Carbon dioxide (CO2) is used by plants in photosynthesis, or the conversion of water, atmospheric carbon dioxide and light in the plant’s chloroplasts into food energy (simple carbohydrates), with oxygen as a byproduct. Resins and saps in the plants stems and branches then transmit this food around the plant to promote growth, reproduction and prevention of disease.
Photosynthesis stops at night, thus plants do not use CO2 during the night, or lights-out stage. Although enrichment of the atmosphere during the night cycle will not harm the plants, efficient CO2 systems are regulated so that when the lights go out, CO2 emissions stop.
Ambient air at sea level contains approximately 350-500 ppm of carbon dioxide. Higher altitudes and rural locations typically have a lower presence of CO2, while lowlands and urban areas have a higher presence. CO2 can be measured, in parts per million (ppm) of air, using an inexpensive device available in hydroponics supply catalogs and garden shops (approx US$20).
Carbon dioxide enrichment involves increasing the concentrations of CO2 to 4-5 times the normal atmospheric levels, to between 1200-1500 ppm in an enclosed space. Enrichment has been shown to promote faster growth, higher yields, and stronger, healthier plants. Levels higher than 2000 ppm have been shown to retard plant growth. Low levels of CO2 (below 200) have been show to halt vigorous growth, even when all other conditions are ideal. Because of this, any enclosed space requires replenishment of the internal CO2 as it is used by plants, either from ventilation or from CO2 supplementation.
Temperature, humidity, and CO2 concentrations form a triangular relationship in a greenhouse or indoor grow. If all 3 factors are not in equilibrium, there is a risk to the plant in terms of stunted growth, toxicity, or death/disease.
Standard growing conditions typically include concentrations of CO2 at 300-500 ppm, temperatures between 65-80°F, and relatively low humidity (20-40% rH). Studies have shown optimal growth and yields at 90-95°F, 1,500 ppm CO2, 45-50% relative humidity, 7,500-10,000 lumens/square foot of light, and vigorous air movement both above and below the canopy. CO2 enrichment under 80°F, under 7500 lumens/sf, or above 50% humidity is not recommended because plants will not be conducting photosynthesis quickly enough to benefit from the enrichment.
Internal air movement in the grow room is critical to CO2 enrichment. Carbon dioxide is a slightly heavier molecule than other molecules floating around in the gaseous mixture we call air. Thus, CO2 enrichment without air movement will result in the gas settling out of the atmosphere before it has a chance to reach the plants. High temps and humidity without air movement can also encourage mold and bacteria growth.
To calculate the amount of Carbon Dioxide needed to enrich a room to 1500 ppm, first calculate the volume of the growing space. For instance, an 8×8 foot room with an 8 foot ceiling would contain 512 cubic feet of space. Determine the CO2 needed to enrich to 1500 ppm by multiplying the volume of space by .0015.
512 x .0015 = 0.768
Thus, 0.768 cubic feet (or rounded up to 0.8 cu ft ) of carbon dioxide will be needed to enrich this room at 1500 ppm. 1 lb of CO2 is equal to about 8.5 cubic feet at normal temperature and atmospheric pressure.
The rate at which carbon dioxide needs to be replaced is purely a function of how much ventilation the space receives and how many plants are consuming CO2 in the grow space. Only testing monitoring will ensure CO2 levels remain somewhat constant. Grow rooms that rely heavily on external ventilation to control temperatures or smell should not consider CO2 enrichment, because any gas introduced to the space will be blown out as quickly as it’s created. A sealed room that relies on no external ventilation is ideal for CO2 enrichment. Since the ideal temperature for CO2 enrichment is much higher than normal, growers who employ this technique will need much less ventilation (if any).
