WE’RE ALL PRETTY HI-TEC THESE DAYS. However, when we dive most of us are still using technology developed in the middle of the last century, when most people didn't even have a phone in the house, and got their news via something called a wireless!
Jacques Cousteau and gas engineer Emile Gagnan, who invented the Aqualung in war-torn France in the 1940s, would be flattered yet probably horrified that we have not progressed far beyond their invention.
In every other area of our lives we have embraced new technology, and other adventure sports have seen huge changes.
The canopies used by skydivers, for example, bear no resemblance to those used by their predecessors a generation ago, and modern technology has given these products previously unimaginable safety features.
Yet in the scuba-diving world, other than a little tinkering, there has been a strange resistance to moving forward. As far as the equipment we use is concerned the only significant change over the past two decades has been in rebreathers, and this remains a very small niche market.
Rebreathers seem to offer huge advantages. Gas supply is no longer a consideration on most dives; the no-decompression dive-time is enormous compared to standard open-circuit scuba; and the environmental impact of a rebreather diver is also greatly reduced, given the absence of both noise and bubbles. With the advent of mainstream dive-industry acceptance and an avalanche of publicity, it might seem that rebreathers have arrived and that, if you are a serious diver, you just have to get one.
Many think that in today’s rebreathers we could be seeing the seeds of the technology that our children will one day use for diving. However, if these devices have been around in the modern era for more than 20 years, why are we not already seeing them everywhere?
Are they perhaps not as wonderful as the publicity suggests? Are there inherent problems in the way they work?

THE STORY SO FAR
Rebreathers are not a new idea: the concept has been around since at least the late 17th century, when the Italian Giovanni Borelli first considered the idea of a diver swimming under water while breathing from a bag of air.
The navies of the world have been equipping divers with oxygen rebreathers for shallow underwater assault operations and deep mixed-gas units for deep missions for decades, and submarines have been fitted with rebreather escape units for almost as long.
In the late 1960s, a US company introduced a closed-circuit system called the Electrolung, and marketed this to sport-divers. Early accidents and subsequent legal action ensured that this experiment was short-lived, and that when the idea of rebreathers for sport-divers next surfaced it would be driven by European rather than US companies.
This was in the mid 1990s when, encouraged by the development of technical diving and the growing public and industry acceptance of nitrox as a breathing gas, Dräger, a world leader in rebreather technology for military divers, introduced the Atlantis semi-closed rebreather (SCR.)
This was heralded as the dawn of a new era for recreational divers.

SCRs
SCRs work with a single pre-mixed cylinder of nitrox. An injector allows gas to pass from the cylinder into a bag called a counterlung, from which the diver breathes via a mouthpiece fitted with twin hoses, one for inhalation, one for exhalation.
When the diver breathes out into the mouthpiece, his exhaled gas passes through a canister containing sodium hydroxide, which removes the carbon dioxide (CO2.) The remainder of the exhaled gas then returns to the counterlung ready to be used again; that is, “rebreathed.”
The combination of counterlung, mouthpiece, canister and diver is called the breathing loop. A constant trickle or electronically controlled injection of fresh gas from the cylinder ensures that the O2 level in the loop remains breathable.
The diver’s lungs are the engine that drives the process. If there is too much gas in the loop, it is vented into the water via an exhaust valve.
If there is not enough gas in the loop, an over-ride valve opens and adds a stream of fresh gas directly from the cylinder, bypassing the injector.

MACHINES OF THE FUTURE
In 1995 this sounded very clever and modern, and many professionals bought into the technology. Sadly, however, as it turned out, the predictions of a new dawn were wrong. Market demand did not match the media interest.
Divers quickly learned that the advertised benefits of these systems did not stand up in practice. They found that an open-circuit set of double cylinders gave a diver about the same extended duration as the Atlantis and other similar machines, but without the increased cost, risk and complexity.
They also discovered that the much-touted silence was interrupted too frequently by the periodic release of a stream of bubbles from the exhaust valve.
So sport divers at that time decided that they didn’t really need rebreathers, and continued using open-circuit scuba like their predecessors.

