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Osprey
09-05-2007, 22:24
So what's the scoop on rebreathers? I admit I don't know a lot about them.. I was supposed to get help from a LDS last spring, they were going to teach me how to use one for a big underwater photography shoot, but the funding fell through and.. well that was that.

I know that they are more compact, produce less noise, but not a lot else. Is there a difference in the gas? Why don't more people use them.. expense?

I sort of browsed online, but I'd like some personal stories if anyone has them, good, bad, or other.

Thank you!

Flatliner
09-05-2007, 22:49
Lots of good stuff here...

www.rebreatherworld.com (http://www.rebreatherworld.com)

Osprey
09-05-2007, 23:18
More popular than I had thought.. thanks!

woody
09-06-2007, 00:01
Basic Function of a Rebreather
To recycle some or all of the exhaled gas back in to the breathing ‘loop’ which allows the gas to be breathed again, hens Rebreather. As you inhale any gas, your body seeks to pull any available oxygen out of that gas and use it to sustain your life, so you must replenish that used oxygen and that is where the designs differ.

Types of Rebreather Designs
Rebreather design falls into 2 major categories, Semi-Closed Circuit Rebreathers (SCR) and Closed Circuit Rebreathers (CCR).

In a Semi-Closed Circuit Rebreather, as you exhale the expired gas is sent to a Counter-lung (a flexible storage container for your breathing gas). A small portion of that gas is expelled in to the surrounding water via a pneumatic or mechanical process. The volume of gas that is released is replenished with fresh higher O2 content gas to restore the total system volume. So as you breathe in you will metabolize some of the oxygen available in the breathing loop and exhale a lower oxygen content breathed gas. This gets mixed with the fresh higher O2 content gas from the cylinder and sent through a ‘scrubber’ to remove the carbon dioxide. The result will be an increase in the overall Oxygen concentration in the breathing gas from what was exhaled.

In Closed Circuit Rebreathers the functions are relative the same but carried out in different manners. The first is that you have 2 cylinders with different gasses in them. First is the ‘Diluant’ cylinder, this cylinder contains your breathing gas of choice, Air for nitrogen based diving or Trimix for Helium/Nitrogen diving. The second cylinder contains pure oxygen. Regardless of the unit or gas you are breathing the body functions the same, you breath a gas in and exhale a gas of lower oxygen concentration out with increased levels of carbon dioxide. Your metabolized oxygen must still be replenished back in the breathing loop, but instead of adding a gas mixture of inert gasses and oxygen to compensate for the used O2, Pure Oxygen is added to replace just what was used. Since the volume of the breathing system it returned to where it started and not any larger there are no bubbles from every breath.

Now to your questions,

I know that they are more compact
Than what? As far as weight goes they typically any where from as much as a single AL80 setup to less than a set of AL80 doubles. My SCR (Dreager Dolphin) is as wide as a normal persons back but a little shorter that an AL80 about as long as an AL63.

produce less noise
They are still noisy. But just to you. Once you get used to that quite you will start to hear the gas moving around the system and any gas additions. The fish don’t seem to notice anything.

Is there a difference in the gas?
Yes in an SCR you wi8ll typically dive a nitrox 32 as the minimum mix and dive EAN mixes up in to the 60% range for some real shallow extended time dives.


Why don't more people use them.. expense?
Yes, Unit and Training Expense is part of it. Other than that it comes down to care and maintenance issues. While everyone else has quickly rinsed their gear (yeah right) and gone of to enjoy the local flair. You will be cleaning, disinfecting, drying, and reassembling your unit. Don’t forget you also have to repack the scrubber too.

I'd like some personal stories if anyone has them, good, bad, or other.
Yes they are extra work,
Yes they are expensive,
Yes they are more dangerous that regular scuba.
Despite all that I love both mine!

JCAT
09-06-2007, 04:55
Well written post by woody.

Osprey
09-06-2007, 08:58
Woody, that was exactly what I needed, thanks :) I appreciate it!

creggur
09-06-2007, 10:05
Woody, are those things computer controlled to adjust your mix "on the fly"?
Can you program it for different mixes during different stages of your dive?


Not a technical diver or anything, but the technology is quite interesting..

frankc420
09-06-2007, 10:25
What makes the rebreather that much more dangerous than scuba? Is it the extra components that are in play?

ianr33
09-06-2007, 10:42
What makes the rebreather that much more dangerous than scuba? Is it the extra components that are in play?

The pO2 needs to be controlled within reasonable close limits. Less than about 0.14 and you risk drifting off into unconsciousness. More than 1.6-2.0 and you risk a seizure from oxygen toxicity.Most rebreathers measure the pO2 with 3 independant sensors.But if something goes wrong (and salt water and electronics are not a good combination! ) and you do not react appropriately to it then you are probably going to die.

At one time I was kinda,maybe ,thinking about looking into getting one some day. Came to the conclusion that as long as I can carry enough gas for a dive then I will do it on Open Circuit.

Lack of bubbles/noise for photographers may be a good reason to use a rebreather on recreational dives (dont know much about that)

Now if I moved to grand Cayman and started doing 300 foot dives every weekend i would probably reconsider.

Osprey
09-06-2007, 11:13
I read that something can go wrong with the scrubber which can cause all kinds of dangers as well, correct?

ianr33
09-06-2007, 11:21
I read that something can go wrong with the scrubber which can cause all kinds of dangers as well, correct?

Yes. If you dive longer than the capacity of the scrubber,or you do not pack it properly then it will not filter the CO2 that you are breathing out.That is bad. Not a rebreather diver but my understanding is that in that situation you go to a bail out tank and call the dive. The problem might be in recognising what is happening before you pass out.

