What is a rebreather

[Derek M.]

Thank you guys. I do not really understand why they seem so dangerous. What is the most common danger when using a rebreather?

 

[battles2a5]

Thank you guys. I do not really understand why they seem so dangerous. What is the most common danger when using a rebreather?

There are several dangers. The 3 primary textbook dangers are hyperoxia, hypoxia, and hypercapnia. But the greater practical danger in rebreathers is complacency, followed closely by lack of experience and lack of proper training.

 

[RJP]

I just wanted to know what is a rebreather?

It is a device that recirculates the air you exhale, removing the CO₂ out and adding O₂ back in. From what I've been able to gather it does this by passing the gas through your wallet where something called "scrubber" removes all the money.

:shocked2:

What is the most common danger when using a rebreather?

Your spouse finding out how much it costs!

 

[battles2a5]

Your spouse finding out how much it costs!

My revo cost $1000

 

[Cave Diver]

 

[RJP]

My revo cost $1000

Custom fit drysuit: $2,100
21w HID canister light: $1,259
Gas-switching computer: $1,500
Miscellaneous bits and bobs: $4,500

Your wife believing the whole lot was "$300 on ebay" - PRICELESS!

"If I die, please don't let my wife sell my gear for what I told her I paid for it!"

 

[kathydee]

Custom fit drysuit: $2,100
21w HID canister light: $1,259
Gas-switching computer: $1,500
Miscellaneous bits and bobs: $4,500

Your wife believing the whole lot was "$300 on ebay" - PRICELESS!

That’s a good reason to have a wife who doesn’t dive!!

 

[Cave Diver]

Here is a good write up on CCR's:

Rebreather Fatality or Simple Pilot Error :: Seaduction ®

By Mike Ange

This is an opinion piece and the views are solely those of the author. The views expressed are in no ways indicative of the views of this publication or its principles.

Rebreather fatalities are largely caused by primary equipment failure.

Now that I have the attention of every avid rebreather diver (and personal injury attorney) out there – BEFORE you get the tar and feathers out – let me clarify that in accident after accident the primary equipment failure generally lies directly between the auditory input sensors on the diver’s head! I recently had a conversation with a reporter for a popular news source that sent me over the edge – when he asked why no one had taken steps to make this horribly dangerous technology illegal in the US. I have known it for a while but this “interview” drove home an important point. Rebreathers like American made cars, generally get a pretty shoddy deal from the diving media and the media in general. I think this occurs because deaths on this “exotic technology” are more sensational than the “garden variety accidents” that occur on scuba and because in the US we live with a mentality that no one is required to actually have personal responsibility. In this article, we will look at the technology very briefly for the uninitiated and the realities of why most accidents occur on this technology that predates regular SCUBA by more than 50 years.

What is closed circuit breathing? Rebreathers are a form of scuba – self-contained underwater breathing apparatus – that attempts to make breathing underwater more efficient by recycling used gas. The average adult inhales approximately two liters of gas every time he breathes. But, in reality, he only uses about 4% of that gas. The other 96% is exhaled as a waste product. On the surface, this is no big deal as the exhaled gas mixes back into the atmosphere and is available for use again. But underwater we have to carry our gas supply with us and to make matters worse, the deeper we go, the higher the volume of gas we waste with every breathe. As you know, according to the principles of Boyle’s Law, a diver at 33 feet inhales with every breath, twice as much gas as he did at the surface. However, metabolically, he still consumes approximately the same amount of oxygen. So, at 33 feet, 98% of the gas used is wasted and the deeper you go the more pronounced this waste becomes leading divers to use increasingly heavier and larger cylinders in order to complete their dives.

In general, rebreather technology seeks to correct this problem by capturing the used gas and recycling it. But there are a couple of issues with this. The waste gas provides a vital function for the diver – it provides the volume necessary to inflate the lungs so that they can properly function to exchange the minimal amount of gas actually required for body function. In addition to bringing good gas in, the lungs also expel waste gas, most notably carbon dioxide, and this is the primary problem that the rebreather must solve. If the carbon dioxide levels in your lungs are allowed to build, hypercapnia or carbon dioxide poisoning will result. The other problem that divers have is that as they recycle the air in and out of their lungs, the oxygen content naturally falls so that the second vital function of a rebreather is to replenish the oxygen supply so that the diver is breathing a mixture safe for vital life functions.

