Controls - Simple flow sensor

So when I reassembled the GW last night, the damper wouldn't open. When I skipped the flow sensor, everything else - including damper - worked as expected. Since I was sure I was full and was pumping, I re-wired around the flow sensor and away we went.

Anyone know where to obtain a replacement? And I'm curious if there is one that actually MEASURES flow, not just an on/off. Seems to me that a variable speed pump could be used in conjunction with this. Slowing the pump down on start-up should speed up the start up, yes? Anyone currently doing this?

Jimbo