Figured it was time to put all of the CO2 info I use into one thread, so I can reference folks back to it. If I missed anything please let me know and I'll add it to this initial post! Some of this I wrote, some of this I stole from other sites/posts. I'm more of a growroom design guy than a botantist, so where I've poached botany info I've provided the source links as well.
Photosynthesis and the C3/C4 Plant Classification (taken from http://www.sjsu.edu/faculty/watkins/CO2plants.htm )
Life is base upon chemical reactions; many, many chemical reactions; but the chains of chemical reactions known as photosynthesis are the basis in one way or another of all life. Photosynthesis involves the input of carbon dioxide and water with radiant energy and the presence of a catalyst called chlorophyll. The outputs are carbohydrates and oxygen. The formal statement of the process is:
6CO2 + 6H2O + ν → C6H12O6 + 6O2
where ν represents photons of radiation.
The catalyst for the reaction, chlorophyll, is an organo-metallic compound containing magnesium. It is one of the three organo-metallic compounds which are the basis for life. The other two are the vital elements of the blood of mammals, hemiglobin, and of crustaceans, hemicyanin. Just as chlorophyll contains magnesium, hemiglobin contains iron and hemicyanin contains copper.
The process of photosynthesis is very complex and chemists could find little about the processes until radioactive isotopes became available. First, the radioactive isotope of oxygen, 18O, was used to create water, H2O. When plants were exposed to this radioactive water the radioactivity showed up in the oxygen exhaled from the plants. This showed that the oxygen created by plants comes from the water it uses rather than from the CO2. The oxygen in the CO2 gets incorporated in the carbohydrates created by the plants.
Second, a radioactive isotope of carbon, 14C, was used to create carbon dioxide. Plants were exposed to this radioactive CO2 for a few seconds and then the leaf material was chemically analyzed. In most plants the radioactive carbon showed in a compound called phosphoglyceric acid (PGA). The molecule of this compound contains three carbon atoms and one atom of phosphorus:
H H H
| | |
O- C - C - C -H
|| | |
O O O-P-O
| |
H O-H
Most plants, including trees and flowering plants, produce PGA as the first step in photosynthesis. A few plant species, including tropical grasses such as sugar cane and corn (maize), produce malic acid or aspartic acid as the first step. The molecules of these compounds contain four carbon atoms and one nitrogen atom. The aspartic acid molecule is:
H H
| |
H-O-O-C-C-O-O-H
| |
H N-H
|
H
Because the initial products of photosynthesis for plants in this category involve compounds containing four carbon atoms this class is called C4. The other category of plants produces PGA which contains three carbon atoms so it is called C3. This classification is important because the responses of the two categories of plants to increased CO2 is different.
Why we need supplemental CO2 in our grow rooms:
In most indoor grow environments, atmospheric levels of 300-330PPM will be quickly used up by a room full of plants, as they transpire and absorb these atmospheric levels. If replenished regularly with fresh outside air the 300ppm level can be maintained, but is not an ideal PPM level for accelerated growth. C3 plants such as cannabis will typically benefit with 20-30% larger yields, faster growth, and faster finishing than non-supplemented plants. By artificially increasing the CO2 levels from 300 to 1500PPM, we can maximize plant uptake of CO2 and enjoy the benefits of such.
CO2 is NOT a magical cure-all for low yields! If your room or plants have unresolved issues such as high humidity, nutrient deficiencies, or anything else, CO2 will probably make things worse. When a room is dialed-in and running more or less perfectly, adding supplemental Co2 can have a notable increase on plant yields and growth.
A grow is only as good as its weakest link. FIx your weakest link over and over until there is no longer a weak link. Then you are ready for CO2.
