Stage bottles are used for decompression gasses as well as when needing additional air to complete the dive, such as penetrating further into a cave than back mounted tanks only would allow. Stage bottles are generally aluminum 40 or 80 cubic foot tanks. Oxygen stage bottles are generally steel low-pressure tanks so that you can take advantage of a complete fill. Pumping oxygen to 3000 psi (AL40 or 80) is not recommended and could be dangerous. Staging tanks larger than aluminum 80s or using steel tanks is not recommended due to weighting and balance characteristics. If more gas is needed, you should use multiple stages.
Stage bottles must be rigged appropriately so that they can be attached to your harness. Many manufactures make stage bottle rigging kits or you can make your own. Note: There should never be metal-to-metal connections on your stage bottles, such as using a brass ring around the neck of the tank and then clipping to that ring. If the clip gets jammed, you won’t be able to remove the stage tank in the event you get entangled or trapped.
Stage bottles must also be properly marked with the maximum operating depth (MOD) the air or gas in the tank is rated for. The MOD, in three-inch tall numbers, should be placed horizontally on both sides of the tank with the correct orientation so that the you and your buddy can identify the MOD at all times.
Stage bottles require their own separate regulators. A stage regulator should have a pressure gauge on a six-inch high-pressure hose that can be bent back on itself so that you can read the gauge at all times. The gauge can be held in place with line or bungee cord. The length of the regulator hose is typically octopus length so that you can route the hose around your neck and place the regulator in your mouth. The second stage should be attached to the stage bottle until it’s ready to be used and the tank should be turned off until use.
Stage bottles should be worn on your side and clipped to a shoulder D-ring at the top and a waist D-ring at the bottom. Typically, stage bottles are worn on the left side, but they can also be worn on the right side. If you mount your primary light on your right side, it’s more difficult to clip off the bottom of the tank and it may interfere with your light. Practice attaching and removing a stage bottle before attempting to do it on a bigger dive.