If you want the Dywer flowmeter, get the dywer RMA-151-TMV, or RMA-151-SSV(or BV), it is the model with 5-50cc/min AIR flow rate at standard psig.
The RMA-151-TMV is top mounted valve, you can lock the steady pressure (regulator output pressure) in the flow tube, and get the actual flow rate according to specific pressure.
The RMA-150-SSV(or BV), is bottom mounted valve, the pressure inside the flow tube is the same as back pressure from your diffuser/check valve.
I don't have calibration sheet for actual CO2 flow rate at different pressure range though.
get the key instrument flow meter, higher precision than the RMA151, evil bay item number: 322765167020
there are not many left...
here is the calibration data for co2 at different pressure settings.
♣ if you don't know what I am talking about, don't mess with dywer or any other flowmeter, just get a bubble counter and you will be settled.