After reassessing my method for culling feeders, I have come to the realization that my current method is no longer the most efficient for the amount of rats that I am producing.  I decided, after breeding rats for three full years, to build my own carbon dioxide (Co2 ) chamber. Before getting to the nuts and bolts of the project, first a little information on carbon dioxide and how it works for this application: Carbon dioxide is a tasteless, colourless, odourless, non-flammable liquefied gas that is heavier than air (1.555:1).  Humans and animals exhale it, while trees and plants absorb it to release oxygen. Carbon dioxide is used on many applications around us including, but not limited to; carbonation for beverages, dairy products and fruit juices to extend their shelf life, cooling products for transport, metal fabrication and for fighting fires. For the project I am covering here, carbon dioxide can be had in two forms; SOLID:  Also called “dry ice”.  Dry ice is a solidified carbon dioxide.  Differing from “normal” ice, which exists at 0 degrees C, dry ice exists at -79 degrees C.  Unlike “water ice”, dry ice does not melt into the atmosphere but sublimates (changes from a solid to gas), releasing Co2 vapour.  In enclosed environments, it can displace oxygen and cause asphyxiation. LIQUID/VAPOUR:  Liquid carbon dioxide, recovered as a by-product of manufacturing processes or from natural wells is purified, liquefied and compressed into tanks for various uses. I have chosen to use liquid carbon dioxide for my purposes for a few reasons; firstly, a 10 lb. tank of Co2 can be purchased or rented fairly inexpensively and stored safely as Co2 is inflammable.  Secondly, Co2 in solid form is dangerous to handle, difficult to store, and sublimates in normal atmospheric conditions, whereas it’s compressed liquid form can be stored at room temperatures and used only when needed.  A normal 10 lb. tank provides enough carbon dioxide to be used for 3-4 months without refilling (at a minimal cost of $15) if used 2-3 times per week. Now that we’ve got the technical information out of the way, here is a list of the items (with cost) that you will need to build and assemble your own Co2 Chamber.  All of these items (aside from the Co2 tank itself) can be purchased at any well-stocked hardware or “big box” store (Home Depot, Rona, Canadian Tire, etc):   10 lb. tank (filled with Co2):  Can be found at any fire and safety supply stores, look in your Yellowpages.  I was lucky enough to have found a refurbished 10 lb. canister (filled) for $100.00. Holding Tank:  You can use any size enclosure that you feel will fit your need.  For this application I used a Rubbermaid tall #2220 (16X11X9) purchased at Wal-Mart for $7.00. Tubing:  Since my connections were threaded to 5/8”, I picked up 10 feet of 5/8” tubing (inside dimensions) at Home Depot for $6.00 ( I only ended up using 3.5 feet).   Flourescent Light Grating:  To lie on the bottom of the holding tank to keep the feeders elevated for better airflow and cleanliness. I purchased a 2’X4’ panel (only size that is available) at Home Depot for $6.00. Adapter Valve:  To connect the Co2 tank to the hose.  I purchased the adapter valve from the same place I purchased my tank for $3.00. 2 “O” Clamps:  Used for tightening the connection between hose and adapters.  I purchased them at Home Depot for $0.59 each. Connector/Coupler:  Used for connecting the hose to the holding tank.  Found at Home Depot for $4.00.     Total cost for supplies:  $127.18 Cdn. (~$100 US) That’s it for supplies, now I’ll get to what you’ll need to do to put everything together: Firstly, attach the adapter valve to the side of the Co2 tank.  Hand tighten only, there’s no need to use pliers or a wrench.  This connection will allow you to “screw” one side of the hose onto the exposed threads of the adapter valve. Next, attach the “O” clamp over one side of the hose and begin “screwing” the hose onto the adapter valve.  Once the hose is attached, bring the “O” clamp over the hose at the threads and tighten with a screwdriver to form a tight seal between the threads of the adapter and hose.  This will ensure that no leaking Co2 gas will escape during normal use. Once the hose is attached at one end, take your holding tank, in this case the tall #2220 Rubbermaid and with a drill and bit, make a ¾” hole in the middle of one of the wide sides (to allow more air flow in to the holding tank) close to 2” up from the bottom.   The connector that I purchased has one side threaded at ¾” and the other side at 5/8”.  Use the ¾” side, and insert it (you might have to screw it) into the hole.  Once it is through and snug, attach the cover over the ¾” threads (which should be on the inside of the holding tank).  Form a nice, tight seal so that once the hose is attached at this end there is no Co2 leaking. Place the “O” clamp over the loose length of hose that Is attached to the canister and place (or screw) the hose over the threads, screwing it into the hose until the threads are covered.  Place the clamp over the hose at the threads and tighten snug with a screwdriver. Almost done!  Cut your pieces of plastic grating (I stack two pieces) to the dimensions of the bottom of your holding tank to keep the rats elevated both for better airflow and to keep them off their waste.  The plastic grating should be an inch or so from the connector attached to the side of the holding tank underneath it. The last thing that needs to be done is Quality Control; turn on the Co2 tank SLOWLY!!!  Pressure in the tank is over 800 PSI and can be dangerous  if turned on too quickly.  You should only have to turn the knob one half to three quarters of a turn after you hear the hissing of gas.  This is all the gas you will need to have on when the chamber is in use.  The less pressure you have the gas at as it enters the holding tank, the more comfortable the rats will be as they fall asleep.  Now check all of your connections to make sure that there are no leaks. You will now notice that the hose will get cold while in use; Co2 gas is cold!  To avoid cracking or splitting of the hose, try not to move the set-up too much.  I have found that it is best to not move your set-up for at least a half hour after use. With proper maintenance this set-up should last for many, many years. . updated Feb 1, 2011