The article Global Temperature Change and Terrestrial Ecology in the Encyclopedia of Water Science (CRC Press, 2007) has the matter stated correctly:
[It is a] well-established fact that CO2 is a powerful aerial fertilizer, which when added to the air can substantially increase the vegetative productivity of nearly all plants…numerous studies have demonstrated that the percent increase in growth produced by an increase in the air’s CO2 content typically rises with an increase in air temperature. In addition, at the species-specific upper-limiting air temperature at which plants typically die from thermal stress under current atmospheric CO2 concentrations, higher CO2 concentrations have been shown to protect plants and help them stave off thermal death…[and] increase the species-specific temperature at which plants grow best. Indeed it has been experimentally demonstrated that the typical CO2-induced increase in plant optimum temperature is as great as, if not greater than, the CO2-induced global warming typically predicted…Hence, [with] an increase in the air’s CO2 concentration – even if it did have a tendency to warm the earth (which is hotly debated) – …[plants] …would grow equally well, if not better, in a warmer and CO2-enriched environment.
The Effect of Temperature on Plant Response to Higher Levels of CO2
Photosynthesis consists of chemical reactions. Chemical reactions procede at a higher rate at higher temperatures. The rule of thumb is that there is a doubling of the reaction rate for every 10°F rise in temperature. Plants grow faster at a higher temperature providing they have adequate levels of CO2, water, sunlight and plant nutrients. The C4 plants have a great response rate for a higher temperature than does the C3 plants.
A higher temperature without adequate level of the necessary ingredients for growth might produce no response or even damage. Sylvan Wittwer, quoted above, states that under most circumstances the availability of CO2 is the factor which limits growth. Thus with a higher level of CO2 in the air plants can grow faster with a higher temperature.
Plants transpire water vapor to keep an even temperature. There are tiny holes on the underside of plant leaves, called somata, which are the openings through which the plant absorbs CO2. With higher level of CO2 concentration in the air the somata do not have to be open as wide. The narrower opening means that less water is transpired and thus less water is required by the plants. In other words, higher levels of CO2 increase the efficiency of water use by plants. This was confirmed in experiments reported by K.E. Idso and S.B. Idso. They found that enhanced CO2 increased growth by 31 percent in plants with adequate moisture but it increase growth by 62 percent for plants in moisture-stressed condition. In effect, enhanced CO2 by reducing water loss created the same effect as providing more water. Thus the effect in moisture-stressed plants was the effects of enhanced CO2 plus the effect of increased water.
The effect of increased CO2 in narrowing the stomata of plants has the additional benefit that a lesser amount of pollutants in the air will make it through the narrower openings. Thus enhanced CO2 has the effect of protecting plants against damage from air pollutants such as ozone or sulfur dioxide.
The effect of enhanced CO2 is even greater for plants grown under low light conditions. The enhance growth is greater than 100 percent for a 100 percent increase in CO2. This compares to less than 50 percent for plants grown in normal light conditions.
The evidence that clinches the argument is that some greenhouse owner artificially elevate the CO2 level to triple what the level in the atmosphere is.
How to prepare your grow for CO2
It should go without saying that a well-prepared grow room should be as airtight as humanly possible. Any leaks or ventilation that exchanges room air with outside air must be sealed or reconstructed. Any air that comes in dilutes the concentration, and anything that leaks out must be replenished. Replenishing this CO2 costs either expensive tanked CO2 or expensive propane. Seal all gaps, cracks, vents, doors, windows and air conditioners as tightly as possible for best results and lowest operational costs.
There are means to increase Co2 using Co2 Boost buckets, fermentation, and by using exhaust from natural gas or proane-fired appliances, but they will not be discussed in depth here as they generally lack precision and are not as easily controlled, if at all.
Since CO2 supplementation requires a sealed grow room, powered exhausts and intakes for temperature control can not be used. Air conditioning (cooling and re-cooling grow room air) is a must. An air conditioner of sufficient size to cool all of your equipment on the hottest day of the year is required. If this is a window unit it will be more difficult to seal, as these often require a carefully constructed box for the A/C to reside in with fans moving the necessary air in and out. Mini split air conditioners are ideal, and the newer dual-hose portable air conditioners are also adequate for this use as long as they are connected properly.
