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BUBBLE FREEPaulo Vincenzo ToomerHow many of you have heard of Jacques Cousteau? All of you, right? If you don't know who he is, you need to be reading a copy of "Horse and Hounds" or "Amateur Rambling in the Cotswolds" rather than this fine publication. He invented SCUBA didn't he? Didn't he also write a book called The Silent World?Now, we all know he didn't invent SCUBA. That was actually some geezer whose name we can't remember and Mr C took all the glory (another reason to hate the French, Jules Eden?). Right? ...Wrong! Well partially wrong. Mr C gained the glory for open circuit but not SCUBA. SCUBA is Self Contained Underwater Breathing Apparatus and it was invented a long, long time ago and it was a Closed Circuit Rebreather (CCR). That is why his world was SILENT. Bloody open circuit is hardly silent. If you don't believe me, take a try dive on a rebreather and listen for when the hordes of wildebeest come charging past you blowing those confounded bubbles. |
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What is a rebreather? A rebreather is a closed breathing circuit (loop) enabling gas to be recycled, therefore giving the user an extended use of the limited gas that he/she carries. There are two primary problems that have to be overcome to make the rebreather work. Firstly, we metabolise oxygen and if we breathe too little oxygen it can cause unconsciousness which could lead to an untimely death. A by-product of oxygen metabolism is carbon dioxide and this insidious gas can also lead to unfortunate circumstances, especially underwater as I'm sure you can imagine. The loop and how it works When a diver exhales, his breath is forced through a non-return valve which forces the exhaled air through a carbon dioxide removal system known as a scrubber (I love that word). The scrubber is made up of a chemical called Sofnalime, which is primarily calcium hydroxide. The exhaled breath is cleaned of carbon dioxide and is pushed through an area where oxygen analysers check how much oxygen is left in the 'cleaned' breath. These analysers talk to a computer, (the unit holds a constant PO2 or oxygen partial pressure) which then tells a solenoid to open and add oxygen to the loop if required. This can also be done manually in the event of any problems. |
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There are also counterlungs on the
unit, which allow the diver to breathe
gas around the loop. Without the
counterlung, the diver would be
unable to breathe. It's similar to trying
to breathe out of a glass bottle –
impossible. However breathing out of
a plastic bag is possible, although very
unsafe (unless you are Rob, in which
case this is standard practice on a
Saturday night). As the diver descends
in the water, hydrostatic pressure acts
on the counterlung and compresses
the gas within it. To enable the diver to
breathe, diluent gases (containing one
or more inert gases, normally air
or trimix) are manually or automatically
added to the loop.
Finally, because the CCR holds a constant PO2, compared to open circuit where optimum PO2 is only achieved at the Maximum Operating Depth, rebreathers will penalise the diver less in terms of decompression obligation. Another huge plus. Gas, gas, gas! Rebreathers work on metabolic rate and not surface air consumption rate. So, if a diver uses 20 litres of air a minute but only metabolises 1 litre of oxygen, the rebreather is far more effective at conserving gas. An example of this based on the above gas usage rates would be: An open circuit diver is at 30 metres on the Thistlegorm and is there for 30 minutes. He uses 20 litres/min x 4 ata (30 metres) x 30 minutes = 2400 litres of gas, or a full 12 litre cylinder pumped to 200 bar. If a CCR diver was at the same depth and for the same time he would use only 30 litres of oxygen. Therefore, if the CCR diver had a 2 litre cylinder filled to 200 bar, he would have 400 litres available. Based on a 1 litre metabolic rate, that diver could stay on the Thistlegorm for 400 minutes or 6.66 hours. 6.66 hours compared to 30 minutes? A gigantic difference and this is the primary reason that deep technical divers prefer CCR. It's all in the gas. Aquatic life I took a CCR to the Maldives on a liveaboard a few years ago. I never went past 40 metres for the two weeks I was there and although I never had any decompression schedule I was swamped by the aquatic life: I was chased by barracuda; nibbled by moray eels; had a two metre stingray push me down the reef as if I was not there; mantas thought I was some kind of weird yellow box fish and finally, the sharks... oh my God, it was unbelievable! I had white tips on a night dive try and beat me up, chewing my fins and rubbing themselves on me as they went past, banging into me to see what I was... just incredible. It was one of the best dives of my entire life. This dive alone was enough reason to spend all that money on my unit. Here is a little history for you:
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All that is left to do now is pick your unit,
get some top end training and go and
play. I assure you this is the future.
In the next issue, we will look at some of the CCRs available, features, recreational and technical CCR and of course training options. Please note that this article covers the basics of rebreathers. I really hope it gives you a little understanding of rebreathers, as I believe they are the way we will all dive in the future. I say BRING IT ON FAT BOY! You can email Auntie Toomer with any of your dive queries and you might also like to check out The Diving Matrix. |
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