Halcyon Symbios CCR Off-Board Gas Connection Explained

Off-board gas is your way out when the onboard oxygen cylinder runs dry. You connect an external source, add gas manually and hold your PO2 where you want it. I carry 100% and 50% for this. That lets you finish the dive on the unit instead of bailing out — which saves gas and saves a lot of options you would otherwise lose.

The problem: the Halcyon Symbios has no connection for it as standard.

Why the button on the unit is not enough

The Symbios has a button for manually adding oxygen. It works fine, but it is fed from the onboard cylinder. There is no way to attach an external hose to it, which makes that button useless for off-board gas.

Open the back and you see the manual addition runs through a hose. Some people take that hose out and build in a one-way valve, a T-piece and some further parts.

I did not want that. I like to keep a unit the way I bought it.

The solution

Together with Stefan at Galicia Diving in Belgium I went looking for a manual addition valve that lets you connect off-board without changing anything on the Symbios. What we ended up with is four parts:

• a very short inflator hose
• a one-way valve with an inflator connection
• a T-piece
• an external connection

The assembly connects where the standard hose enters the head. The original hose then goes back onto this adapter. So you keep using the hoses the unit shipped with.

Why on top

I mounted it on the upper side, and that is not an accident. The unit sits in front of you, so you keep room to work with both hands. And when you look down you can still see what you are doing. On a valve you would otherwise have to find by feel in the dark, that is the difference.

An extra benefit: it comes off just as easily. For an ordinary dive I leave it at home.

After the dive

I hold the button, press the inflator, and blow out any water that got in using a little oxygen. That dries the assembly out somewhat.

No interest

The assembly can be ordered as the Symbios offboard MAV.

I am not sponsored. I paid for this in Stefan’s shop like anyone else. Worth saying where a recommendation is involved.