***Just a quick note for a product I'm a fan of, check out http://www.coolerado.com and check out the M50, a 5-ton air conditioner that draws 600W of power, and only wastes 1 gallon per hour for cooling. Only good for larger grows of course, but they have a new 3-ton model as well. Price for the M50 was $4999 but requires no big power line/draw OR an HVAC technician to install it.***
Dehumidification is also important, as RH levels of 50% or thereabouts are required to prevent bud rot, powdery mildew and slow growth. By reducing RH plants are encouraged to take up more water from the roots and transpire it from the leaves. A dehumidifier may be required for any grow that exceeds 50% relative humidity even on occasion. In my room I use two medium-sized units, plumbed into a full-time drain so I don't have to empty catch basins of water. Some re-use this water for gardening by dumping it back into their reservoirs.
Another preparation that must be made is for plant growth. Since growth rates in all stages of life will be accelerated, the grower must be prepared for plants that require additional water, nutrients, pruning and training. Lights may need to be raised to compensate for additional height.
Air-cooled light hoods are only recommended with CO2 if they are pulling in outside air, running it through the lights, then back outside. Room air should never be used to cool hoods in this case, as you would be drawing precious CO2 outside.
A dedicated day/night programmable temperature controller is strongly recommended, as the recommended daytime temperature with supplemental CO2 is approximately 82*-85*F. Since nighttime temperatures this high would be excessive, it's recommended to have a night program that drops the temperature to approximately 75*F.
A side effect of the higher temperatures is insect reproductive rates are often increased. Spider mites thrive in warmer temperatures, and what was a manageable threat before may well get out of hand with CO2. Be prepared for outbreaks, use pyrethrin foggers regularly as a preventative measure, and (my personal recommendation) use Floramite SC foliar sprayed every 3 weeks or so, except the final 3-4 weeks of flower of course.
In some rooms I've built, I used a CAP AIR-4 to control a CO2 setup as well as an emerrgency powered exhaust. If the room temp rises above 88* (say due to a failed AC or the like) a powered exhaust fan will keep room temps from getting out of control until the situation is remedied. The CO2 is turned off automatically by the AIR-4 if this occurs, though other similar controllers exist.
How to calculate CO2 needs
Every room is different, and will have different requirements for adding CO2. First, you need to calculate the volume of your grow room, needing just width, length and height to use the following calculator. Click here to go to the CO2 grow room calculator: http://www.hydroponics.net/learn/co2_calculator.asp
Flow settings for the CFH are found on your CO2 regulator packaging, or on the CO2 generator you will use. Most regulators can do at least 50 cubic feet per hour, and generators range from 6 to 46 cubic feet per hour of CO2 generation.
This calculator will tell you how long your rig needs to run to increase the room from 300 to 1500 PPM, but not how often, or how to maintain that level. It will also tell you (in hours) how long your tank will last at those rates. It is recommended that CO2 generators do not run for mroe than 15 minutes to minimize heat buildup in the room.
How to add CO2
I do not advocate the use of timers to control CO2 levels, as they completley lack the ability to compensate for actual CO2 usage, and leakage. The only accurate way to measure and regulate CO2 dispersion is with a high-quality CO2 controller, most of the good units from Sentinel, CAP and GenHydro use an infrared beam to measure and regulate the tank or generator to appropriate levels. These units are not inexpensive, but will pay for themselves with higher yields, precise CO2 metering, and fewer CO2 tank/propane tank changes.
For small grows where excessive heat is a problem, but your AC is able to keep the room to ~85*F, adding supplemental CO2 via a tank is the best option. If your AC can not keep your room to 85 or less you need a bigger AC, period. I usually recommend tanks for grows/rooms under 100 square feet, if adequately sealed. A benefit of tank systems is the ability to use fuzzy logic controllers, which predict CO2 usage and compensate accordingly, reducing the sometimes large offsets that occur with simpler metering plans.
