The heart of this system is the scrubber canister. The top section of this canister – the head unit – is fitted with two hose attachment points and an over-pressure exhaust valve. The cylindrical bottom, which threads into the scrubber head, is sized to hold a single Micropore ExtendAir 801C large bore CO2 absorbent cartridge.
Assembled, the scrubber stands approximately 13.5 inches (34.3 cm) in height with an overall diameter of approximately 5.5 inches (14 cm) – about the same thickness as an aluminum 40CF cylinder. Included with the canister is a mounting system that ties to a BCD’s primary cam strap, allowing the system to attach to standard aluminum or steel scuba cylinders.
The rest of the system is front-mounted, and includes two counterlungs fitted with protective coverings and mounting hardware with Velcro flaps for attachment to the 2-inch webbing on a technical style harness. There are 4 loop hoses in all. The first pair (one 17-inch, one 22-inch) connects the scrubber canister with the counter lungs, while the other 2 (each 17-inches in length) run from the counterlung’s T-pieces to the GEM’s mouthpiece.
In addition to serving as recipient connection between the loop hoses and counterlungs, the T-piece atop the right hand counterlung features a built-in gas addition system (ADV) in the form of an Aqualung Mikron 2nd stage regulator. The left hand side T-piece houses a single oxygen sensor for monitoring the your PO2 in the loop.
The remainder of the GEM’s gas delivery system utilizes a standard open-circuit regulator. To integrate an existing regulator system with the GEM, simply attach the hose that connects the ADV to the first stage. There is no need to remove the alternate air source (octo) from your regulator, unless you want to.
GEM is an acronym for Gas Extending Mechanism. The more technically minded will refer to it as a Passive Addition Semi-Closed Rebreather, or PASCR for those who enjoy even more acronyms.
Unlike a fully closed rebreather, which uses both oxygen and diluent gasses, semi-closed rebreathers require only one gas supply. In the GEM’s case, this means the same Nitrox mixes of 32 percent and above that are readily available to recreational divers. And unlike active-addition semi-closed rebreathers, which use gas at a constant rate, the GEM’s gas delivery system is keyed directly with the diver’s breathing at a ratio of approximately 3 to 1 – extending the duration of any cylinders gas supply by a factor of three.