semi-rebreather

[Centrals]

It was also not unheard of to convert the dosage unit and add a manual addition valve to convert it to an mCCR.

I can still recall a case in Philippines that happened many yrs ago in Cebu with deadly consequence.

 

[telemonster]

OP, I had thought about the same thing as you before. But I think a number of issues would come about:

1. Keeping the setup small. Pendulum configuration would have dead space. How do you keep things streamlined?

2. I assume co2 removal is done, so then you would need absorbent. If it's close to the regulator then caustic cocktail / flood risks present themselves. Farther away then positive buoyancy.

3. How would it be switched? Mechanical, ehh. Electronics are dirt cheap but then you've got a microcontroller and electromagnetics. Water, batteries, harsh environment, complexity is skyrocketing.

4. Since the breathing gas is now changing, the bottom time calculations by any dive computer would be off. Do you active monitor to improve accuracy on calculation, or do you just average it out? More complexity unfortunately.

Neat idea, but not sure it's practical. So someone should build it to figure it out!

 

[MegDiver792]

This already exists with KISS GEM. Active PPO monitoring, pretty streamlined, etc. Its just the cost of it. But used can be found under 2k, then you just need a fischer connected computer, and you are good to go. Uses your existing OC tank, regulator, etc.

 

[Paul Haynes]

I have always wondered why someone has not come up with a system that allows a diver to breath the same air twice before it is exhaled. This would allow us to use half sized tanks and no O2 tank with the same bottom time and half the bubbles. Also it would not cost thousands like the rebreather systems presently on the market. If i recall, a person only uses about 17% of the O2 in a breath leaving 83% remaining. Some kind of expanding bag possibly with scrubber chemical in it for the CO2. Cycle: breath in from tank, breath out into expanding bag, breath in from expanded bag, breath out into water, repeat.....guess i am dreaming huh?

A system to extend open circuit gas supplies was developed by the French at the end of the 1940's. It was a remains a very elegant solution - what today we refer to as a passive semi-closed circuit rebreather. The French system designed and built by La Spirotechnique was later designated the DC 55, DC after the surname initials of its two principle designers and 55, the year of its introduction to the market place. It was a military rebreather, principally used for mine hunting diving although I used it for years in Swimmer Delivery vehicles before we converted to a fixed oxygen partial pressure electronically controlled system. Being a rebreather for military use it was not freely available to to the public, however the DC55 went on to used by navies throughout the world - and still is by some nations. Its present day manifestation is the Aqualung CRABE - plenty of info on-line for both the DC55 and CRABE. The counter lung was a concertina bellows within which was an inner bellows that was 10% of the volume of the outer bellows. Through a simple system of one-way check valves, on each inhalation, the inner bellows discharged to ambient 10% of the outer bellows volume. This discharge ratio remained fixed regardless of tidal volume. Approximately every third breath, a simple demand valve would be activated by the bellows finally 'bottoming out', thus injecting fresh nitrox or trimix gas (the French are the only nation to have a successful and long history of using trimix gas for both open circuit and rebreathers). Why did such a system not be adopted by the sport diving industry? Well despite its many advantages, a passive SCR has a number of disadvantages that potentially exclude its safe use by recreational divers, where as an 'active' constant mass flow SCR is arguably more suited to the recreational diving environment. That said, as mentioned by someone here, a line of passive SCRs was developed back in the late 1980s / early 1990s that eventually culminated in the RB80 used exclusively by highly trained GUE cave explorers.