CO2 is heavier than air, and should be dispersed from the top of the room down. Oscillating or circulation fans should be employed to keep the heavier CO2 on the floor stirred and mixed in the room. In more than one grow I've hooked the CO2 discharge hose to the back of an oscillating fan so it could "spray" the room with it as it turns.
Generators are slightly more involved, but always follow manufacturers instructions and read the entire manual before proceeding. Generators produce a large amount of heat, some humidty, and copious amounts of CO2. Propane generators are more beneficial and economical for large grows where heat is not typically an issue. A 5 gallon propane tank will last 4 times as long as a CO2 tank at the same flow rate, and costs less.
Ducted generators are now available so 50% of the heat can be ducted outside, as arte water-cooled units from HydroGen (Hydro Innovations.) The hydrogen is basically a small tankless water heater that produces roughly 36 cfh of Co2, but should only be used with tap or well water, pump-driven hydrogens almost always fail to light properly after a while, even with large pumps. My advice is to run these units to waste if at all possible, chilling and reusing this water is not economical.
How long will my CO2 tank or Propane tank last me?
Using the calculations gathered above, divide the total runtime (in hours) by 60 minutes, then figure out how many cycles that would be. for example:
28 hours of runtime X 60 minutes is 1680 minutes of runtime. If the calculator says my generator needs to run for 12 minutes to fill the room, then divide by that:
1680 / 12 = 140 "on" cycles.
If you figure this will need to occur at least 4-5 times per light cycle, we can get a rough estimate of how many days the tank will last:
140 cycles / 5 cycles per night = 28 days.
Again, this does not account for actual plant use, or leakage, so it should only be used as a rough ballpark.
For propane users, a dual-tank kit is available to connect two tanks, doubling time between tank changes. I believe a similar device exists for bottled CO2 users as well.
I've also seen great deals on Craigslist for 100# RV propane tanks, two of those would CO2 my room for over 6 months, and it's a huge room!
The only accurate way to measure actual tanked CO2 or propane tank use is the use of a bathroom scale (preferably digital.) Weigh the tank at the same time once per day and you can quickly determine how much gas you're using up, and multiply X 30 for a monthly average.
Generators vs tanks, pros and cons
Generator Pros:
Large CO2 output
Supplemental heat for cold-weather grows
New pilotless models are safer and waste less gas than pilot models
Propane tanks last longer and are cheaper than bottled CO2
Many models can run off natural gas, great for garage/basement grows where gas-fired appliances are nearby (furnace or water heater)
SIlent
Propane is cheap, $17.99 at my local wal-mart for a 5 gallon tank
Propane is much easier to find than CO2.
Generator cons:
High heat output must be compensated for with larger air conditioners
Propane tanks must be changed every so often
Generator and controller is expensive
Tanked CO2 Pros:
No heat produced at all
Very precise metering
Silent
Many hydro shops carry refill/exchange bottles now
Ability to use fuzzy logic to maintain perfect level of 1500 ppm
Tanked CO2 cons:
CO2 is more expensive and doesn't last as long as a propane tank
Harder to find tanks and more suspicious than a propane tank
Valves/regulators can freeze or stick open
Fuzzy logic controllers are very expensive
Where to get CO2 tanks or propane tanks
CO2 tanks:
Beverage supply stores
Welding supply stores
Hydro stores (some)
Propane tanks:
Most gas stations have an exchange cage
Wal-Mart
Target
Home Depot/Lowe's
Pretty much anywhere.
Using CO2 to kill bugs:
If having a bug infestation, turning the CO2 up to ~10K PPM for 1 hour will kill all the bugs in a room, as well as any humans. This is easier with tanked CO2 than a generator (due to the heat) but will not kill egss from mites. A repeat dosage ~4 days later will deal with new hatchlings before they have time to reproduce.