For those who still want or need external ventilation, CO2 enrichment will only succeed if exhaust and enrichment are timed and set on opposing cycles. For instance, in a flowering room an exhaust fan timed to operate during the night would not conflict with CO2 enrichment during the day, when plants can use the additional gas. In vegetative growth rooms, the fans and enrichment would need alternating cycles to make enrichment worthwhile. For those growers using unregulated sysems, CO2 output should be adjusted for both speed and volume to make up for the exhaust.
There is some anecdotal evidence that charging nutrient solutions with seltzer cartridges will encourage plant growth in some hydroponics systems. The CO2 is released into the atmosphere as a byproduct of nutrient movement in the hydro system. This method has not been scientifically proven, nor would not be effective in aeroponic systems where nutrients are largely contained in separate tubs from the leaves and branches of the plant. Spray ring and ebb/flow systems may have the best potential for success with this method.
METHODS OF CO2 PRODUCTION
Tanked CO2 is by far the most reliable and controllable method of CO2 enrichment. Bottled CO2, usually available from welding supply and bottled gas vendors, is metered out via regulators and solenoids. It is possible to very finely regulate the amount of CO2 in the atmosphere using technologically advanced digital regulators. In many areas, licenses or permits are required to obtain bottled compressed gasses due to safety regulations.
-Very fine control of CO2 using regulators
-Easy to automate, hassle free once set up
-High initial cost of equipment
-Logistics of delivering and returning heavy bottles to a secure grow area
-The tank becomes a deadly projectile in a catastrophic failure, or can cause a significant and dangerous explosion in a fire.
-Rapid, unexpected release of CO2 can cause over-enrichment and asphyxiation of room occupants.
-Permit/license requirements may make bottled gas difficult to obtain
Fuels such as ethyl alcohol, natural gas, or propane produce CO2 as a byproduct of combustion. Burning of one pound of clean burning heating fuel will produce 3 pounds of carbon dioxide gas, 1.5 pounds of water vapor, and approximately 22,000 BTU of heat.
Devices which help attract and kill mosquitoes in outdoor yards use propane fuel tanks to create carbon dioxide. The insects are attracted to the CO2, which in nature is an indication of a food source. These devices burn propane in a tightly regulated, low temperature combustion chamber. Although these would probably be the lowest temperature application of this method, any indoor storage of propane, natural gas or other bottled, explosive gasses is highly discouraged.
Ethyl alcohol (available as denatured alcohol in hardware stores) is a readily available material and can be safely burned indoors in small stoves or lamps. Ethyl alcohol is also the primary reactive component of Sterno and similar gel fuels.
In our sample room (8x8x8), we would need to create about 1 lb (8.5 cu ft) of CO2 over a 24 hour period. To find the volume of ethyl alcohol, we first need to find out how much ethyl alcohol weighs. Water has a specific gravity of 1.0, but ethyl alcohol’s specific gravity is .79. Since one gallon of water weighs 8.33 lbs:
8.33 x 0.79 = 6.58 lbs
Thus, 1 gallon of ethyl alcohol weighs 6.58 lbs. Since 1 lb of fuel creates 3 lbs of CO2, only .333 lb of fuel would be needed to create 1 lb of C02.
By ratio and proportion:
6.58 lbs * X gals = .333 lb * 1 gal
X = .333/6.58 = .051 gal
Since 1 gal = 128 fluid ounces:
.051 gal * 128 ounces = 6.48 ounces
Thus, we would need to burn 6.48 ounces of ethyl alcohol per day (a little more than 3/4 cup) to enrich a completely sealed room. The amount of CO2 needed (and thus fuel) would increase with any supplemental air changes. There is some evidence that active combustion can help control odors in enclosed spaces.
Coleman stoves, bunsen burners, portable propane space heaters, and other similar devices are all potential sources of carbon dioxide as long as they are used safely.
-Inexpensive to set up, depending on method chosen.
-Heat can be beneficial if temps are low, such as in a cold basement grow room.
-Output can be regulated by size of flame
-Can provide slight odor control.
-Open flames in enclosed spaces create a fire hazard
-Additional heat produced by combustion adds to heat already produced by HID lighting.