Capt Hook
09-06-2007, 12:04
I just wish they could bring the price down to a reasonable cost. I would love to try one but the classes I've found are way to expensive.

woody
09-06-2007, 14:02
Woody, are those things computer controlled to adjust your mix "on the fly"?
Most CCRs are computer controlled, but not all are. Most SCR units are electronically monitored. The mantra of the rebreather diver is “KNOW YOUR PO2!”

Most CCRs are Constant PO2 rebreathers. Meaning the PO2 is maintained at a constant level, usually a PO2 is in the .7 to 1.4 range. This is what is referred to as a set point. This is the target PO2.
Think of the best mix formula from your nitrox class FO2 = PO2 x P. Well since the PO2 is constant, the FO2 in the mix will move opposite to the Pressure. As you descend you will increase the pressure and the unit will target a lower FO2 in the breathing mix.

SCR rebreathers are more of a guess what you will need and a monitor what you have and are not adjustable while diving. A considerable amount of math is done on the front end to guestimate what you will have in the breathing loop. With those values you will compare them to actual readings during the dive. But the planning is a lot simpler than it sounds.


Can you program it for different mixes during different stages of your dive?
Some units allow you to adjust the set point for different phases of the dives. Some do not. You can manually adjust the breathing mix / PO2 by using the O2 bypass (Similar to a BCD Inflator) and manually add additional O2 in to the mix to increase its FO2 / PO2.

woody
09-06-2007, 14:11
What makes the rebreather that much more dangerous than scuba? With SCUBA you pretty much just have to remember to breath. With a rebreather it is possible with a malfunctioning / improperly configured unit to have it not supply enough oxygen to replenish what you are using. Over time you will lower the available oxygen to the point where it will no longer sustain life. But before that you will loose consciousness. You will start getting sleepy and eventually end up going to sleep underwater and then die (couldn't think of another word to use) from lack of oxygen.


Is it the extra components that are in play?
Some what, The extra components require you to monitor more of what is going on with your system other than breath in / breath out / and check gas supply.

ianr33
09-06-2007, 14:24
What sort of dives do you use your rebreathers for woody?

woody
09-06-2007, 14:27
if something goes wrong (and salt water and electronics are not a good combination! ) and you do not react appropriately to it then you are probably going to die.
True nuff


At one time I was kinda,maybe ,thinking about looking into getting one some day. Came to the conclusion that as long as I can carry enough gas for a dive then I will do it on Open Circuit.

True Nuff. It leads to a less complicated dive. All you have to do is just breathe and switch when the time comes.
But it does offer some advantages in terms of deco profiles. As you ascend your unit will continue to raise your FO2 in the mix, effectively doing continues gas switches for your entire ascent. This creates an optimum ascent profile.


Lack of bubbles/noise for photographers may be a good reason to use a rebreather on recreational dives (don’t know much about that)

Other than getting experience with a unit and extreme NDL bottom times. But some people are just more comfortable in their units than in OC Scuba.


Now if I moved to grand Cayman and started doing 300 foot dives every weekend i would probably reconsider.
Consider the price of Trimix and the drastic reduction in deco time and you certainly would. In some parts of the world you would be better of diving a CCR as diving OC can get expensive and dangerous. In the South Pacific for example helium is hard to get and very expensive when you do get it. The reduced gas costs would allow you to run Trimix in the CCR while on OC you might reconsider it for economical reasons.

woody
09-06-2007, 14:42
I read that something can go wrong with the scrubber which can cause all kinds of dangers as well, correct?
Yes! that too.
The 3 main worries for the scrubber are water, channeling, and time/usage.

First-Water
If the scrubber material get flooded you will create a highly alkaline solution referred to as a 'caustic cocktail' High alkalinity just like acids can cause burns. With water in the scrubber this cocktail will find it's way in to your mouth and possibility your lungs.

Second- Channeling
This is a result of a poorly designed scrubber or an improperly packed scrubber. Channeling is when the gas that moves thought the scrubber moves across the same parts of the scrubber medium. CO2 is removed from the gas by the scrubber medium. The more that it absorbs the less effective it becomes. The gas must also stay in the scrubber medium for a little bit for that reaction to take place. This is call dwell time, and the longer the better. So if the gas follows the path of least resistance and short cuts its way through the scrubber medium it retains a higher percent of CO2 than a properly working unit. This is call break through.

Third – Time / Usage
Again effectiveness of the scrubber medium is a function of how much CO2 has become absorbed in the medium. The total effectiveness of the scrubber medium is based on temperature. The warmer the medium the better it reacts with the medium.

Sorry for all the posts but I gotta get my count up! It's shamefully low.;

woody
09-06-2007, 14:48
I just wish they could bring the price down to a reasonable cost. I would love to try one but the classes I've found are way to expensive.

Good News they are! Some of the first commercial units used to cost $20,000 or more. These days, You have eBay (Get the unit serviced as soon as you get it!). Dreager is also closing out the Dreager Ray units and are having a fire sale on the remaining units. You can find new units in the $4,000 to $8,000 range and some of the more expensive ones are around $10,000. So the prices are dropping. It will just take some more time.

woody
09-06-2007, 14:51
What sort of dives do you use your rebreathers for woody?

Dreager – It’s just a toy that got me into the rebreather diving.

Prism Topaz - Practice and Trimix diving with the appropriate team.

quarrydiver
09-11-2007, 22:10
We used to have a guy up here that did classes in em but he quit due to the cost.