By Piers Boileau-Goad

By Piers Boileau-Goad

23rd June 2021

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.

As you descend, the volume of the breathing loop will reduce, but the PO2 increases (a la Boyle and Dalton respectively). This means that you need some way of increasing the breathable volume of gas using a diluent gas. Diluent is added via a diver operated demand valve – If you breathe and there is not enough gas in the bag, much like a regulator, a diaphragm dips and diluent is squirted in. On the Redbare, there is a 2 litre diluent cylinder fitted as standard.

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?

You have something called a Set Point on a CCR which is the pressure (in BAR) of Oxygen that you want to breath. For example, if I want to breath a Set point of 1.3 (my usual) at 20m then using daltons triangle (PO2/PxFO2) I would be breathing 1.3/3 (20m=3 ata) which equals 0.43 or 43% Oxygen.
If I now ascend to 10m the Set Point will remain the same but the gas I breath now changes:
1.3/2 (10m = 2ata) which is 0.65 or 65%.
Voila, you now have a personal decompression table as well as an onboard gas mixer.
Secondly, and a major factor in my opinion is the reduction in gas consumption. My personal consumption of O2 per dive is roughly 0.7 litres per minute. If I also add in my Diluent consumption I use roughly 1.2 litres per minute of gas, total. Diluent is used on the descent to increase volume as previously mentioned but on the bottom is barely used. Therefore, 2 x 200 bar cylinders provide 400 bar x 2 = 800 litres. If you use 1.2 litres of gas that would in theory and assuming a 30m dive, give you 667 minutes for the dive. My 0.7 ltr of O2 consumption is steady and not affected by depth. To put this into perspective, Vobster Quay charge £19.80 for a 3 Litre 18/45 Trimix Fill. Assuming you did this dive on twin 12’s it would have cost you £113.30 – in addition to stages/bailout cylinders!
Divers Underwater 2
Divers Underwater 1
Divers Underwater 3

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.

In the meantime, I have found a very attractive playground in Cornwall with some absolutely amazing wrecks and some very thorough local knowledge, quite by accident! I might not even want Malin Head for a while as Cornwall was fantastic!

So What did I learn and what did I dive down there?

The first dive was SS Indus, the shallowest of our three dives at 42m. This was a checkout dive. Getting off a RIB with two stage/bailouts is interesting and required a change of mindset. Two cylinders on one side is not always the best way so we then opted to try the local way of one underslung on each side. This helped later.
Dive Profile 1

Gas consumption:
O2: 50 bar
Dilluent: 87 bar

Dive Two was on U1021 with a bailout ascent planned from 50m. Again we tried to sling both cylinders on the same side but it just go to plan. Boat assistance is a wonderful thing but team work in the water is invaluable! One diver lost his fin on the descent so ended up not really going anywhere while I ended up with the bailout cylinders attached to the wrong rings. I started sorting this myself as I suspected something was wrong but then a buddy told me to stop and let him sort it as it would be easier for him than for me. Teamwork is absolutely essential. Time up and fin recovered by a second pair of divers my bailout ascent is due. My CCR is then switched to open circuit bailout. That means the O2 injection is off, but I still have both of the handsets providing decompression information in OC mode, its just the O2 monitoring and maintenance which is not functioning.
As you ascend pressure reduces so volume increases, hence the need to dump gas from your suit. You also now have to dump gas from the rebreather by venting the loop through the mouthpiece. 50m is a long way to be doing this for, mercifully it doesn’t need to be done as much as people think.

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.

This is a tad long winded but shows the pressure changes and therefore the amount of loop venting is not really that much until you reach the top 10-20m where the big increases happen.

Gas consumption
O2: 42 bar
Dilluent: 71 bar

Dive Profile 2
Dive 3 was to UB65 – the Haunted U Boat (SM UB-65 – Wikipedia) which was again, a lumpy day. Skills required here were a Hyperoxic O2 Flutter (Something has gone wrong and too much Oxygen is being allowed into the loop making it too oxygen rich) as well as some more drills on the deco hang. Starting the dive with a cylinder on each side helped kitting up become so much easier and less stressful and can then be switched back to my preference underwater of having both on one side – deep bailout on the top, shallow bailout on the bottom. For the return to the RIB, it would then have been necessary to switch back from one side to each side again.
At 56m we did our bailouts and I started the flutter, all in the shadow of the conning tower and the deck gun. A very eerie setting.
Deco for this dive was done mostly at 9m because at 6m the swell was pushing us up to 4 and back down to 8/9m. 25minutes of deco at 9 was the better option. I could have increased my setpoint from 1.3 to 1.4 in order to increase the Oxygen content of the gas but didn’t really see the need. All three of us were using the same setpoint and at 9m the swell was bearable.

Gas Consumption
O2: 67 bar
Dilluent: 67 bar

Dive Profile 3

More Detail

Visibility on all three dives was great, 10-15m whilst apparently this is unusually poor. I will however be going back as soon as possible. U 1021 and UB 65 beckon!

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):
Dilluent 14/40
Deep bailout 20/52
Shallow Bailout 50/20

So what if this has wetted your appetite for trying it all out?

VMS build all their units onsite at Vobster Quay. Tim Clements is their head of training and the chap who ran my course. It was him who gave me the confidence to use my unit safely at the depths I am now able to go to. All maintenance is done on site by a truly wonderful chap whilst communication from user to VMS is quick and very easy. I have asked questions from a boat out of Portland considering bailing on the next dive only to have my query answered before the kit up time and able to be back in the water for dive two. I don’t know another organisation who genuinely make you feel like part of the family! I have also heard stories of parts breaking and being sent to arrive next morning ready for diving.
If you want to stay around 10-20m that’s fine, but consider a rebreather. It can make your diving much much safer as well as cheaper! If you want to go deeper I would definitely say that a rebreather is the way forward, gas prices will reduce hugely and, depending on how much you dive, the price of the unit will be offset by the saving in gas. The fact that I have a portable gas mixer on my back means I will not be going back to open circuit unless I am having to bailout.
I would urge you to speak to the team at VMS, and if nothing else, give it a try dive. Awareness doesn’t cost a bundle and its also pretty fun!