Recently I learned a new word for what I'm looking for -- rotameter -- a device that uses a weight in a conical bore to measure fluid flow.
I've found many for gasses, but not many for liquids, and none for the range I'm looking for.
To first order approximation do I need to do more than compare the viscosity of air to that of water to use an air calibrated one for water?
The viscosities differ by about a factor of 50. Would a 5 liter/minute gas rotometer be a 6 liter per hour water rotameter.
Caveats: In a gas meter the bead may be plastic, which has no buoyancy effect in air, but a substantial one in water. Indeed the plastic might float, and I'd have to use the thing upside down. Plastic with a specific gravity of .9 would make the device 10 times more sensitive.
Depending on what's dissolved in the water, the viscosity can change somewhat. I will be using more or less pure tapwater with about 600 ppm of assorted hardwater crud.