CCRs
At the same time, other people were developing rebreather systems that were more complex and offered significant advantages. However, they also presented divers with new problems. These were fully closed-circuit rebreathers (CCRs).
Typically, these have two cylinders, one with oxygen and the other containing a gas to be mixed with the oxygen.
This second gas is called the diluent and is either air, trimix or heliox, depending on the depth of the dive.
A CCR diver presets the desired partial pressure of oxygen (PO2) required to be maintained during the dive, and the rebreather injects little spurts of oxygen into the breathing loop from time to time to maintain the required PO2.
The rebreather’s computers constantly adjust the level of oxygen in the breathing mixture to ensure that the diver is always breathing the optimum gas for the current depth, thus extending no-decompression times to the maximum or reducing decompression-stop times to the minimum.
The diluent gas is added on descent only to maintain the volume of the breathing loop, so once CCR divers are at the maximum depth of the dive, unless they lose gas as a result of mask-clearing or have lots of ups and downs, the only gas they use is the oxygen that the unit adds to replace the oxygen they have metabolised.
This is usually around one litre per minute at any depth, so a small 4-litre/200bar oxygen cylinder fitted to a closed-circuit rebreather will provide pretty much anyone with enough gas for well over 12 hours.

A DIFFERENT WORLD!
A CCR can give you quite astonishing performance. Imagine you are on a dive along a reef wall using air as the diluent gas and a preset PO2 of 1.3 ATA.
When you are at 30m you will be breathing nitrox 32. You are almost a completely silent observer. You hear the sounds of the sea, parrotfish munching on coral, shrimp crackling, dolphins calling.
The natural sounds are punctuated by a slight hiss every few seconds as the rebreather’s solenoid opens to allow a tiny amount of oxygen into the loop to replace what you have metabolised.
Watch your no-decompression time and move gradually shallower. As you ascend, the rebreather will add oxygen to your mix, and the nitrogen level will drop as the breathing loop vents the expanding gas.
Swimming at 20m, you will be breathing nitrox 43, at 10m you will be on nitrox 65 and by the time you eventually reach your safety stop at 3m, you will be breathing almost 100% O2. A two- to three-hour no-decompression dive is easy to achieve. It really is a different world.
For technical divers, this was the holy grail. They decided that they did need rebreathers, and now CCRs have succeeded in dominating the world of technical diving, mainly because of their phenomenal advantages in gas economy.
They bring the cost of trimix diving down to manageable levels, and enable explorers to undertake dives that would be impossible on open-circuit.

THE DOWNSIDE
There is a downside. While simple in concept, CCRs use complex electronics, and are expensive to buy. The models technical divers traditionally use are mostly built by specialist boutique companies or by expert and enthusiastic individuals in small workshops, so it can be hard to find maintenance support outside the country where the manufacturer has its headquarters.
CCRs also require divers to be meticulous in their preparation and constantly focused during the dive. They are highly unforgiving and allow virtually no room for diver inattention. This is mainly because they expose divers to a couple of insidious threats that can easily sneak up on them.
With the level of oxygen a diver is breathing on a CCR controlled by the unit’s electronics systems, this can fluctuate considerably. Oxygen is essential for life, but in too-high or too-low quantities it is toxic to humans.
The first rule of rebreather diving is “always know your PO2.” Divers must make sure that the oxygen sensors they are using are functioning correctly, and always monitor how much oxygen is in their breathing loop.
Too much oxygen can cause hyperoxia, whereby the diver’s central nervous system short-circuits, leading to underwater convulsions and, often, death by drowning. Too little oxygen, or hypoxia, causes the central nervous system to shut down completely, and the diver blacks out. There are no warning signs.
CO2 also represents a danger to the rebreather diver. Too much CO2 in our bodies, a condition known as hypercapnia, results in uncontrolled breathing, makes us confused and disorientated and can be fatal, particularly if the diver is alone. Divers suffering from advanced hypercapnia are unlikely to be able to rescue themselves.
Technical divers may be prepared to deal with these additional risks in return for the cost benefits, but most of us want to relax when we dive, and are not drawn to equipment where the science and preparation time get in the way of the fun, or where we have to spend more time watching the machine than watching the fish. This was a key reason why rebreathers did not catch on in mainstream diving in the 1990s. 
However, in the second decade of the 21st century a series of developments took place that pushed rebreathers back into the mainstream limelight. I'll take up the story in part two next month.

Read more from Simon Pridmore in:
Scuba Confidential – An Insider’s Guide to Becoming a Better Diver
Scuba Professional – Insights into Sport Diver Training & Operations
Scuba Fundamentals – Start Diving the Right Way

All are available on Amazon in a variety of formats.