Carbon Dioxide (CO2) is a colorless, odorless gas that is found in small quantities in the air, and is essential for plant life, without it plants could not survive.
Carbon dioxide is absorbed by the plants and during photosynthesis the CO2 is is split into it's basic elements, carbon and oxygen. Small amounts of oxygen are used by the plant but most of the oxygen are released back into the atmosphere.The carbon is combined with water (H2O) in the presence of light to form a sugar molecule. The plants then convert the sugar into carbohydrates. When the plant absorbs nutrients (primarily nitrogen from the roots) they are combined with the carbohydrates to form new plant tissue. This process is called photosynthesis. The entire process is only as good as its weakest link. If any of the required ingredients (light, CO2, water and nutrients) are at a level below that which the plant can use for maximum efficiency, the plant will not perform at it's full potential.In other words, if you inject CO2 into a system that is not receiving enough light or nutrients the results will be disappointing.
We here at Hydro-U recommend that CO2 injection should only be done by experienced gardeners with a good working knowledge of their gardening system.
Once a gardener is comfortable with the workings of their system and plant growth, CO2 can be a great benefit, however there are a lot of variables involved with using CO2 and beginners can really have their hands full, increasing the likelihood of a disaster (like total death of the entire crop).
There are several conditions that must be met for the plants to be able to use the increased CO2 levels properly. The most important is lighting. Light levels must be very high (more than 20 watts per square foot) or there will be little or no increase in plant growth rates. The plants will like slightly higher temperatures than normal (approx. 3 - 5 degrees higher). The plants will also metabolize water and nutrients faster, so reservoirs may need a little more attention.
Plants can absorb and process very large amounts of CO2. There is usually about 300 to 600 p.p.m. (parts per million) of CO2 in the atmosphere. Most plants can use 1500 p.p.m. in optimum growing conditions. When using elevated levels of CO2 the growth rate can be increased by as much as 100% to 200%. Most studies report increases in the 40% to 50% range.
The ideal situation would be to keep the CO2 levels at optimum at all times. This would require constant injection of low levels of gas (constantly replacing what the plants are using). This is not practical in most situations as venting of the growing environment is often needed to control heat build up. In these cases CO2 injection should be done immediately following venting.
The biggest problem that people encounter when using CO2 is that they get carried away, they think that a little is good so a lot is better....NOT! When CO2 levels approach 2000 p.p.m. most plants will die. High levels of CO2 are also toxic to humans, primarily due to oxygen deficiency. Before injecting CO2 the room should be vented to remove excess CO2 that might be left over from the previous injection, this prevents the build up of CO2 that could harm the plants.
There are several ways to get extra CO2, the two most common are using bottled CO2 and using CO2 generators. These are the automated ways to add CO2 to the growing environment. Getting precise control of the CO2 levels in your growing environment can be rather expensive, CO2 monitors are the best method, these monitors keep a constant reading of the C levels and automatically adds gas when needed. These monitors are fairly expensive so most people opt for a more inexpensive method (like timers).
There are also several "low-tech" ways to increase CO2 levels. Additional information about using all these methods follows:
Using Bottled CO2
There are several ways to introduce CO2 into the growing environment. Probably the most popular method is to used bottled gas. This type of CO2 injection consists of a CO2 tank, a pressure gauge (monitors how much gas is remaining in the tank), a flow meter (to monitor the amount of gas being released), a solenoid valve (turns the flow of gas on and off as needed) and a method of controlling the solenoid valve (a timer or other controller).
Using CO2 Generators
Until recently CO2 generators were used primarily by commercial growers, but with the advent of smaller, less expensive units, many hobby growers now use generators. CO2 generators burn either propane or natural gas. They have a pilot light that ignites the gas when a timer has opened a solenoid valve to release the gas into the system (similar to a gas bar-b-que grill). The generators come in many different sizes.
Producing CO2 with a generator is more cost effective than using bottled CO2. The generators can produce aprox. 26.1 cu. ft. of CO2 per pound of fuel burned.
The generators work very well and are cheaper to operate than the bottled system, however, they burn the fuel so they give off a lot of heat as well as CO2. This may or may not be a problem for you. If you live in a colder climate and you need to add heat to your growing environment than a generator might be just what you need. If you live in a warmer climate you may already be battling the heat and adding more might send you over the top. With devastating effects on your crop.
Another word of caution about CO2 generators: If they are not working properly they can give off Carbon Monoxide (CO) instead of Carbon Dioxide (CO2). Carbon Monoxide is very dangerous! It is deadly even in small doses (CO2 is deadly too, but only in higher concentrations). You should only use top quality generators that have been built specifically for gardening and have been checked out by the manufacturer. When using generators make sure that the flame is burning blue, this means that there is complete combustion and the generator is putting out CO2 and not CO. If the flame is burning yellow or orange there is a problem with the unit. Turn it off immediately and do not use the generator until a qualified service person has fixed it! Yellow or orange flames mean that the combustion is not complete and the main by product will be Carbon Monoxide instead of Carbon Dioxide.
OTHER METHODS OF ADDING CO2
There are several ways to add CO2 to your garden that do not require a lot of expensive equipment. These are not automated so require more "hands-on" attention.
Controlling the amount of CO2 is difficult at best with these methods, however there is little chance of accidentally adding too much CO2.
Decomposing organic matter gives off CO2 as a normal bi-product of decomposition.
Use materials like leaves, sawdust, manure, hay, etc.This method of generating CO2 is certainly inexpensive, with most materials being free, but it can be kind of gross. Besides being messy there is a possibility of disease and fungus forming in the decomposing material.
The fermentation process also gives off CO2 as part of the natural process (that's what puts the fizz in beer). Mixing water, sugar and yeast together will cause fermentation to begin and for CO2 to be generated.
Dry ice is solid CO2 that is made by compressing the gas until it "freezes" into a solid mass. CO2 is a rather unique substance in that it goes directly from a solid to a gas, which means that you can simply set a block of dry ice in your growing environment and it will slowly "boil" off CO2 gas. Dry ice is probably one of the most expensive ways to produce CO2 in your garden.
One pound of dry ice is equal to one pound of liquid CO2 (bottled). One Pound of either is equal to 8.7 cu. ft. of gas.
If you drip vinegar onto backing soda you will produce CO2 as well.
ADDITIONAL INFORMATION
If you would like to know more about using C in your garden there is an excellent book about it called "Gardening Indoors With CO2" by George Van Patten.
There is a lot of good information about CO2 equipment as well as other environmental controllers available at the following link. which is the website for GreenAir Products, which manufactures what most people consider the finest environmental controllers available for hobby gardeners.
for the find Xlr8! It's cheaper than more CO2 things also!
EDIT: I wrote the company, will let you guys know my findings.
Unless you tell me not to.. Glad your liking indoor growing! It sure seems the girls like it! 
You gave'em a hell of a home!
