Many different hobbies, from home brewing to playing paint ball, make use of carbon dioxide (CO2). If you use enough CO2, you can refill your own tank instead of continually running to the local sports or hardware store -- wherever you get your CO2 refilled now. You will need to purchase a CO2 refill station to do it at home.
Place the CO2 bulk tank in a secure place. If the tank is not secured properly and it is allowed to fall over, the valve can break off, releasing pressurized CO2 and turning the tank into a potentially-lethal rocket. A metal frame can secure the bulk tank, as can a tight strap wrapped around the tank and attached to an heavy object.
Attach the refill line to the CO2 bulk tank. Place the plastic ring that came with the refill line between the tank output and the line input when attaching these two parts.
Attach an empty CO2 tank to the other end of the refill line. The line will screw into the knob on the tank. Make sure the line is tightly attached to both the bulk tank and the smaller tank before doing anything else.
Secure a section of hose over the small white barb on the empty tank in order to redirect the gas and help avoid any contact with the extremely cold CO2.
Slowly open the purge valve on the dual valve refill line to make sure all of the CO2 inside the small tank is released. A single valve line will not allow you to do this. Close the purge valve once the tank is completely empty.
Place the empty tank on an electronic scale. Zero the scale -- set it so the weight of the empty tank is equal to zero -- if you are able. If you can't zero your scale, note the weight of the empty tank and use that number when figuring the weight of the CO2 in the tank.
Open the valve on the bulk tank and then open the on/off valve on the refill line. When the scale reads an ounce or two, shut off the on/off valve. Purge this CO2 from the tank. This time, do it quickly in order to chill the tank.
Open the on/off valve on the refill line. Don't let the small tank fall over as it fills. Once the tank is within a few ounces of its capacity -- the larger the tank, the more space you should leave -- stop the flow of CO2 by shutting off both open valves.
Unscrew the refill line from the newly filled tank and then open the purge valve on the refill line. Once the line has been completely purged, remove it from the bulk tank. Make sure all of the valves are closed on your various pieces of equipment before finishing up.