Or alternately what is the name of the device I'm looking for?
My first thought is to make a narrow V shaped chamber by gluing 1/4" acrylic together with 2 face plates and a mid plate with a triangular hole removed. The liquid comes in at the bottom and lifts a slightly heavier than water bead. Faster flow has to get further into the V chamber before it spreads far enough to match the net gravitational down on the bead.
At 10 liters per hour, flow through a 1/4" pipe is about 9 cm/s.
This seems like such a simple way to do it, I'm sure there are off the shelf ones available for a few bucks. But often searching depends on knowing what it's called.
This is one that I found on ebay, but intended for gasses, not liquids.
this sort of design with 2-4" plumbing is used for pneumatic seed sorters. However in those, you are indifferent to turbulence. For this application to have something reasonably repeatable, I think I need non-turbulent divergence.
Purpose: I'm trying to make a fertilizer injector with minimal pressure drop. Overall the notion is to put a slight constriction in a 3/4" pipe creating a small (few oz) pressure drop. Two small diameter hoses connect to the bottom and top of a chunk of PVC pipe with the fertilizer inside it. Since the pipe is pressurized to the line pressure, the differential across the constriction will move water through the device.
A small valve on the 1/4" line can be used to adjust the flow to put the bead at about the same place each time.
Another thought would be to use something like the IV drip chamber. This is a comparable flow rate. For long duration, I don't know if you could maintain air in the chamber.
Are there other effects I could use?