Calculating CO2 Enrichment

Many marijuana growers know that enriching the air in their grow rooms and greenhouses can really affect yield.  Operating CO₂ generating equipment properly can be a bit tricky, and dangerous if done incorrectly. To address this issue several companies offer fantastic equipment designed to control CO₂ generating appliances. These controllers are really the best way to sequence your enrichment cycles but are rather expensive. here is of course another way, we can approximate appropriate enrichment periods using this simple formula.

Room Volume (adjusted target CO2 Concentration)
----------------------------------------------  = Burn time*, in minutes to reach target
CO2 input/min

*If you are using bottled CO₂ just replace the term ‘burn’ with ‘on’, the math works exactly the same any enrichment method what has a measurable output. Use this link if you don’t like math.

Hypothetical Scenario

Before we begin with the equations lets establish a hypothetical situation. Let us say you just purchased a propane burning CO₂ generator which produces 13 cubic feet of CO₂/hour, in fact lets say it’s a C.A.P. GEN-1. Next lets pretend your grow room or greenhouse is 10′ x 10′ x 10′, I chose these even numbers to simplify the math. Finally we should establish an approximate ambient CO₂ concentration for your area. Atmospheric CO₂ tends to range from 300-600ppm, expect to find higher levels of CO₂  in busy cities and lower levels in rural areas. For this example lets assume your grow room is in the country with an ambient CO₂ concentration of 300ppm.

Lets recap the parameters we’ve established;

• Output of CO₂ generating appliance = 13 cubic feet/hour
• Grow room or greenhouse dimensions = 10′ x 10′ x 10′ 
• Ambient CO₂ concentration = 300ppm

Calculating Room Volume

So we have some numbers to work with but they are not quite ready to enter into my equation.  Before we begin, we need to convert your room dimensions into a volumetric value expressed in cubic foot units. I will demonstrate;

Length x Width x Height = Volume
10'x10'x10'=
1000 cubic feet

Calculating target CO₂ Concentration and Adjusting for Ambient

Marijuana plants are theoretically capable of processing CO₂ in concentrations as high as 2000ppm, however, this is dependent on many factors and requires that every other facet of plant health is in perfect balance. Laboratory trials on plants often fail to meet these requirements or are only able to maintain the balance for a short time, for this reason 1800ppm is often considered to be ideal CO₂ level. Concentrations beyond 1800ppm offer only diminishing return. For this tutorial let us use 1800ppm as our CO₂ concentration target.

We have already established your ambient atmospheric level as 300ppm so we can now subtract that from the target CO₂ value.

Target CO2 concentration - ambient atmospheric = adjusted value
example:
1800ppm - 300ppm = 1500ppm

Next, to greatly simplify the math we will convert our CO₂ value from parts per million into a decimal value.  To do this simply multiply your ppm value by 0.000001, or use this idiot proof calculator. Lets input our values;

ppm value x 0.000001 = decimal value
example:
1500 x 0.000001 = .0015

Converting CO₂ generator output

Now we need to convert our burner output into cubic feet per minute. The generator used in this tutorial has an output of 13 cubic feet per hour, to convert simply divide this value by 60.

output in cubic feet per hour/ 60 = output in cubic feet per minute
example:
13 / 60 = .216 cubic feet per minute

Build your Equation

Room Volume (adjusted target CO2 Concentration)
---------------------------------------------
CO2 input/min

Next, lets insert our example variables and solve;

1000(.0015)
-------------
.216=

1.5
---- = 6.9
.216

What it all means

So according to the math our pretend grow room should reach 1800ppm CO₂ concentration after a 7 minute burn from our C.A.P. Gen-1. This may be the case if our room was sealed and if several other variables like elevation, temperature and barometric pressure are kept nominal. But, its close enough, and as good as we’re going to get without using specialized equipment.

There are other factors to consider too, a 7 minute burn will likely raise room temperatures which could trigger an automated exhaust cycle. Also your plants will be busy converting the CO₂ into oxygen which will lower the overall saturation. Finding this balance between temperature and atmospheric enrichment is part of fine tuning the room, and every room is different.

CO₂ Generating Appliance output

Most of the time you can find the output rating of your CO₂ generating appliance printed on it’s packaging, housing or attached literature. If your propane or natural gas burning appliance was not built with CO₂ enrichment as it’s primary function you can still determine its CO₂ output by using it’s BTU output. This is rarely the case and probably only applies to growers who want to use their propane burning water heater or furnace to enrich their garden or greenhouse.

Marijuana Growers HQ does not recommend use of household, gas burning appliances for CO₂ enrichment but it can be done. Diverting vent gas from your water heater or furnace into your home is VERY dangerous and has lethal potential.

To convert BTU output to CO₂ output use the following formulas;

Propane (LP)

Btu/Hour(1.18)
-------------
1000

Natural Gas

Btu/Hour
---------
1000

Of course, different ways of introducing these exhaust gasses would be needed. Water heaters turn on and off on their own schedule. Perhaps another reason not to try something foolish like this, but above are the conversions anyway.

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