive, machinist-oriented) Pi Tape. The tape is has linear measurements (times pi) etched on it. There's an offset and the witness line has verniers. You can read the diameter directly. Work great, but they cost hundreds of bucks. In use:
This interesting gizmo I take it you don't need to wrap around a pipe, it is enough to have access to just a part of the pipe's curved body, right?
From the preceding, I like most the spanner wrench method. That's exact enough to make you decide which of the two kinds of diameter you have.
As to calipers, everybody should have one of those cheap plastic ones at minimum that sell for several dollars. I picked up ones with mechanical readout on round dial gradated 1/64th inches, because it was on sale but having grown up in metric world, I still have problems with imperial readouts. Takes me some time squaring it in my head and sometimes I can't make it LOL. Next time I see some good sale on digital, I will get it.