ZRDR CO2 High Pressure Reducing Cylinder Valve CO2 Control (W21.8) Part #R101 
 
CO2 Bottle attachment is W21.8 fitting
Accurate fine-tuning (regulate to 1 bubble/2-3 sec)
Fixed Fine Tunning Value
Material: aluminum alloy and acrylic
Shows only the pressure inside the bottle for the mini single pressure gauge
Includes - Single head bubble counter - Part # W00-02A
Approx size: Height 3" inches, Width 1" inch
 
Note - This is used for low level constant flow (no  solenoid to control on / off)