-Can be difficult to burn enough fuel to achieve optimal enrichment without adverse side effects, such as carbon monoxide.
-Indoor storage of bottled fuels is potentially dangerous.
It is widely known that CO2 is a byproduct of fermentation. CO2 is the gas found in bubbly beverages, such as champagne and beer. The same process that “carbonates” these beverages can be harnessed to create CO2 for a grow area. A pound of sugar will ferment into approx. 1/2 lb of ethyl alcohol and 1/2 lb of CO2. We’ve determined that we need 0.8 cu ft of CO2 for our 512 cu ft grow room (see above.) Then calculate the size container needed by dividing the size of the grow room by 32.
512 / 32 = 16 gallons. (A tall kitchen garbage can would make a good 16 gal. bin)
Assuming that the bin will produce half alcohol and half CO2, the bin will consume .16 lbs of sugar every four hours, which is roughly 1 lb per day. This means that about 45 lbs of sugar will be used over 6 weeks (assuming that not all sugar is completely converted to alcohol).
To get the process started, mix a pinch of yeast, 12 ounces of warm water and a half-cup of sugar and keep warm and covered until bubbles form in a day or so. Use this mixture to inoculate the main bin.
To create a yeast bin mix, dissolve 3 lbs of sugar per gallon of boiling water. Cool the mix to 80°F before adding the yeast. Locate a container with a tightly fitting lid. The lid should be equipped with a hose to direct CO2 gas towards a fan for distribution into the space. Increased air pressure in the bin will force the gas out of the hose.
Both canister and lid should be thoroughly cleaned with hot soapy water and rinsed well before use. Start off the bin a little more than half full (10 gallons of water and 30 lbs of sugar). Every week, add another gallon of water and 3 lbs of sugar. The yeast bin must remain at 80-85°F for the reaction to continue.
To monitor activity and prevent contaminants from entering the bin, create a fermentation lock by placing the end of the hose into a glass of distilled water. The bubbling water will be an indicator that there is still a reaction in the bin and prevent bacteria from entering the bin through the hose.
Our bin will need to be completely replenished every 6 weeks, or when the bubbling slows. A simple taste test will tell if the bin needs replenishing. If the taste is sweet, there is still sugar in the water and the reaction should continue. If the taste is dry like wine, the bin is mostly alcohol and should be replenished. Some growers preserve a cup of liquid from the old bin and use to inoculate the new bin, however if an infestation is starting to occur, this can contaminate an otherwise fresh bin with bacteria. It’s just as easy to inoculate with new yeast as above, and extra yeast stores easily in the refrigerator for months. Corn sugar (available at wine making shops) is a less expensive fermentation medium than regular cane sugar. Other fermentation mediums can be used depending on materials cheaply and readily available to the grower. Corn syrup, maple sap, even old fruit juice can be fermented, although with increased odors and more waste cleanup when the bin is refreshed.
-Easy to create with simple materials
-No safety dangers
-Inexpensive materials when purchased in bulk (sugar)
-Ethyl alcohol byproduct can be siphoned off and burned in alcohol lamps for supplemental CO2 enrichment
-Difficult to regulate
-Fermentation can produce odors
-Large yeast bins are heavy and hard to move.
Dry ice is nothing but carbon dioxide in its solid form. Dry ice is commercially available nearly everywhere for industrial, medical, and theatrical (fog machine) applications. One pound of dry ice is equal to 8.5 cubic feet of gaseous CO2. Create a CO2 chamber by poking holes in the sides and top of an insulated box, foam cooler, or similar container that can insulate the material from human skin and plants. The box also helps insulate the solid ice so that it vaporizes more slowly. Ideally it should take an entire day for the chunk of ice to vaporize, although smaller chunks may need to be added at intervals through the day to maintain 1500 ppm.
Some growers place their containers of dry ice directly over grow lights. The falling CO2 bathes the plants beneath them and also helps control temperatures from hot lights.
