5lb c02 tank with regulator...is it possible?

I'm about to start my grow with 40 plants. 20 sour d and 20 headband. I have 2 5lb tanks of c02 from my kegerator and was wondering if it's possible to use these for inside the room. It has a regulator, but not the timer and exact measurement. Is it possible to just turn the tank on for a few minutes and shut it off to work, by guess work? can i just open the valve and let it go or is there something else i'm missing? any help would be greatly appreciated.

oh yeah the veg room size is 3x8x8 and the flower room is 6x8x8. (wxlxh)

I wouldn't....I mean you can vastly underdose, and it may help a little, but it's so hard to tell without the scfm measurement on your regulator. I don't have scfm, I have a bubble counter (5lb co2 as well, but for fish tanks, we grow mad plants too)...anyway, I tried it for about a week, until I left it on too high, almost passed out in the grow room, left my nice hanna meter sitting in the open drying out, and passed out in the hallway a few rooms up n' over. Luckily I didn't pass out in the co2 room, cause I would have been a goner. It's too risky without proper measurement cause otherwise you could really kill yourself.

i thought you said you had a regulator, no.If not ,get one.then you can look at the co2 caLculator in indoor, its a sticky. just put in your room size and put in what setting you will use and it will tell you how long to run it. here it is.http://www.hydroponics.net/learn/co2_calculator.asp

regulators do not all have flow readings and the flow readings are different depending on application...that's the problem..he has a regulator, I have a regulator, that doesn't mean it measure scfm, if his does he's golden...

then he will need a flow meter eh

im' going to take a pic of the regulator and tank later tonight.

found a fish supply shop that can get me a 5lb tank regulator w/timer.