Well that's all I can think of off the top of my head, but again, please let me know if I forgot to add anything and I'll put it in here with a writer's credit!
Thanks all, LM
Photosynthesis and the C3/C4 Plant Classification (taken from http://www.sjsu.edu/faculty/watkins/CO2plants.htm )
Life is base upon chemical reactions; many, many chemical reactions; but the chains of chemical reactions known as photosynthesis are the basis in one way or another of all life. Photosynthesis involves the input of carbon dioxide and water with radiant energy and the presence of a catalyst called chlorophyll. The outputs are carbohydrates and oxygen. The formal statement of the process is:
6CO2 + 6H2O + ν → C6H12O6 + 6O2
where ν represents photons of radiation.
The catalyst for the reaction, chlorophyll, is an organo-metallic compound containing magnesium. It is one of the three organo-metallic compounds which are the basis for life. The other two are the vital elements of the blood of mammals, hemiglobin, and of crustaceans, hemicyanin. Just as chlorophyll contains magnesium, hemiglobin contains iron and hemicyanin contains copper.
The process of photosynthesis is very complex and chemists could find little about the processes until radioactive isotopes became available. First, the radioactive isotope of oxygen, 18O, was used to create water, H2O. When plants were exposed to this radioactive water the radioactivity showed up in the oxygen exhaled from the plants. This showed that the oxygen created by plants comes from the water it uses rather than from the CO2. The oxygen in the CO2 gets incorporated in the carbohydrates created by the plants.
Second, a radioactive isotope of carbon, 14C, was used to create carbon dioxide. Plants were exposed to this radioactive CO2 for a few seconds and then the leaf material was chemically analyzed. In most plants the radioactive carbon showed in a compound called phosphoglyceric acid (PGA). The molecule of this compound contains three carbon atoms and one atom of phosphorus:
H H H
| | |
O- C - C - C -H
|| | |
O O O-P-O
| |
H O-H
Most plants, including trees and flowering plants, produce PGA as the first step in photosynthesis. A few plant species, including tropical grasses such as sugar cane and corn (maize), produce malic acid or aspartic acid as the first step. The molecules of these compounds contain four carbon atoms and one nitrogen atom. The aspartic acid molecule is:
H H
| |
H-O-O-C-C-O-O-H
| |
H N-H
|
H
Because the initial products of photosynthesis for plants in this category involve compounds containing four carbon atoms this class is called C4. The other category of plants produces PGA which contains three carbon atoms so it is called C3. This classification is important because the responses of the two categories of plants to increased CO2 is different.
Why we need supplemental CO2 in our grow rooms:
In most indoor grow environments, atmospheric levels of 300-330PPM will be quickly used up by a room full of plants, as they transpire and absorb these atmospheric levels. If replenished regularly with fresh outside air the 300ppm level can be maintained, but is not an ideal PPM level for accelerated growth. C3 plants such as cannabis will typically benefit with 20-30% larger yields, faster growth, and faster finishing than non-supplemented plants. By artificially increasing the CO2 levels from 300 to 1500PPM, we can maximize plant uptake of CO2 and enjoy the benefits of such.
CO2 is NOT a magical cure-all for low yields! If your room or plants have unresolved issues such as high humidity, nutrient deficiencies, or anything else, CO2 will probably make things worse. When a room is dialed-in and running more or less perfectly, adding supplemental Co2 can have a notable increase on plant yields and growth.
A grow is only as good as its weakest link. FIx your weakest link over and over until there is no longer a weak link. Then you are ready for CO2.