Courtesy Dräger Dive Americas

Contrary to common logic, the CO2 is the more critical piece of the puzzle. A diver can die from CO2 poisoning much more rapidly than he will die from hypoxia (breathing a low concentration or partial pressure of oxygen). This math is not exact, but for simplicity let’s assume that for every liter of oxygen consumed the diver produces one liter of carbon dioxide. Therefore, if a diver on the surface uses 4% of the total gas inhaled and all of that gas is oxygen, his exhaled breath would contain approximately 4% of CO2. If the diver is breathing a Nitrox mixture of say 40% oxygen, he could re-breathe the same breathe between 4 and 5 times and still have an oxygen level that is equivalent to the level usually breathed on the surface at sea level. (A 4% decrease with each breathe X 5 breathes = 20%, 40 – 20 =20 or slightly less than atmospheric air.) Unfortunately, that 4% gas is also being replaced in our example with approximately 4% CO2. A CO2 concentration of 3% will create symptoms of hypercapnia. Sustained breathing of concentrations even slightly higher than 3% can be fatal. So using our example, the diver’s recycled breath could be dangerous to re-breathe after only one breath. Rebreathers deal with this problem by passing the exhaled air through a powdered or granular mixture commonly called scrubber. There are a number of different chemical compounds that will accomplish this and the compounds in use vary slightly from manufacturer to manufacturer and they change over time. However, the general mechanism is the same. The gas is passed through a granulated material where the CO2 bonds with the grains releasing heat and moisture in the process. The clean, warm, moist air is then passed out the other end of the scrubber material into a breathing bag or counter lung. This bag pressurizes the gas to the same level as the surrounding water pressure so that it can easily be inhaled by the diver.

The other issue that the rebreather must deal with is the depletion of oxygen and there are three different methodologies for dealing with this issue. Rebreathers are classified generally into three broad categories: semi-closed circuit (SCR), oxygen closed circuit (O-CCR), and electronic closed circuit system (E-CCR or just CCR). O-CCRs use, as the name implies, pure oxygen as the only gas and they are basically volume controlled. The diver breathes pure 02 and as the O2 is used up by the diver, a demand valve very similar to a regulator second stage kicks in and replenishes the supply of gas to maintain the volume of the counter lung. Because the divers are using pure oxygen and oxygen becomes toxic at very shallow depths, these rebreathers are generally limited to military or public safety applications and dive depths in the 6 to 9 meters (20 to 27 feet) range. Semi-closed circuit systems work on a constant flow methodology where a very slow trickle of gas is “leaked” continuously into the breathing loop to replenish the O2 content. Although much more efficient than open circuit, these systems are not quite as efficient as electronic units and they do produce a small discharge of bubbles throughout the dive. These units are a good choice for recreational divers because they do deliver very efficient gas use with durations of two and one half to four hours depending upon the unit, depth, etc on tanks containing as little 27 cubic feet (3 liter cylinders) of gas. They do require the use of a pre-blended nitrox mixture which means that the dive depth must be planned before the dive commences. A simple formula or table is used to predict the gas mixture that will actually be inhaled by the diver during the dive. Because they are relatively simple mechanical devices, SCRs are relatively easy to maintain and much cheaper to operate than electronic units.

The electronic CCR is the state of the art, at this time, and it works exactly the same way as a semi-closed unit with one exception: instead of injecting a continuous flow of gas, the rebreather uses electronic technology and sensors to continuously adjust the gas mixture in the breathing loop. The E-CCR uses two cylinders of gas instead of one. One cylinder is pure oxygen and depending upon the dive and the unit used, the other cylinder, commonly called the diluent cylinder, can hold a number of different gases ranging from air to trimix or heliox. Gas from the oxygen cylinder is passed through an electronically controlled valve into the breathing loop whenever the sensors indicate that the O2 levels have dropped below the set point prescribed in the E-CCR’s computer system. Therefore you only use an amount of gas equivalent to the metabolic consumption of the diver for most of the dive. Depending upon the level of sophistication, the E CCR will either use a manual push button valve, a demand valve, an electronic control valve or some combination of these to add gas from the diluent tank to the breathing loop when required. Generally, this occurs when the volume of the loop drops (during descent or after mask clearing for example) and whenever the oxygen content in the loop is higher than the unit’s pre-programmed set point.

