I have a flow meter that looks just like the picture you posted and so far I think it is junk. The ball bounces up and down so much it is hard to tell what the reading is. I use the bottle test method, but do stick the meter in everyonce in a while just to see how it reads.
I will say this, as I have increased the production of my cells, I have noticed that the guage works better. I suspect it may work fairly well upwards of 3 liters or so. I am going to observe a dyno test on a large HHO system next week, and I'll try to remember to take that thing with me and see how it works with higher producing cells.
1991 Plymouth Acclaim 3L V6.
1 dry cells with nineteen 6"x8" 316L ss plates, driven by constant current PWM set at 35 amps (13.3V at PWM). 28% KOH electrolyte. Total measured output 2.5 lpm. Mileage went from 18 to 26 mpg, all city driving (44% increase). EFIE set at .370 and I still need to play with ignition timing.