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Yep, it's an EK flow indicator with the rotor taken out. Definitely not my idea (sadly I don't remember where I saw it first, so I can't give credit), but it is indeed genius - works really well, especially with that foam inside (otherwise the small volume makes it likely that air bubbles will be pulled back into the pump). Though I would probably recommend going with something other than this EK unit (it was the only one I could easily get ahold of): it has a really thick back wall - nearly half the unit's thickness - which takes away a lot of what could have been usable volume. I've considered drilling it out, but I'd need a drill press and even then I'd likely crack it (and it would require tearing down the loop again), so I'm leaving it as is. Something like this Alphacool unit makes much better use of its total volume. Anything with three ports and where the rotor can be taken out should work well.