You need to have a flow meter with that unit or use a PPM controller. Search eBay for flow meter but make sure it has the proper scale. You want a small scale and not one for an air compressor.

http://www.hydroponics.net/learn/co2_calculator.asp

Use that calculator to figure your flow and how long it need to run.
Zandor Reviewed by Zandor on . CO2 CONTROL QUICK,(I HOPE) QUESTION. I JUST RECIEVED MY CO2 REGULATOR. SHOWN IN PIC. THE BUBBLER HAS NO MARKINGS ON IT. HOW DO I CONTROL THE FLOW TO BE AT 1500ppm WITH OUT A CO2 CONTROLER? CAN I WEIGH THE TANK EVERY DAY? CAN I GO BY THE AMOUNT OF BUBBLES? THE ROOM IS 6'x6'x5', SO I NEED .27 lbs TO WORK IN THAT SIZED ROOM. I JUST DONT WANNA DIE OR WASTE ANYTHING I DONT NEED TO WASTE. THNX Rating: 5