Alternately, if you don't need to actually measure flow so much as merely know that it's happening, I've been very happy with a Gems flow switch as a safe-guard on my RIMs recirculation. The body is stainless, though the actual mechanism is a polymer of some sort.
Arduinos, as others have said, can handle this and much, much more. I'll never understand the antipathy towards them.
"Be excellent to each other." -Benjamin Franklin