The article Global Temperature Change and Terrestrial Ecology in the Encyclopedia of Water Science (CRC Press, 2007) has the matter stated correctly:
[It is a] well-established fact that CO2 is a powerful aerial fertilizer, which when added to the air can substantially increase the vegetative productivity of nearly all plants…numerous studies have demonstrated that the percent increase in growth produced by an increase in the air’s CO2 content typically rises with an increase in air temperature. In addition, at the species-specific upper-limiting air temperature at which plants typically die from thermal stress under current atmospheric CO2 concentrations, higher CO2 concentrations have been shown to protect plants and help them stave off thermal death…[and] increase the species-specific temperature at which plants grow best. Indeed it has been experimentally demonstrated that the typical CO2-induced increase in plant optimum temperature is as great as, if not greater than, the CO2-induced global warming typically predicted…Hence, [with] an increase in the air’s CO2 concentration – even if it did have a tendency to warm the earth (which is hotly debated) – …[plants] …would grow equally well, if not better, in a warmer and CO2-enriched environment.
The Effect of Temperature on Plant Response to Higher Levels of CO2
Photosynthesis consists of chemical reactions. Chemical reactions procede at a higher rate at higher temperatures. The rule of thumb is that there is a doubling of the reaction rate for every 10°F rise in temperature. Plants grow faster at a higher temperature providing they have adequate levels of CO2, water, sunlight and plant nutrients. The C4 plants have a great response rate for a higher temperature than does the C3 plants.
A higher temperature without adequate level of the necessary ingredients for growth might produce no response or even damage. Sylvan Wittwer, quoted above, states that under most circumstances the availability of CO2 is the factor which limits growth. Thus with a higher level of CO2 in the air plants can grow faster with a higher temperature.
Plants transpire water vapor to keep an even temperature. There are tiny holes on the underside of plant leaves, called somata, which are the openings through which the plant absorbs CO2. With higher level of CO2 concentration in the air the somata do not have to be open as wide. The narrower opening means that less water is transpired and thus less water is required by the plants. In other words, higher levels of CO2 increase the efficiency of water use by plants. This was confirmed in experiments reported by K.E. Idso and S.B. Idso. They found that enhanced CO2 increased growth by 31 percent in plants with adequate moisture but it increase growth by 62 percent for plants in moisture-stressed condition. In effect, enhanced CO2 by reducing water loss created the same effect as providing more water. Thus the effect in moisture-stressed plants was the effects of enhanced CO2 plus the effect of increased water.
The effect of increased CO2 in narrowing the stomata of plants has the additional benefit that a lesser amount of pollutants in the air will make it through the narrower openings. Thus enhanced CO2 has the effect of protecting plants against damage from air pollutants such as ozone or sulfur dioxide.
The effect of enhanced CO2 is even greater for plants grown under low light conditions. The enhance growth is greater than 100 percent for a 100 percent increase in CO2. This compares to less than 50 percent for plants grown in normal light conditions.
The evidence that clinches the argument is that some greenhouse owner artificially elevate the CO2 level to triple what the level in the atmosphere is.
How to prepare your grow for CO2
It should go without saying that a well-prepared grow room should be as airtight as humanly possible. Any leaks or ventilation that exchanges room air with outside air must be sealed or reconstructed. Any air that comes in dilutes the concentration, and anything that leaks out must be replenished. Replenishing this CO2 costs either expensive tanked CO2 or expensive propane. Seal all gaps, cracks, vents, doors, windows and air conditioners as tightly as possible for best results and lowest operational costs.
There are means to increase Co2 using Co2 Boost buckets, fermentation, and by using exhaust from natural gas or proane-fired appliances, but they will not be discussed in depth here as they generally lack precision and are not as easily controlled, if at all.
Since CO2 supplementation requires a sealed grow room, powered exhausts and intakes for temperature control can not be used. Air conditioning (cooling and re-cooling grow room air) is a must. An air conditioner of sufficient size to cool all of your equipment on the hottest day of the year is required. If this is a window unit it will be more difficult to seal, as these often require a carefully constructed box for the A/C to reside in with fans moving the necessary air in and out. Mini split air conditioners are ideal, and the newer dual-hose portable air conditioners are also adequate for this use as long as they are connected properly.