For our 512 CF grow room, about 1 lb of dry ice per day would be needed to keep CO2 at 1500 ppm. At $.60/lb, dry ice would be a very cost effective solution. Storage of dry ice in a home freezer will slow it’s vaporization, but dry ice is hard to store ahead because doesn’t have a long shelf life. Not many homes have freezers capable of maintaining -109°F.
-Inexpensive, widely available material
-Easy to construct and maintain
-No risk of catastrophic failure
-Dry ice has slight cooling effect
-Impossible to regulate evaporation
-Must be used immediately – has no shelf life
-Can harm skin if handled without gloves.
Baking soda and acetic acid solution, such as white vinegar (5% acetic acid), will bubble and foam when mixed. The bubbles produced are carbon dioxide. Unfortunately, large quantities of materials are required to produce carbon dioxide adequate for enrichment, making this solution viable only for very small closet grows.
To produce 1 lb of CO2 every day for our 512 cu ft test grow room, we would need to mix about 2 lbs (1.91 to be exact) of baking soda with 3.25 gallons of 5% acetic acid vinegar. As you can see, the costs for baking soda and vinegar would add up quickly. For a small closet or cabinet operation, it may be a workable solution though. A small drip setup can be placed on a top shelf of the closet, with the CO2 cascading down onto the plants (so long as it’s not sucked out by vent fans).
Mixture of appropriate amounts of vinegar and baking soda will quickly fill a small room to acceptable enrichment levels. From there, a simple drip irrigation system can be created to steadily regulate CO2 levels, using a reservoir of white vinegar suspended over a tub of baking soda. A hose with a small pinhole is a good way to create a steady regulated drip. Calibrate the drip with a pushpin or small nail until the hole allows the desired amount of vinegar to drip through in a 24 hour period. An added bonus to this method comes from baking soda’s odor neutralizing effect when left open to the air.
For slightly larger operations, 1 lb of carbon dioxide can be created from 2 lbs of baking soda and 1/2 gal of 33% muriatic acid, which is an chemical additive used in swimming pools. Although this is more cost effective, it is still more expensive than some of the other methods mentioned. Muriatic acid (a.k.a hydrochloric acid) is also highly caustic which can cause serious chemical burns if mishandled.
There are commercially available machines which produce CO2 this way, by mixing baking soda with muriatic acid using mechanized agitators. These units do not have regulators, solenoids, or pressurized compartments to store gas during the off cycle. Any jug made from plastic that can withstand a caustic material such as muriatic acid would be equally effective.
-Easy to set up with simple, readily available materials.
-No risk of catastrophic failure
-Slight odor control benefit from baking soda.
-Difficult to regulate during off cycle
-Can take a long time to build up a proper CO2 enrichment
-Materials can be expensive over time unless purchased in bulk.
-Some chemicals can be caustic.
The natural breathing of air by people is also a way to contribute carbon dioxide to an enclosed space. Some quick calculations show that one person breathing can actually provide a significant amount of CO2. Although the total lung capacity is approximately 7 liters, the natural tidal volume (each normal breath at resting) is about .5 liter (5000 cubic centimeters) per breath.
To convert cc to cubic feet, multiply by 3.531 x 10^-5
0.00003531 x 5000 = 0.17655 cubic feet of air
Since each breath made at a rest is 5% carbon dioxide:
0.17655 cu ft air x .05 = .0088275 cu ft of carbon dioxide
And since a person breathes approximately 14 times per minute at rest:
.008275 x 14 = 0.123 cubic feet of CO2 per minute.
