The rotor turns at a speed directly proportional the velocity of the fluid.  The rotor has a magnet on it which talks to a magnetic pickup on the meter body so every revolution of the rotor makes a pulse.  The pickup then goes to a converter which counts the pulses and (usually) converts the pulse signal to a 4-20mA output signal.  

It's just an impulse-momentum thing.  Not rocket science.  Get out your fluid mechanics book and look at the chapter on fluid machines.  It's a propeller operating in reverse.

You can also contact the manufacturers of those things, like Halliburton or Brooks; they are usually a lot of help.  I have a TON of practical experience with these meters, so just ask if you need any more help.

Thanks!
Pete
P. J. (Pete) Chandler, PE
Principal Engineer
Mechanical, Piping, Thermal, Hydraulics
Processes Unlimited International, Inc.
Bakersfield, California USA
pjchandl@prou.com

Hi Pete.  

I'm just a chemie, but havn't you described a turbine meter?

I understand a rotameter as a variable area flowmeter ... like a heavy ball sitting in the bottom of a vertical cone; the cone that gets wider the higher up you go inside of it.  Fluid flow coming into the bottom of the cone drags the ball up from the bottom.  The cross sectional flow area of the cone increases with height, such that fluid velocity reduces around the ball as the ball rises.  Where the ball comes to rest in equilibrium with the moving fluid depends upon the flowrate .. which, for example, is written on the side of a glass rotameter (or magnetic measuring device etc).  

Excellent, easily found, description is at:  http://www.svce.ac.in/~msubbu/FM-WebBook/Unit-III/Rotameter.htm