I would also question the wisdom of buying a $300 flowmeter for just adding into a system to know flow rates. For around $400 you can get a Promethia, right? Or if straight water is your thing, just pick up a positive displacement pump on ebay and you should be able to know your flow rate from the fact that they stay more or less constant over a wide range of restriction.
If you DO want a flowmeter, and don't mind the paddle wheel types, just check out ebay or labx for used ones. $30-100 instead of $300 and up (way up).
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