So, that is the two minute and much simplified tour of rebreather technology. For safety, it is vitally important that the diver understand the pre-dive safety checks and the specific nuances of the operating system used by the rebreather. That is why every rebreather requires a distinctive certification course before the diver can use the system. It is true for all kinds of diving, but rebreather diving is more unforgiving and as a result safety shortcuts kill here more quickly than with open circuit systems. Here is the primary difference between open circuit and closed circuit technology: if your open circuit system fails, you will either get an uncontrolled violent flow of air or nothing to breathe at all. Given those two possible scenarios, any type of catastrophic equipment failure is certain to meet a concerted and immediate response from the diver. The rebreather diver, however, doesn’t have the luxury of a kick you in the teeth warning. The gas delivery systems on any of the rebreathers can quit functioning and the diver will continue to have something to breathe. Unfortunately, the oxygen supply in the loop will continue to drop by about 4% with every breath. It is possible for the diver to continue breathing until he becomes hypoxic, looses consciousness and drowns without ever realizing that the system has malfunctioned. Likewise, the diver can wind up with CO2 buildup causing hypercapnia if the scrubber container on the unit is not packed and used properly or if the recommended duration of dive time for the scrubber material is exceeded. If the scrubber is not packed properly, the granules inside the canister can settle making channels where the breathing gas can pass through the canister without being properly cleaned.

Any of these conditions can create significant and serious safety issues. If the diver runs out of gas, if the electronic control system were to fail or any other problem were to occur that prevented the rebreather from adding gas to the loop, the diver can continue to breathe possibly without a clue that he has a problem occurring. It is for these reasons that rebreather divers have to be meticulous in the set up of their unit, the completion of pre-dive safety checks and monitor the unit’s displays and warning systems routinely throughout the dive. It is rare that all of these precautions are taken in accordance with the manufacturer’s specifications and an accident still occurs. Yet we never read in the popular media about the true cause of many of these accidents. If the semi-closed rebreather diver failed to check the flow rate as required in the setup procedures and the flows are not adjusted properly when the accident occurs, the equipment failure is not in the rebreather – it is in the diver. If a rebreather diver packs his scrubber canister improperly or too soon before the dive and channeling occurs, it is not a rebreather failure, it is a diver failure. If a rebreather diver runs out of gas while he is playing with his video system, does not realize it and dies, that is not a rebreather failure, that is a diver failure.

In most of the western world, we live in free and open societies where, generally speaking, people are free to kill themselves through recklessness and carelessness anytime they desire. Don’t get me wrong there clearly have been rebreather failures that caused accidents, but in comparison to the diver errors that have caused rebreather accidents the number is very small indeed. Undertaking dives on closed circuit technology requires that the diver gets proper training and takes responsibility for his or her own safety. If you are not ready to accept that responsibility, do not dive a rebreather. If you do take that responsibility, do not run to the media or your corner ambulance chaser to blame the rebreather for your breach of personal responsibility. It is a wild and woolly world out there and diving, especially technical diving, is an adventure sport. So either grow up and deal with it or else stay at home on your couch and watch Sea Hunt reruns on T.V.

For those of us in the media, we have a responsibility to express opinions as opinions and to be careful with what we reflect as factual. If you cannot verify that the technology failed in a diving accident do not imply that some piece of technology is responsible. Find out what really happened, limit your facts to what you actually know or go write for a supermarket tabloid! AND YES – maybe the American cars do deserve the bad rap but that is an article for another time and venue.

 

[jepuskar]

Took some searching, but I found it.

http://www.scubaboard.com/forums/rebreather-diving/215489-can-someone-explain-rebreather-me.html

 

[Derek M.]

Thanks everyone. I liked the video and that quote Cave Diver gave even though it was a tough read. The diagram helped a lot to. Again Thank you guys