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How do I control the amount of CO2 in my growing area?

There are several ways in which a grower can control the level of CO2 in the growing area, assuming of course that the growing area in enclosed.

Unless you are using a digital CO2 monitor/controller or complete environmental controller, you will first need to determine how many cubic feet per hour (CFH) your CO2 device dispenses.. However, if you are using a CO2 Enrichment System in conjunction with bottled CO2, then the output amount can be varied depending on the flow meter setting.

If you are using a C02 tank with an Enrichment System you will need to use the following formula in order to determine the proper C02 level and flow rate for your needs:

  1. Determine the size of the room in cubic feet (CF). This is done by multiplying the length by the width by the height of your room. If your room measures 10’ long by 10’ wide by 8’ high, then your total CF is 10 x 10 x 8 = 800 CF.
  2. Determine what your desired level of C02 is and subtract the existing amount of C02 that’s already present in your room. Most growers will prefer about 1500ppm (parts per million) of C02. Plants will respond to up to 2000ppm, but this amount is generally not used since plants will utilize such high levels only if every other aspect is in perfect balance (which is extremely difficult to achieve). In general, there is about 500ppm of CO2 already present in the atmosphere if you live in a big city, around 300 if you’re in a less populated area. Assuming that there is 300ppm present where you live, you would have to add an additional 1200ppm in order to reach your target level of 1500ppm.
  3. Determine how many CF of CO2 you need to inject. Multiply the volume of your room by the amount of C02 necessary to raise the C02 ppm to the target level: 800 CF x .0012 = 0.96. We will round 0.96 up to 1. Therefore, in order to raise the ppm level in your room to 1500ppm, you will need to inject 1 CF.
  4. Determine the flow meter setting. Assuming that after 3 hours C02 levels will return to normal due to plant use as well as leakage, we will divide the 1 CF of C02 into three 1 hour increments. 1 CF divided by 3 = 1/3 CF or 0.333. Therefore, every hour an 800 CF room needs 1/3 CF of C02 in order to bring it back up to 1500ppm, and so your flow meter should be set to 0.333.

If you will be using a CO2 generator to supply CO2, refer to the following chart to choose the correct size.

NOTE: This chart assumes that you will only have to raise the ppm in your room 1000ppm, not 1200ppm as in our previous example (multiply numbers by 1.2 to achieve values for 1200ppm).

The first row in the chart represents the total CF of your grow room.

The first column in the chart represents the cubic feet per hour that the generator puts out.

Each intersection will give you the amount of time needed to increase the ppm in your room by 1000. For example, a CD-18 generator will take about 4 minutes to increase the ppm by a 1000 in a 1200 cubic foot room.

NOTE: It is usually recommended that you choose a generator that is large enough so that it can fill your room within 10 minutes or less. If it takes more than 10 minutes for a particular generator to fill your room it is recommended that you choose the next size up.

Timing System

One Timer:

There are three types of timers which can be used: 15-Minute Increment Timer, Digital Timer and Repeat Cycle Timer.  Timers with increments of 30 or longer are less desirable, since we want to inject CO2 quickly and have more precise control. If you are not using an exhaust fan, you can get by with just one timer, otherwise you will need two; see below.

Two Timers:

If you are using an exhaust fan in conjunction with CO2, you will need to synchronize the two so that the fan doesn’t come on at the same time as the CO2 and suck it out of the room before it has a chance to be used by the plants. Because of the precision required to this, you would need to use either digital timers or repeat cycle timers. In this situation, you would set the fan timer to come on long enough to vent the room completely, usually 15-30 minutes provided the fan has been sized properly. The CO2 timer would be set to come on right after the fan turns off. You would program the timers to repeat this cycle every hour during the light cycle.

Digital CO2 Monitor/Controller:

Using a digital monitor/controller offers the most precise control by enabling you to set the desired CO2 ppm level and the controller will automatically turn on and off your CO2 equipment to maintain that level. The controller can also be plugged into  an Off-Switcher can be used as in the previous example (just substitute the repeat cycle timer with the controller in the previous example) in order to synchronize your exhaust fan with the CO2 equipment.

Complete Environmental Controller:

A complete environmental controller has the capability to control not only CO2, but also temperature, humidity, water pumps, lighting systems, etc.