Hydrofarm has a good guide on the details of CO2 injection (Hydrofarm - Carbon Dioxide Enrichment Methods. This is mainly a guide to replicate their system using off-the-shelf parts.

For bottled CO2 injection you'll need:

1. Tank - Usually rented when you get it filled
2. Regulator - Make sure the regulator output is within the working range of your flowmeter and solenoid.
3. Solenoid
4. Flowmeter -You can use one calibrated for air rather than CO2**
5. Short Cycle Timer/Monitor
6. CO2 Tubing
7. Teflon tape
8. hex nipples (male/male)
9. barbed/pressure fitting - For flowmeter->tubing connection

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**If you cannot find an inexpensive flowmeter with direct read scales for CO2, you can use one calibrated for air with a conversion equation:

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
************************************************** **

You will connect the regulator output to the solenoid, which feeds the flowmeter which feeds the distribution tubing. (Pictures are worth 1000 words). The solenoid cord plugs into the timer.

The general idea is pretty straightforward. Figure out how many cubic feet of CO2 you need to add to raise the CO2 concentration from atmospheric (200ppm) to an arbitrary concentration (for our purposes, 1500ppm). Hydrofarm has already done the hard math and you can get it from the link. For the impatient, here's a summary (Substitute your own values accordingly.):

Release Interval: 2 hours
Atmospheric Concentration: 200ppm
Desired Concentration: 1500ppm
Raise concentration by: 1500ppm - 200ppm = 1300ppm
Room Volume: 8x8x8 = 512 ft^3
1300ppm in 512 ft^3 = "1300 millionths of 512" or .0013 * 512 = .66 cu. ft.
So you need to release 0.66 cubic feet once every two hours.

At this point you need to set your timer and flowmeter to a combination that will release 0.66 cubic feet every two hours. Ideally you'll run the solenoid for as short of a time as possible, but this depends on the resolution of your timer and its shortest possible "on-time". You will need to correlate your flowmeter range to the resolution of your timer and the size of your room.

To find the flowmeter range you need take the resolution of your timer (say 30 minute increments) and figure out what flow would be required to reach .66 cu. ft. in 30 minutes. In this case it is 1.32 CFH. (.66 CFH for an hour would give you .66CF. For half an hour, double the rate)

If you have $$ and time, the C.A.P. CO2 monitors are rebranded GE Telaire units. The "portable" monitors that G.A.C sells are also GE.

This isn't very organized. It's mainly just thoughts/notes from my own experience that may help others out. Maybe I'll come back and clean this up and turn it into a proper guide. I think the key points are in sourcing the proper parts and doing the math for flow rate.