I'm using the DIGITEN G1" Water Flow Hall Effect Sensor Switch Flow Meter Flowmeter Counter 1-60L/min from Amazon for $10.99. Threads aren't exactly NPT, but if you need NPT, you can cut them easily by cutting channels in a female NPT connector and threading it on. You can get accurate flow by measuring time between pulses and accurate measure by counting pulses. It needs 5V to operate but doesn't output 5V. It might be a could idea to use a diode to protect the Omega input, although it probably isn't needed.
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RE: Water the Garden