What on earth is a Rebreather?
It's Scuba gear.

Oh. You're still here. So an explanation of breathing gas under pressure is called for. If you're a diver or know diving theory click here to skip it.

I've been Scuba diving for a few years now and I wanted to extend the range of diving sites I visit and my appreciation of them but again and again I was coming up against limitations on breathing gas. There are three problems with what you breathe that all pull in different directions.

The first is pure quantity. You are probably breathing at most 20 litres of air a minute at the moment but the actual amount of air that represents changes as you go under water. Naturally just under the surface it's not much different from normal but at 10 meters depth the pressure is double that on the surface and twice as much air is squeezed into every breath. Your body doesn't know this - it still needs to move its 20 litres a minute or the Carbon Dioxide, a gaseous by-product of just staying alive, doesn't get flushed out of your body so air comes out of your cylinders twice as fast. Make that 20 meters and that's three times as fast, 30 meters four times and up at 50 meters, as deep as I tend to go, six times the rate.
This means that the nice 12 litre tank blown up to 200 times air pressure that would last you two hours on the surface is gone in 20 minutes and that doesn't include the time for getting there and back or any margin for error. You can user bigger tanks and blow them to more pressure but even my twin 10 litre tanks at 300 times atmospheric pressure only double the duration. Also they weigh 40 kilos (that's 6 stone in bathroom scales units) and anyhow, they just make the other problems worse.

The second problem is the coke bottle effect. As you descend you come under more and more pressure and just like the coke in the bottle more and more gas dissolves in you. Remember that human beings are 90% water and liquid fats. This is OK till you come up. Then it doesn't want to be dissolved anymore. Now nobody wants to fizz up like coke. If your blood vessels are full of bubbles they get blocked and bits of you start to die. Divers avoid this fate with tables saying how deep for how long is safe plus wrist mounted computers monitoring our exposure. If we do go deeper than the tables allow we switch to more advanced tables that insist that we come up slowly, waiting at set depths, still under some pressure, until the gas in our bodies has been reduced to a harmless level. We call this decompression diving and we can improve our decompression by breathing gas mixtures with more oxygen, which our bodies can cope with more readily, and less nitrogen which has no significant biological value but increases the fizz factor and makes you feel woozy.

The third problem is that oxygen, the very stuff you must breathe to live, becomes poisonous under pressure. Well if it wasn't divers would breathe 100% oxygen all the time and have a lot less problems with decompression effects. This means we need to carefully control our exposure to what we breathe once we move away from simple compressed air so deep and prolonged diving means careful planning.

Right so now maybe you'll get on to rebreathers?
Well a rebreather addresses the problem that on standard Scuba gear that what you breathe in comes from the tanks on your back and what you breathe out bubbles away as used gas.
Well you may have finished with it but out of the air you breathe your body actually takes the equivalent of one litre of oxygen a minute to keep the internal processes processing and that never really changes unless you start doing a lot of exercise. More significantly it doesn't change with depth. The trick is to keep what you breathe out, spruce it up again and re-breathe it. That is send it round in a loop.

Now this adds complications but let's look at the advantages first.
You need to keep topping up the gas in the loop with oxygen but only with about one litre a minute regardless of depth. Now that's not a big tank - that is a very small tank. Definitely not my 40 Kilogram scuba rig here. Also if we are injecting oxygen we can decide how much to inject and always have the optimum oxygen mix for the depth we are diving so we get the best decompression performance and keep well away from any nasty toxic effects of the oxygen. Basically we can go deeper for longer for less side effects or stay shallow for ages and ages.

But, sadly, you never get something for nothing. We need more complicated equipment as we now have a loop of gas going round. We need chemicals to remove the carbon dioxide we exhaled (nasty stuff if you get too much) and something to inject the oxygen when it's needed. And we are going to take all this down into the sea which is notorious for its detrimental effects on complicated machinery and electronics.
Standard Scuba gear has evolved over the years to be very safe and our practices mean that if you follow the rules you are not taking much risk diving. The more complications you add mean the more risk you get.
So the first rule of diving is know what you breathe. This is why beginners start on nice simple compressed air. Air is relatively simple but as soon as you complicate things the application of the rule means measure it. Measure it yourself everybody teaches.
However with the rebreather what you are breathing depends on what you are taking out of the loop by breathing it and what is getting injected. This means you either use a control system based on the design of the unit eg: the nitrox semi-closed systems or a fully computer controlled device with monitoring eg: the closed circuit systems.

Now I can't claim I'm much of a fan of the semi-closed systems. They inject a constant stream of nitrox - that's air with its oxygen content boosted - and bubble off a constant percentage off the loop gas. There's no argument that the system works but you are pre-setting the percentage oxygen in the mix by selecting the strength of the mix you inject and the rate it is injected at. Living on the south coast of England we have our plans routinely altered by the weather. I can't predict the next dive I'm going to make with much accuracy when I drag down to the shop for a fill. This is one of the problems with Nitrox mixes anyway.

The Electronic Closed Circuit Rebreather is the other end of the spectrum. Rather than a keep-it-simple system you have high technology fixes. You have sensors measuring the oxygen in the mix and electrically injecting pure oxygen to keep it as requested. Because it is injecting pure oxygen I have no need to buy the right mix for the depth as it will inject to make it just right but conversely I am trusting my life to the mechanics and electronics of the system.

A rebreather then is a quite simple concept. The gas you breathe goes round in a loop with one way valves to enforce the circulation. The loop needs a floppy bag that can expand and contract as gas moves into and out of your lungs, called the counterlung as it is working exactly opposite to the real lungs. It also needs a chemical canister to react out the carbon dioxide and finally it needs a means to inject oxygen.
It's just a bit more complex as a means to inject ordinary air or a substitute is needed for diving or any loss of loop gas could leave you breathing pure oxygen, which would be quite dangerous, and the loop oxygen measuring instruments need to be fitted in somewhere but that's not too bad.

Once you have this you just jump in the water and dive. As you descend you have to inject air as the gas in the loop compresses and you must watch the computers so if they stop reading correctly you can take over and drive the system manually as you get out of the water. Once you are at the desired depth you just let the system inject oxygen as required, still doing your monitoring of course, and when time comes to ascend you will need to dump gas from the loop as it expands just as you do from the BCD and drysuit. All in all quite civilised.

Back to the Rebreather page.


Counter
by Nigel Hewitt