Measuring Tubes With Large Diameter

Speaking off the top of my head, I imagine you're referring to an encoder.

A rotary encoder is a device with an input shaft which, when rotated, results in an output of electrical pulses. Various types are available, giving typically 100 to 1000 pulses per revolution (but lower and higher resolutions are also available). If an encoder was fitted with a wheel of known circumference, bearing against the rotating tube, the circumference of the tube could be calculated by counting the output pulses for one revolution of the tube.

For something with a large outside diameter, you can purchase a "PI TAPE" (a steel tape for a specified range of diameters that have been mutiplied by PI and are accurate to the thousandth of an inch). For large inside diameters you can get a 2" inside micrometer "head" that is spring loaded and has a .0005" increment dial indicator on the other end, this combination is connected to a piece of conduit or you can purchase the body (same function as conduit) from a supplier. The micrometer head with indicator is probably about $1000.00 US. I have used them to inspect diameters up to 300 inches and are about as good as a FARO Laser Tracker. email questions to phoenix66109@hotmail.com

Or you could measure the OD with an ordinary tape & divide by pi.

wouldn't you multiply by pi ?

I'd just eat the pie......

GUEST ! is correct, a good quality PI tape is a very accurate bit of gear, often supplied with a calibration cert. and calibrated @ 68.4 F, used correctly they will measure quite accurately. I would not recommend using an ordinary tape measure they are no where near accurate enough. I have used PI tapes to measure 2 metre dia. parts on a machine where a replacement part has to be made and the machine could only be stopped for a few minutes to allow me to take sizes.