Wednesday 3 July 2013

Rebreather Diving... Is it the future of Scuba?

rebreather divers in Cyprus on the Megalodon
It seems like a strange question since I am a Megalodon Rebreather Instructor. Am I questioning whether Diving a Rebreather is the way forward? Certainly not! But, is Rebreather Diving the future of Scuba?

Since I have a TDI Megalodon Rebreather Mod 1 course running this week, this is at the forefront of my thoughts.

What Is a Rebreather?

In the most basic terms, a Rebreather is a breathing apparatus that recycles the air that you exhale, removing the Carbon Dioxide (CO2) and re-injecting Oxygen (O2) to replenish that which you metabolise with each breath.

Rebreathers are not limited to the Underwater World. You can also find them being used by Firefighters, Miners, up mountains, in hospitals and even outer space! In fact, you can find rebreathers being used in many places where the breathing supply is limited or the environment is toxic.

But we are talking about rebreathers that are used for Diving purposes.

How does a rebreather work?

When we breathe underwater on regular open circuit scuba, we actually waste a lot of gas. With every breath we only metabolise around 4-5% of the inhaled Oxygen and the rest is exhaled into the water column and is wasted.

Rebreathers are a means of making this gas management more efficient and since I am a Megalodon Instructor, I will focus on this unit.

The Megalodon is comprised of 2 cylinders of gas; one Oxygen to replenish the Oxygen that our bodies metabolise as we breath and the second can be either a trimix or air, which is used for inflation and to "dilute" the oxygen in the mix. Therefore, we call it Diluent.

These Cylinders are attached by mounts to an aluminium canister with a scrubber inside. So called because the Scrubber unit contains a lime product which reacts chemically with our exhaled Carbon Dioxide to "scrub" it from the breathing mix. We use sofnolime 797 in the Meg and there are 2 types of scrubber you can have; Radial or Axial but this is another topic all of its own.

At the top of the scrubber can is the "Head", which contains the Oxygen Sensors (3 of them) and the battery packs and is attached to the Primary and Secondary hand sets, which give you the information you need to dive the rebreather. On the Meg, both handsets are completely independent so that, were one to fail, you could surface safely on the other.

Fixed to the can and the cylinders is the loop and the counterlungs, which are the parts we breath from.

The counterlungs are collapsible bags and are needed because without them, it would be like trying to breathe from a plastic bottle and the work of breathing would go through the roof! The loop itself has the mouthpiece and a shut off valve to prevent water entering the loop when it is out of our mouths.

Obviously, there are a few other bits and pieces but these are the main components needed to explain how the rebreather works.

As you can see from the diagram above, as we exhale, the exhaled gas is passed through the scrubber unit to remove the Carbon Dioxide from the mix. It then passes along where a solenoid injects oxygen to replace that which has been metabolised and makes the gas breathable again.

The amount of Oxygen injected is dependent on how you set the rebreather initially.

The Megalodon is a fully electronic closed circuit rebreather, which means it is run by the electronics. We set them to work by Partial Pressure of Oxygen (PO2) and usually tell the electronics to maintain a PO2 of 1.2 or 1.3. This is called the set point and, once this is set, the electronics can detect how much Oxygen to add to provide you the diver, with the optimum breathing mix for the depth you are at.

It is that simple!

Are Rebreathers the future of Diving?

 I believe that rebreathers are the future of diving but they are also the past, as CCR was around long before the advent of the Aqualung, single tank BCD and regulator set up.

The oldest known rebreather prototype was patented in France back in 1849 by Saint Simon Sicard (although the ideas were around from as early as 1808) and the earliest models were based on an oxygen reservoir and used sponges soaked in calcium carbonate to absorb the CO2.

Obviously things have become a little more refined and reliable since those days but the principles remain the same.

Rebreathers have amazing benefits over open circuit diving

Your mix is dynamic, so you are always breathing the optimum mix for the depth you are at. It is like having a nitrox/trimix mixing panel on your back as you dive. Obviously, this has an effect on your decompression obligations, reducing the time you have to sit on a line completing decompression stops.

The gas you are breathing is humid and warm rather than the dry, cold air from a compressed air scuba cylinder and regulator set up. Again, this is a benefit as it helps you to stay warmer and is less dehydrating than using the aqualung, which is great from a health and decompression point of view.

You don't use as much gas as you do on open circuit. Since the exhaled gas is recycled, not wasted, your limited gas supply will last much longer underwater, giving you better dive times. In fact, you are often only limited by the duration of the scrubber material in the can. However, remember that you must carry bail out gases and so, the dive plan must account for the gas you will need to ascend on open circuit completing any and all decompression obligations along the way.

In fact, the only real down side of the rebreathers is the initial set up cost to buy. They are still relatively expensive to purchase, although if you are an open circuit trimix diver, you can make the money back on difference in price to fill 3l cylinders with a Helium mix on the rebreather to filling twin 12s with the same mix.

Where in the past, rebreather diving has been something for Technical, mixed gas, decompression diver only, in recent years, a number of companies such as Poseidon and Hollis have been looking at opening it up to the recreational market by developing "plug and play" rebreathers for use down to 40m. While these can still set you back around £4000, it is hoped that the price will eventually drop and make it more accessible to the everyday scuba diver.

ISC has even brought out their "recreational" rebreather called the Pathfinder, which is a lovely little unit perfectly suited to the travelling diver.

So, while rebreather diving is still quite specialist at the moment, I believe this is one niche to keep your eye on. Who knows in a decade or so, you may be able to walk into a dive centre and say "I want to learn to Dive" and the Instructor will say, " sure...

on this?.....

Or this?...

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