Guten tag to my non existant german readers, and konichiwa to the rest of you.
Lets face it, I’m envious of those techies on the quayside in there cool gear however what i find that i yearn for the most is a bad ass re-breather. Not only does it add to your street cred when you bust out one of those bad boys but it’s actually and incredible clever and elegant piece of kit which has a fair few benefits to my faithful Apeks regulator.
Let us begin.
In conventional scuba gear, as you breathe in air is taken from the tank on you back and delivered to you through your mouthpiece. Then you breathe out the waste air and it’s expelled through the regulator forming pretty bubbles in the surrounding water. Simples.
A Closed circuit re-breather (CCR) allows you to breathe the same air again and again, without expelling any gases in the surrounding environment. However, in order for this to happen some clever magic has to go on in a funky hard-case which you carry on your back instead on a tank.
1) Remove exhaled carbon dioxide: To achieve this, exhaled gas passes through a cannister of Soda lime which contains several chemicals, the most important being Sodium hydroxide and calcium hydroxide. What goes in in this cannister is a simple neutralisation reaction. Remember back to chemistry class!
Acid + Alkali = Salt + water
Carbon Dioxide + Soda Lime = Salt + water
(for you chemistry nerds like me, I will post a tasty strong base catalysed, water facilitated reaction at the end!)
Sweet! Calcium carbonate forms as a solid but what about that pesky water? It’s actually quite handy as it helps to fight dehydration from breathing dry air for long periods.
2) Replace oxygen you have used: Small tanks carrying normally close to the hard-case inject oxygen (or mixed gases, normally Nitrox or Heliox) inhaled by you back in to the loop.
3) Control oxygen concentration in the breathing loop: too much or too little oxygen is bad, mkay. Therefore a clever little gadget called a ‘solid state oxygen sensor’ monitors the partial pressure of oxygen and sends results to a microprocessor in the gas delivery system. Cool huh?
All pretty common sense if you take the time to sit down and struggle with it for a while. This is all very good and clever but how does it benefit us, the diving community?
As far as I can see, there are 4 main bonuses to this system:
- Improved gas efficiency – normal SCUBA systems waste a lot of oxygen considering our lungs aren’t great at absorbing oxygen, therefore exhale a lot of it which is then wasted! Rebreathers only replace absorbed oxygen so don’t waste gas.
- Lighter – normal air contains about 78% oxygen therefore the majority of weight in a normal cylinder is useless nitrogen. Rebreathers don’t have to carry nitrogen so for the same amount of oxygen, can be lighter.
- Less Decompression – Because the nitrogen in the system, which is involved in “the bends”, is kept to a minimum, decompression is less complicated and divers can stay down longer than with conventional scuba.
- Stealth: Rebreathers produce few or no bubbles, so they don’t disturb marine life or reveal the diver’s presence.
There we have it folks, the basics (and I do mean the basics). After trolling for hours through swathes of really complicated PDF files, forums and website i think I’ve separated the wheat from the chaff and given you a little bit of ammunition to impress you friends or deflate the ego of a tec diver and their debonair hardhats.
More rebreather stuff will follow this considered I’m obsessed with the thing at the moment. Hope you enjoyed this relatively theory heavy post!
WARNING: CHEMISTRY BANTER FOLLOWS
Welcome friends to the nerd zone. Here as promised is that cheeky reaction in full. Enjoy.
The overall reaction is:
CO2 + Ca(OH)2 → CaCO3 + H2O + heat (in the presence of water)
The reaction can be considered as a strong base catalysed, water facilitated reaction.
1) CO2 + H2O → CO2 (aq) (CO2 dissolves in water – slow and rate determining)
2) CO2 (aq) + NaOH → NaHCO3 (bicarbonate formation at high pH)
3) NaHCO3 + Ca(OH)2 → CaCO3 + H2O + NaOH (NaOH recycled to step 2) – hence a catalyst)
Each mole of CO2 (44g) reacted produces one mole of water (18g)