Our room requires 0.8 cubic feet of CO2 to reach 1500 ppm, which it will attain after only 6.5 minutes of normal breathing. However, that enrichment is quickly absorbed by the plants. Assuming that we require 1 lb (8.5 cu ft) of CO2 per day for our 512 cu ft grow room:
8.5 cu ft / 0.123 cu ft per minute = 69.1 minutes
Thus to enrich our room to 1500 ppm day, one average sized person would need to spend approximately 70 minutes per day in the grow room assuming the room was completely sealed. Spending this much time at once could elevate carbon dioxide to unhealthful levels, but several stops in the grow room spaced out during the day (perhaps 35 minutes in the morning and 35 minutes in the evening) would keep CO2 concentrations elevated to optimal levels.
Of all the methods mentioned, breathing for CO2 enrichment is free and requires no special tools, additives, equipment, or skills. Breathing produces no unhealthful byproducts or hazards. Most gardeners spend a good amount of time in a grow area looking over the plants for bugs/disease, pruning them, mixing nutrients, admiring, etc. Entry to the room should minimize CO2 loss, through an airlock for example. As long as the space is well sealed and the air is vigorously circulated, normal breathing could produce all the C02 needed to enrich a small to medium sized room if it’s visited and tended daily. One of the other supplemental methods can make up for times the gardener is away from the room for extended periods oftime. Working in any enclosed space requires caution and alertness to avoid asphyxiation.
-Requires no tools, equipment, or setup
-Byproduct of being in the garden working
-Multiple stops into the garden daily are required
-Slight risk of asphyxiation from being in an enclosed space too long
-Entry to room without an airlock will eliminate any gains.
Cost & Security Benefits of CO2 Enrichment
Plants in a CO2 rich environment can withstand and need much higher temperatures to derive any benefit. Inversely, CO2 enrichment can help mitigate ventilation and air conditioning challenges in grow rooms, common challenges faced by growers looking to minimize costs and maximize security.
Ventilation to the outdoors is a weak link in any secure grow operation. Exhaust to the outdoors can be detected by close neighbors, especially for growers in townhomes and apartment complexes. In many areas, a tip from a neighbor and detectable smell to the local constable or sheriff could constitute “probable cause” to get a search warrant. CO2 enrichment eliminates the need for excessive exhaust and thus the need for this breach in your security.
The primary operating cost of a residential grow operation is electricity. Reliance on high intensity discharge lights, fans, humidifiers, and pumps for hydroponic systems can nearly double a residential electric bill. Cooling a hot grow area to 75-80°F for normal growing adds another important but potentially expensive challenge. In many older homes, this could require additional electrical circuits, since each standard (15 amp) residential circuit should only power devices totaling about 1500 watts. CO2 enrichment eliminates the need for additional cooling above what’s needed to maintain 95°F.
Notes & Warnings
CO2 is widely considered to be a “greenhouse gas”, which is thought to be responsible for trapping the sun’s radiation in the atmosphere and causing global warming. Commercially available CO2 is the by-product of industrial applications which reclaim gas that would have escaped into the atmosphere anyway. CO2 produced from combustion, fermentation or other means further increases the amount of CO2 in the atmosphere, albeit minutely. Enrichment with reclaimed CO2 is a more environmentally responsible method, however it is also the most expensive and logistically difficult.
Although CO2 is not a deadly gas, it’s presence in an enclosed space can deplete the atmosphere of oxygen needed for human occupation, causing asphyxiation. Signs of asphyxiation include weakness, lethargy, dizziness and loss of consciousness. If a grower notices any of these signs for any reason, immediately leave the room and go to a safe space. If these signs then subside, the CO2 in the grow room is too highly concentrated and should be vented immediately.
Many of the methods described in this guide can be harmful or fatal if used improperly. The grower should use extreme caution when using any volatile compound, flame, or hazardous material. Consider emergency situations when designing your system. For instance, bottled gasses will explode or become deadly missiles when punctured or heated by fire. Fuel vapors in the atmosphere can explode suddenly from electrical arcs, open flames, even static electricity. Asphyxiation resulting in unconsciousness and death can occur quickly when a room is over-enriched. If you suspect any form of danger, get to safety first. No plant, CO2 system, or even a whole house is worth a human life.
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