***Just a quick note for a product I'm a fan of, check out http://www.coolerado.com and check out the M50, a 5-ton air conditioner that draws 600W of power, and only wastes 1 gallon per hour for cooling. Only good for larger grows of course, but they have a new 3-ton model as well. Price for the M50 was $4999 but requires no big power line/draw OR an HVAC technician to install it.***
Dehumidification is also important, as RH levels of 50% or thereabouts are required to prevent bud rot, powdery mildew and slow growth. By reducing RH plants are encouraged to take up more water from the roots and transpire it from the leaves. A dehumidifier may be required for any grow that exceeds 50% relative humidity even on occasion. In my room I use two medium-sized units, plumbed into a full-time drain so I don't have to empty catch basins of water. Some re-use this water for gardening by dumping it back into their reservoirs.
Another preparation that must be made is for plant growth. Since growth rates in all stages of life will be accelerated, the grower must be prepared for plants that require additional water, nutrients, pruning and training. Lights may need to be raised to compensate for additional height.
Air-cooled light hoods are only recommended with CO2 if they are pulling in outside air, running it through the lights, then back outside. Room air should never be used to cool hoods in this case, as you would be drawing precious CO2 outside.
A dedicated day/night programmable temperature controller is strongly recommended, as the recommended daytime temperature with supplemental CO2 is approximately 82*-85*F. Since nighttime temperatures this high would be excessive, it's recommended to have a night program that drops the temperature to approximately 75*F.
A side effect of the higher temperatures is insect reproductive rates are often increased. Spider mites thrive in warmer temperatures, and what was a manageable threat before may well get out of hand with CO2. Be prepared for outbreaks, use pyrethrin foggers regularly as a preventative measure, and (my personal recommendation) use Floramite SC foliar sprayed every 3 weeks or so, except the final 3-4 weeks of flower of course.
In some rooms I've built, I used a CAP AIR-4 to control a CO2 setup as well as an emerrgency powered exhaust. If the room temp rises above 88* (say due to a failed AC or the like) a powered exhaust fan will keep room temps from getting out of control until the situation is remedied. The CO2 is turned off automatically by the AIR-4 if this occurs, though other similar controllers exist.
How to calculate CO2 needs
Every room is different, and will have different requirements for adding CO2. First, you need to calculate the volume of your grow room, needing just width, length and height to use the following calculator. Click here to go to the CO2 grow room calculator: http://www.hydroponics.net/learn/co2_calculator.asp
Flow settings for the CFH are found on your CO2 regulator packaging, or on the CO2 generator you will use. Most regulators can do at least 50 cubic feet per hour, and generators range from 6 to 46 cubic feet per hour of CO2 generation.
This calculator will tell you how long your rig needs to run to increase the room from 300 to 1500 PPM, but not how often, or how to maintain that level. It will also tell you (in hours) how long your tank will last at those rates. It is recommended that CO2 generators do not run for mroe than 15 minutes to minimize heat buildup in the room.
How to add CO2
I do not advocate the use of timers to control CO2 levels, as they completley lack the ability to compensate for actual CO2 usage, and leakage. The only accurate way to measure and regulate CO2 dispersion is with a high-quality CO2 controller, most of the good units from Sentinel, CAP and GenHydro use an infrared beam to measure and regulate the tank or generator to appropriate levels. These units are not inexpensive, but will pay for themselves with higher yields, precise CO2 metering, and fewer CO2 tank/propane tank changes.
For small grows where excessive heat is a problem, but your AC is able to keep the room to ~85*F, adding supplemental CO2 via a tank is the best option. If your AC can not keep your room to 85 or less you need a bigger AC, period. I usually recommend tanks for grows/rooms under 100 square feet, if adequately sealed. A benefit of tank systems is the ability to use fuzzy logic controllers, which predict CO2 usage and compensate accordingly, reducing the sometimes large offsets that occur with simpler metering plans.
