> I'm looking for insight on measuring water flow rate inexpensively.

"Rate" may mean volume per second or velocity. The former is more
rigorous but the latter not unknown.
Mass rate may also be used.

> To choose an appropriate method, fill in the blanks:
> a) max/min flow rates?
> b) required accuracy?
> c) low cost means max price < ?
> d) fresh water, salt water, waste water?
> e) any harmful chemicals present?
> f) temperature range of flow?
> g) ease of installation/replacement?
> h) reliability? (just curious about the flow or do your fish die if it's
> wrong?
> i) ???

- In a pipe / In an open channel.

- Flow can be assumed to be constrained by gravity to bottom of pipe
or channel  / no assumptions on where flow is is allowed

- Flow cab be partially obstructed / greatly obstructed / may not be

- Large / moderate / some / no pressure drop allowed at measuring point.

Orifices, notches and weirs with head and/or pressure change or level
are easy and time honoured and may not be appropriate.

Propellors and similar allow inferred flow.

Positive displacement systems allow great accuracy.

Ultrasonic systems and correlation systems allow obstructionless flow.

....

    R
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