This clearly demonstrates that the world is even more weird than I imagined (clicky for more dots):
I don’t measure that many big cylinders that I can’t figure out their diameter by hand.
The prices suggest folks who need Pi Tapes need them bad.
Pi Measuring Tape
The Smell of Molten Projects in the Morning 
Pi tapes are expensive because there really is no other alternative except a big old micrometer which costs really big bucks and takes up a huge amount of space. Drop a big mike accidentally and there is a good chance that it is ruined. Pi tapes are easy to use, pretty accurate and occupy little space when rolled up. Of course a pi tape can’t measure out of roundness and they do seem to be a tad pricey though.
Not that I’ve ever deliberately dropped a precision tool…
Or, for that matter, a small part. [heavy sigh, searches for flashlight]
this is a variation of Circumference ruler used by sheet metal workers, I have a three foot Circumference ruler that I used to use for sheet metal work – one edge is standard inches and along side it is the circumference length – nice for making circular duct work –
Long ago, I built a sheet metal adapter box from the clothes dryer’s 4 inch round exhaust port to the 3×10 inch rectangular hole in the house wall, offset by half the dryer width and jammed into the available three inches of free space. That project gave me a profound respect for sheet metal workers, a deep appreciation for all my fingers, and a desire to never do that again.
A tip o’ the cycling helmet to you for needing a Circumference Ruler!