What is a rebreather

[Derek M.]

I just wanted to know what is a rebreather?

 

[battles2a5]

There are a few different types of rebreathers. The main categories are semi-closed and closed. A semi-closed rebreather uses a single gas and recirculates a portion of the gas to increase gas efficiency and extend the range of a dive with a given volume. A fully closed system recirculates the entire gas volume and requires the use of 2 gases; pure oxygen and a diluent. The diluent is used to dilute the pure oxygen and the system injects oxygen to replace the volume that your body metabolizes. The net effect is a massive increase in gas efficiency and range.

Semi-closed have 2 main categories, constant mass flow and PASCR (passive addition semiclosed rebreather). The former relies on a constant flow orifice tuned to the gas, the later uses a counterlung that is smaller than the diver's respiratory volume which forces the system to expel a portion of the gas which is replaced via demand valves during the breathing cycle.

Closed circuit (CCR) can be defined as mCCR (mechanical CCR), eCCR (electronic CCR) or hybrid (mix of mCCR and eCCR). All require electronics to monitor oxygen levels. mCCR uses the same principle as the CMF semi-closed but the orifice feeds pure O2 and tuned to deliver slightly less O2 than the body consumes. Therefore, the diver must add o2 to maintain desired levels but the system is very simple and easy to run. eCCR uses electronics to monitor the O2 levels and automatically injects oxygen to a desired set point.

This is a gross generalization. Check out wikipedia or Rebreatherworld.com is you want to learn more.

 

[idocsteve]

The post above may be a bit technical as a response to the newcomers question, so I'll provide one in 'laymans terms.

A rebreather is different from "regular Scuba gear" in that rather than exhausing your breath as bubbles, most of it is recirculated and filtered to maintain it's oxygen level. By reusing your gas it lasts much longer and you don't make as much noise underwater so you can get closer to the fish and other aquatic life. Because you're getting such long bottom times, you will most likely find yourself in "deco" more often than not and this needs to be considered as part of your dive plan.

Rebreathers are much more complicated (and expensive to buy and maintain) and unfortunately there have been more than the "typical" numbers of accidents which usually result in fatalities, as compared to diving tradiational "open circuit" Scuba gear. These accidents are almost always due to a diver screwing up when it comes to maintaining or using the rebreather properly, and the most common result of that is a "blackout" where the diver loses consciousness due to not having enough oxygen or perhaps it's because of too much C02 buildup.

 

[battles2a5]

These accidents are almost always due to a diver screwing up when it comes to maintaining or using the rebreather properly, and the most common result of that is a "blackout" where the diver loses consciousness due to not having enough oxygen or perhaps it's because of too much C02 buildup.

No no no, they are all heart attacks, drownings, or some other medical event that only seem to effect rebreather divers. Rebreathers are safe... really

 

[idocsteve]

Rebreathers are safe... really

So are cars and airplanes and guns and farm equipment.

The problem with all of the above and most everything else is usually "operator error". Well with guns it might not be an error but you get my point.

 

[battles2a5]

and I was hoping you would catch my sarcasm...

 

[idocsteve]

and I was hoping you would catch my sarcasm...

I tried to catch it but I was holding a rebreather in both hands.

 

[Rainer]

It goes on your back (or chest...).

I tried to catch it but I was holding a rebreather in both hands.

 

[Rainer]

I know you're generalizing, Ryan, and your post is a really good summary, but just wanted to point out that you don't need two gases to have a fully closed system. Pure O2 rebreathers are actually pretty common (especially in the military). Understanding how they work is actually significantly simpler (IMO) than those which add diluent. Basically (and this isn't aimed at you Ryan, I know you know how these work!), you breathe in and metabolize O2, breathing out non-metabolized O2 and CO2. The rebreather "scrubs" the CO2 and recirculates the remaining gas (not wasting the O2) and you add extra O2 as needed as it's metabolized.

A fully closed system recirculates the entire gas volume and requires the use of 2 gases; pure oxygen and a diluent.

 

[battles2a5]

I know you're generalizing, Ryan, and your post is a really good summary, but just wanted to point out that you don't need two gases to have a fully closed system. Pure O2 rebreathers are actually pretty common (especially in the military). Understanding how they work is actually significantly simpler (IMO) than those which add diluent. Basically (and this isn't aimed at you Ryan, I know you know how these work!), you breathe in and metabolize O2, breathing out non-metabolized O2 and CO2. The rebreather "scrubs" the CO2 and recirculates the remaining gas (not wasting the O2) and you add extra O2 as needed as it's metabolized.

That's a good point. You just don't see a whole lot of O2 rebreathers out there outside of military use. When you make the jump, most people don't want to limit their diving to 20ft (or whatever depth the military lets these guys dive pure O2). Same theory applies, you just don't need the dil given the depths.