On the 13th June 2006 I obtained my Open Circuit Trimix ticket down in Plymouth. This gave me the ability to dive to 100m, the big ticket as far as many people were and perhaps still are concerned. The cost however precluded too many dives, efficiency therefore became the watchword. Therefore when I discovered the Redbare from VMS during TekCamp 2018 my interest was peaked due to the massive reduction in gas usage (therefore cost) and the huge increase in safety that a CCR provides.
How does it work?
A Closed Circuit Rebreather, CCR, or just plain rebreather, works by ‘capturing’ the exhaled Oxygen in your breath, sending it around a loop of technical wizardry, injecting enough fresh Oxygen into it at the end and allowing you to re-breathe this exhaled gas.
But what about the Carbon Dioxide?
Trying to keep it simple, we need a certain amount of carbon dioxide in the body, an excess creates problems, not enough CO2 decreases the impulsion/natural reflex to breathe (under normal circumstances, we maintain around 40mm Hg (mm of Mercury) in arterial blood). This exhaled carbon dioxide is therefore potentially harmful if it builds up so we need to scrub it from the breathing gas. This is done by various products in many different guises available on the market from Lithium Hydroxide (used in Submarines and Space applications) to Sodium Hydroxide (commercial and military absorbents). The pellets of Sofnolime used in the Redbare are in a certain shape to allow it to scrub as much CO2 as possible while packed correctly into the scrubber. It is therefore imperative that both the manufacturer’s and the CCR unit makers guidelines are adhered to.
To monitor CO2 gas build up, divers have traditionally relied on self monitoring for symptoms of hypercapnia (too much CO2). Given the individual unpredictability of Cor susceptibility and the combination action with high PO2, this has led to unexpected incidents and deaths, which have contributed to the image of rebreathers as ‘only for experts’. These days, reliable CO2 monitoring is available and some CCR’s have a CO2 detector which uses infrared beams to measure the greenhouse gasses which pass over it. While not completely foolproof in itself it provides a very good way to measure gas levels. Diver self awareness remains as an important backup.
Once the gas has been cleaned of CO2 it flows over Oxygen sensors which check the amount of O2 remaining. If this falls below a certain point then the unit will inject enough O2 in order to reach the level required by the user – the Set Point. The user then breathes this gas and the whole process starts again.
This diluent always has less oxygen than the gas we actually wish to breath, which means it needs ‘topping up’ with O2. The onboard electronics works out how much O2 to ‘inject’, to top up. As you metabolise oxygen during your dive, the sensors detect this lower level of O2 and adds more. This means that the only gas added to the loop while at a constant depth is about 1 litre of O2 per minute – a 60 min dive = 60 litres of gas. This is where the savings and safety come in. No more feeling desperate as the cylinder gauges drops.
To put this into perspective, a 12 litre steel cylinder with an average 210 bar fill, will total 2,640 litres. Let’s say you exit the water with 50 bar, that’s 160 bar breathed, or 1,920 litres breathed.
So what are the other benefits?
Why did I do this Normoxic course?
Malin Head. I have a thing for rusty wrecks and History. Consequently, Malin Head is a really attractive proposal for me and something to aim for, those guns of HMS Audacious and the Sherman tanks onboard SS Empire Heritage are just calling my name like a siren song.
So What did I learn and what did I dive down there?
O2: 50 bar
Dilluent: 87 bar
Assuming that I have a ten litre loop volume and nothing changes:
50m /6 ata, volume would be 1/6 of that at the surface or 1.6ltrs.
40m/5 ata would be 2 litres.
30m/4 ata this would be 2.5ltrs,
20m/3 ata this would be 3.3ltrs,
10m/2 ata this would be 5 ltrs while at
0m/1 ata this is now back to the original 10 litres.
O2: 42 bar
Dilluent: 71 bar
O2: 67 bar
Dilluent: 67 bar
Total Dive time was 167 minutes. Total Deco time was in the region of 75 minutes. Two other people were diving with me. Diving through ‘Harlyn Dive School’.
Average gas consumption:
O2 – 53 bar
Diluent – 75 bar
Gas used (O2/Helium):
Deep bailout 20/52
Shallow Bailout 50/20