Gas switching procedure
Breathing the wrong gas at the wrong depth will kill you.
This is a simple procedure yet it can cause big problems for those trying to do it fast and mess it up. Remember, slow and deliberate is always fast. Also keep in mind that all deco and stage bottles are turned off when not in use. That prevents us from loosing gas without knowing it and is also an additional safety step preventing us from breathing the wrong gas.
These are the most common mistakes for those relatively new to this:
Switch to stage or deco bottle
1. Wait until you reach the switching depth.
2. Hang up you primary light (turned on, pointing down).
3. Choose the proper bottle by looking at the MOD label and show it to your buddy who verifies it with an OK (depth and gas is correct).
4. Grab the second stage with your right hand and route the hose around your neck.
5. Open the valve and purge the second stage.
6. Remove the longhose (with left hand) and put the stage/deco reg in your mouth and breath.
7. Clip of the longhose on the right chest D-ring.
8. Unclip your light and signal you buddy that you are ready with an OK.
Switch back to the longhose.
1. Hang up you primary light (turned on, pointing down).
2. Unclip the longhose and hold it in your right hand.
3. Remove the reg from your mouth with your left and pull the hose over your head.
4. Put the longhose in your mouth and start breathing.
5. Close the valve on the bottle you where breathing.
6. Lift the innertube with your left hand (thumb) and push the hose in there.
7. Pull on the hose and make sure the second stage is secure under the innertube.
8. Unclip your light.
Switching between multiple bottles
If you need to switch from one stage to another, from a deco bottle to another or any combination, you go to the long hose first. Like this:
1. Switch back to the longhose and stow the bottle you are breathing.
2. Move bottles around if it makes things smoother.
3. Switch to the new stage or deco bottle.
And you can have the light clipped off until you have completed the whole process.
If you are switching deco gases, let's say from 50% to Oxygen, you switch to backgas (longhose) at 9m/30ft for the last couple of minutes. That gives you time to stow the 50% and move things around. Then ascend to 6m/20ft and deploy the oxygen. This is the cleanest and safest way to do it.
When doing more than 20 minutes on oxygen you have to do a gas break to keep the gas exchange effective. When you go to breath the longhose you can stow the reg on the oxygen bottle just by clipping the second stage to one of the boltsnaps or the handle. Anyway you chose to stow it, you NEVER let regs hang around your neck. Why? Because when something happens you will not know what you are breathing or even if it is turned on.
When you switch deco gases it's best to do it one at a time, especially if you're not very experienced. The chance of something going wrong is always bigger at the gas switches so it's a good idea to supervise each other. I have stopped people from breathing oxygen at the wrong depth or choosen the wrong bottle several times. Also it's easy to get something into the second stage, like small sticks, sand, clay and if you breath that you may need some help to recover. That's by the way why you need to purge the second stage before breathing it. But we all make mistakes, right?
When you are using stages in the ocean you often suck them dry or almost dry. Then you switch to the backgas (unless you have several stages which is uncommon). If you want to you can signal you buddies, show them the switch sign and everybody can do the switch. Nice if you have similar gas consumption since everybodies stages should be getting close to empty. If you are experienced you can switch on the fly though.