The US Navy has begun pool testing of its new MK29 Mixed Gas Rebreather designed to conserve helium during deep sea dives below 170 ft (52 m). The test dives took place at the Naval Surface Warfare Center Panama City (NSWC), Florida, where it was developed under a project sponsored by the Office of Naval Research Global (ONR Global) TechSolutions program.

Rebreathers are a form of breathing apparatus that has been used for decades. Using a chemical scrubber to withdraw carbon dioxide from exhaled air and return it to the wearer, this closed-circuit system is used by firefighters, mine rescue teams, mountaineers, and others who may have to go into areas with low oxygen or high concentrations of poison gas, where they cannot breath the outside air.

For underwater work, rebreathers are mainly used by special forces dive teams like British Special Boat Service (SBS) and US Navy Sea, Air, Land Teams (SEALs) teams, or by underwater natural history photographers because such rigs don't produce bubbles that might give away the diver's location or startle the local sea life.

However, the new MK29 is designed to solve a different problem. Conventional rebreathers use a mixture of oxygen and nitrogen like scuba gear and are only intended for use down to about 150 ft (46 m). And, like scuba, this mixture becomes increasingly dangerous the deeper one goes, requiring decompression procedures to prevent the onset of the bends, or risking nitrogen narcosis, where the nitrogen concentration becomes toxic and the diver can literally become drunk on air.

Below 170 ft, we are in the realm of what is called technical diving, where the oxygen nitrogen mixture is replaced by oxygen/helium or even more exotic mixtures. Pure oxygen would be immediately toxic at great depth, so it has to be mixed with something. Helium, due to its tiny molecules and inert nature, is better than nitrogen because it seeps in and out of the body tissues more readily and doesn't produce the same narcotic effect.

However, helium has its drawbacks. Aside from making one's voice sound like Donald Duck, it carries off heat at six times the rate as air, so the diver must wear a heating apparatus or risk freezing to death in temperatures that would be perfectly comfortable for a wetsuit diver. Such systems are also extremely bulky.

And reducing this bulk is the rationale behind the MK29 rebreather. Technical diving requires massive amounts of helium that must be supplied by air hoses from pumps on a surface support vessel. The current practice in the US and other navies is to use the Fly-Away Mixed Gas System (FMGS), which vents the diver's exhaled breath into the water. This means every dive requires massive tanks of helium that are expensive, hard to transport, and are a general logistical nightmare.

As in the shallow-water rebreathers, the MK 29 filters out the carbon dioxide using a scrubber and cycles the clean mix back to the diver with a slight oxygen top up. According to the ONR, the recent tests have shown great promise.

"The MK29 decreases helium requirements by approximately 80 percent," says Dr John Camperman, a senior scientist overseeing the development of the MK29 at NSWC Panama City. "Divers can perform more dives with the same amount of gas, or bring less helium."

In addition to these benefits, the results of the prototype tests also showed that the rebreather allows for reduced breathing noise thanks to a new 3D-printed titanium tubing connecting the hoses to the breathing manifold, zero bubbles, and less helmet fogging.

Testing of the MK29 will continue and the Navy hopes to issue the rig throughout the fleet next year.

The video below discusses the MK29 rebreather.

Source: ONR