I don't want to dissapoint you but I had a look at the 0100B110 datasheet on Proteus Industries's website:
1) It's a flow switch with user settable trip point, not a flowmeter, so no way to do RPM monitoring.
2) Output is a 120VAC-3A max SPDT relay contact.
3) Pipe connections are only 1/4" NPT, pressure drop is 6psi!
4) Flow speed is indicated visually by rotor speed.
There is another possibilty if you have some serious electronic skills:
Dismount the electronic circuitry to analyse the actual schematic.
I suspect the rotor to be a sort of a hall effect generator with magnets.
Create your own schematic using the hall generator's signal to output an open collector signal proportional to the flow speed and suitable for RPM monitoring.
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