I've been toying with the idea of a flow meter for months.
However, what puts me off every time is the restriction.
The ones that are within my budget that I can source, in the predicted flow rates (2 - 2.2 GPM) range from .33 to .7 bar pressure drop, depending on model and choice of fittings.
That's the same restriction as adding an extra Apogee GTX (best case scenario) or an extra Stealth (worst case scenario) just for the sake of measuring flow.
And that's forgetting about using a filter, which should be used unless you are extremely confident about the cleanliness of your loop. If you add a filter, you'd rather shoot your own foot
Not an easy choice.
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