CO2 is heavier than air, and should be dispersed from the top of the room down. Oscillating or circulation fans should be employed to keep the heavier CO2 on the floor stirred and mixed in the room. In more than one grow I've hooked the CO2 discharge hose to the back of an oscillating fan so it could "spray" the room with it as it turns.
Generators are slightly more involved, but always follow manufacturers instructions and read the entire manual before proceeding. Generators produce a large amount of heat, some humidty, and copious amounts of CO2. Propane generators are more beneficial and economical for large grows where heat is not typically an issue. A 5 gallon propane tank will last 4 times as long as a CO2 tank at the same flow rate, and costs less.
Ducted generators are now available so 50% of the heat can be ducted outside, as arte water-cooled units from HydroGen (Hydro Innovations.) The hydrogen is basically a small tankless water heater that produces roughly 36 cfh of Co2, but should only be used with tap or well water, pump-driven hydrogens almost always fail to light properly after a while, even with large pumps. My advice is to run these units to waste if at all possible, chilling and reusing this water is not economical.
How long will my CO2 tank or Propane tank last me?
Using the calculations gathered above, divide the total runtime (in hours) by 60 minutes, then figure out how many cycles that would be. for example:
28 hours of runtime X 60 minutes is 1680 minutes of runtime. If the calculator says my generator needs to run for 12 minutes to fill the room, then divide by that:
1680 / 12 = 140 "on" cycles.
If you figure this will need to occur at least 4-5 times per light cycle, we can get a rough estimate of how many days the tank will last:
140 cycles / 5 cycles per night = 28 days.
Again, this does not account for actual plant use, or leakage, so it should only be used as a rough ballpark.
For propane users, a dual-tank kit is available to connect two tanks, doubling time between tank changes. I believe a similar device exists for bottled CO2 users as well.
I've also seen great deals on Craigslist for 100# RV propane tanks, two of those would CO2 my room for over 6 months, and it's a huge room!
The only accurate way to measure actual tanked CO2 or propane tank use is the use of a bathroom scale (preferably digital.) Weigh the tank at the same time once per day and you can quickly determine how much gas you're using up, and multiply X 30 for a monthly average.
Generators vs tanks, pros and cons
Generator Pros:
Large CO2 output
Supplemental heat for cold-weather grows
New pilotless models are safer and waste less gas than pilot models
Propane tanks last longer and are cheaper than bottled CO2
Many models can run off natural gas, great for garage/basement grows where gas-fired appliances are nearby (furnace or water heater)
SIlent
Propane is cheap, $17.99 at my local wal-mart for a 5 gallon tank
Propane is much easier to find than CO2.
Generator cons:
High heat output must be compensated for with larger air conditioners
Propane tanks must be changed every so often
Generator and controller is expensive
Tanked CO2 Pros:
No heat produced at all
Very precise metering
Silent
Many hydro shops carry refill/exchange bottles now
Ability to use fuzzy logic to maintain perfect level of 1500 ppm
Tanked CO2 cons:
CO2 is more expensive and doesn't last as long as a propane tank
Harder to find tanks and more suspicious than a propane tank
Valves/regulators can freeze or stick open
Fuzzy logic controllers are very expensive
Where to get CO2 tanks or propane tanks
CO2 tanks:
Beverage supply stores
Welding supply stores
Hydro stores (some)
Propane tanks:
Most gas stations have an exchange cage
Wal-Mart
Target
Home Depot/Lowe's
Pretty much anywhere.
Using CO2 to kill bugs:
If having a bug infestation, turning the CO2 up to ~10K PPM for 1 hour will kill all the bugs in a room, as well as any humans. This is easier with tanked CO2 than a generator (due to the heat) but will not kill egss from mites. A repeat dosage ~4 days later will deal with new hatchlings before they have time to reproduce.
Well that's all I can think of off the top of my head, but again, please let me know if I forgot to add anything and I'll put it in here with a writer's credit!
Thanks all, LM
