There are small ball flow meters that are very inexpensive that could be used. IDK any names OTOMH, but we used to have a full kit of these. They have a small knob that moves a conical needle in and out of a tapered conical chamber. The water pressure is adjusted by the distance of the needle and the ball floats on the incoming water and rises to indicate the rate of flow.
They are small, about 1/2 inch on each side and about 3" long. They have standard threaded connectors at each end for easy attachment to equipment. The only drawback is that these must be in a vertical position to work.
We had them on every tank of chemicals in our main testing labs so that they knew precise wash and replenishment rates, and this data was used to derive the data sheets you see posted on Kodak publications.