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CO2 Calculations and Setting Timers

The normal atmospheric CO2 level is about 395 ppm. The ideal range for maximum plant growth is 1500 ppm. That means we need to increase CO2 levels by 1100 ppm (or .0011) in the garden area.

First we need the volume of your garden area. This is length x width x height. For our example we will use a 5 x 5 x 8 foot room, which has an air volume of 200 cubic feet.

Next, we calculate how much CO2 to add to the garden area by multiplying the volume of your garden by .0011. For our example this would be 200 x .0011, which equals .22. So, it will take .22 cubic feet of CO2 to bring the garden in our example up to 1500 ppm.

Now to your CO2 tank regulator. On the tank there is a pressure regulator valve and also a flow control valve. The pressure valve is usually set between 50 and 100 psi, depending on the recommendations for the equipment you have purchased. The flow control valve is adjustable, and cubic feet per hour (CFH) is used for the measurement.

In our example, if you set your flow control to .22 cubic feet per hour, and programmed a timer to leave the solenoid valve open for one full hour, you would slowly bring the room up to 1500 ppm by the end of the hour (assuming you didn’t have to exhaust in the meantime to bring your temperature down). The following is a more realistic way to set this up…

Set your exhaust fan to kick on at 90 – 95 degrees, and kick back off at the ambient room temperature. This will cause the room to get a little warm for a short time during the cycle, but it should not do any harm. Watch it for a while and determine how long You have in between exhaust cycles (while the fan is off). For maximum plant growth, you want to achieve 1500 ppm in the garden area at least 2/3 of the way thru this time period. Let’s use our example one more time…

Let’s say you watch the system and determine you have 30 minutes between the exhaust cycles. You need to release .22 cubic feet of gas to bring the room up to 1500 ppm, and you need to do it in less than 20 minutes (2/3 of the way through 30 minutes). Releasing the necessary amount of CO2 within 10 minutes would certainly be within these guidelines, and would give your plants a little extra time in the fully enriched environment.

The relationship between time and your flow rate is inversely proportional. That simply means if you take 1/4 the time, you need 4/1 flow rate (that’s 4 times). In this example, we want to release the required amount of CO2 in 10 minutes, which is 1/6 of an hour (60 mins x 1/6 = 10 mins). We therefor need to multiply our old flow rate by 6/1, or by 6 in other words. The result is…

.22 CFH x 6 = 1.32 CFH

Now you set your flow rate to 1.32 cubic feet per hour and program the CO2 timer to come on for ten minutes immediately following your exhaust cycle. All done, now you are ready to maximize your plant growth!