Flow meter

You don't say what you are measuring the flow of ( and it makes a difference), more information is needed, but let me give you an example.

In my shop we have to measure the flow of natural gas and air. Typical would be a Rosemont flow transducer, (also called a transmitter or sensor). reading the pressure drop across an orifice plate. Often the output is 4 to 20ma, and an instrument calculates the flow. There are other types, like ultrasonic, vortex, magnetic, etc.

You could possibly find what you are looking for here:

http://www.emersonprocess.com/rosemount/products/flow/index.html

or here:

http://hpsweb.honeywell.com/Cultures/en-US/Products/Instrumentation/flow/default.htm