As foxysox said, you can run bottled CO2 injection for under $250.

If someone were looking for CO2 and didn't want to go near a hydro store I would take a look at some aquarium stores . CO2 injection is used in living plant aquariums with a few different/extra parts.

For your room you will need:
- CO2 bottle (20lb is usually enough)
- CO2 regulator
- CO2 solenoid (120V)
- Flowmeter (Ideally calibrated for SCFH CO2, but most likely air)
- 1/8" MPT to 1/8" MPT hex nipple (1" should be sufficient)
- 1/8" MPT to 1/4" Barb fitting
- Silicone or CO2 tubing (1/4")
- Timer or controller

BOTTLE
You can usually buy or rent a bottle at a gas supply.

REGULATOR, SOLENOID and FLOWMETER
Any regulator with standard threads and an output within the working pressures of your solenoid and flowmeter will work.

You will find several brands of CO2 solenoids if you search for aquarium CO2 injection. Any should work.

Flowmeters may be tough to find. Ideally you want yours in "SCFH CO2" but you may only find them in SCFH Air. Not a problem as long as the meters materials can be used with CO2 gas. Use this conversion if you have one for air:

Q2 = Q1 * sqrt(1/1.5189)

Q1 = Observed flowmeter reading
Q2 = Actual flow corrected for specific gravity
1 = Specific gravity of air
1.5189 = Specific gravity of CO2

More specific information on the math in my other post: http://boards.cannabis.com/indoor-growing/121494-diy-co2.html#post1501420"]http://boards.cannabis.com/indoor-gr...ml#post1501420

Milwaukee makes a nice dual gauge regulator combo that includes a 120V solenoid, needle valve and bubble counter. You can unscrew both the bubble counter and needle valve and set them aside. You won't need them.

TIMER/CONTROLLER
You can get an excellent digital intermatic timer (7 day, no astro auto-adjust) for around $30. You just want a timer that can do 15 minutes or less on time, every 2 hours at least 6 times. (Only needed during light cycle of flowering)

The CAP units are GE Telaire sensors. If you're familiar with PIC or something similar you should have no problem setting up a test loop and tripping a relay. Shouldn't need much more explanation if you're comfortable with this.

ASSEMBLY
Regulator connects to the solenoid input, solenoid output connects to the hex nipple, hex nipple connects to the flowmeter input, barb goes into flowmeter output then tubing on the barb. Use thread tape where appropriate. FOLLOW THE INSTRUCTIONS THAT COME WITH YOUR REGULATOR

If you have an adjustable regulator, make sure your regulator output pressure is turned down/off each and every time you open the tank valve. Once the tank valve is open you can slowly adjust the reg pressure.

I'm not going to go into the math of flowmeter selection (based on room size and minimum on-time of timer), time calculations or gas conversions as I already covered these in another post: http://boards.cannabis.com/indoor-gr...ml#post1501420