Question CCR for recreational depths

[L13]

It's about the number of molecules of oxygen in the rebreather and the fact that your body only metabolizes a certain number of molecules per minute regardless of what depth you're at. As you go deeper, to maintain the volume you need to add more molecules of gas. So at 330' you'll have 10x the amount of molecules in a bag as you would at the surface.

Since your body metabolizes the same number of molecules of oxygen per minute, regardless of depth, you run a higher risk of going hypoxic when there are fewer oxygen molecules in the breathing loop to begin with.

For a given ppO2 there is a constant number of oxygen molecules in the loop, regardless of depth. That is what ppO2 means. it is the number of inert gas molecules that changes with depth.

 

[Heat Miser]

The O2 molecules are not as dense when shallow as they are when compressed at depth. This means that the PO2 will decay much faster and there is a real possibility of going hypoxic, passing out, and drowning. Normally the solenoid will fire and keep it breatheable but without close monitoring you might not notice a malfuncion in time. Several very experienced RB divers have died either on the surface or very shallow.

I had never thought of it this way and am somewhat confused.

Are you saying if I take a rebreather with a setpoint of 1.3PPO2 and breath it down to say 0.65 PPo2 (ie without injecting any O2 or dil) that this will happen faster at 10m than it will happen if I did the same thing at 30m.

I allways thought that on rebreather you metabolised the O2 at the same rate across all depths.

 

[happy-diver]

Do you understand that at depth you are diving a far leaner o2 mix as with pressure too much o2 is toxic
When heading up to lower pressure with the mix the same, the potency of your o2 is less, to not enough
Dudes!

 

[huwporter]

I allways thought that on rebreather you metabolised the O2 at the same rate across all depths.

Yeah you will. If you start at a given PO2 and add no O2, the loop PO2 will drop at the same rate due to metabolism whatever depth you are at, assuming the same workrate etc etc. There is plenty of confusion about the difference between PO2 and PPO2/FO2 in this thread. Your (functioning!) CCR varies the FO2 to give you a constant PO2 at any depth, and it is PO2 that is metabolically important. For a 1.3 setpoint with air dil, at 10m you have "Nitrox 65" in the loop, while at 30m you have "Nitrox 32.5"; the total number of molecules of O2 in the loop remains the same, and you metabolise the same number of molecules per minute.

I'd say additional risks of a CCR shallow would be due to;
1. Typically starting from a lower PO2 (e.g a 0.7 setpoint instead of 1.3) so you start with fewer molecules of O2 in the loop; and
2. PO2 dropping more rapidly from the same distance of ascent when shallow compared to deep due to faster % ambient pressure change.

SCRs definitely have added risk shallow compared to deep, but that is a different discussion.

 

[MidOH]

Kinda makes a strong argument for mccr.

 

[huwporter]

Kinda makes a strong argument for mccr.

??? No it doesn't - there is no difference between a failed solenoid (on CCR) and a failed/blocked orifice (on MCCR) in this scenario.

 

[Angelo Farina]

I think the availability of good modern professionally designed and built rebreathers is key. 10-15 years ago a rebreather was pretty much unicorn material. They did exist, but you never saw one. If you did, it was something really special. Right place, right time, right connections.

These days, it isn't that hard to find. I've run into them without looking.

This of course depends strongly on the region. Here in the Mediterranean, and particularly here in Italy, CC pure-oxygen rebreathers (ARO) were the standard scuba equipment for diving in the fifties and the sixties. Compressed air systems were much more expensive, bulky, providing shorter diving times. They were reserved for professional divers hunting coral, for example.
A basic pure-oxygen rebreather is a simple and reliable machine, the max depth at the time was 18m, and it was common practice to dive without purging the bag and your lungs, so actually it was used as an enriched air rebreather. It was totally manual, so the diver had full control of the oxygen being injected in the bag.
The unit was weighting less than 10 kg, and so it was easy to be operated also by tiny females, which revealed to be among the best divers with these units.
Unfortunately a number of accidents occurred (some due to hypoxia, some to hyperoxia, the greatest part due to hypercapnia, which is too much CO2, due to poorly operating scrubber and wrong breathing cycle, particularly on the cheapest Cressi unit which was pendular, single-corrugated-hose, no loop).
So in the seventies the preference shifted slowly to air tanks, which became easier to use thanks to the two-stages regulators, and the rebreathers almost disappeared, remaining in use only for military corps were being silent and without bubbles was very important.
Only in recent years rebreathers started to be fashionable again, mostly the high end ones suitable for helium mixtures and great depth. Some companies, such as Mares, attempted to introduce semi-closed rebreathers for shallow depths, based on the same strong points of original ARO units (lightweight, perfect buoyancy and trim control, simple machine easy to maintain compared to high-end CC rebreathers) but the market did not receive them well. The Horizon looks nice in their web site, but I have never seen them being used in the sea..

Horizon Rebreather - SCR - Mares

Horizon rebreather: incredibly extended dive time. Revolutionary SCR approach in a system that is easy to use, natural to dive, designed with all levels of diver. Discover more on Mares.com

www.mares.com

 

[AJ]

When your counterlung volume is 4L at the surface and you are metabolizing 0.7L of O2 per minute it's about 4 mins to drop to non-life sustaining levels. If your orifice clogs or your solenoid quits near or at the surface. Even faster if your ADV is on and injects low fO2 dil as the loop volume drops.

At 30m depth the loop volume is still 4L but there's 16L of gas in there. Your ppO2 decays much slower if the solenoid quits or orifice clogs. If your ADV fires hypoxic gas its also going to be breathable at that depth.

Bottom line, mixed gas CCRs are particularly dangerous at or near the surface.

Agree, but four minutes not paying attention to your CCR while it's flashing red in your face? Maybe one should pick an other hobby? To me there no valid reasons for not paying attention to the CCR. If you're too busy, bail out and go OC just to be save. A CCR requires you to pat attention all the time!

I know there have been fatalities and yes, I know the risk on the surface is greater. But you have to realise you're not diving OC. The adagium here is: know your ppo at all times, no matter what. The lesson learned in most accidents is lack of attention or/and complacency. Very few accidents can be attributed to mechanical failure.

 

[huwporter]

One way of framing the discussion is to use moles instead of litres, since that is independent of pressure.

one litre of gas at 1ATA (and normal temperatures) contains roughly 0.04 moles. (Molar Mass of Gas Calculator)

Adults at rest at 1ATA metabolise roughly 0.01 moles of oxygen per minute (16 litres per hour Metabolism). In the water, it is quite normal to work at 5 METS or 5x resting metabolic rate (Physical Fitness for Diving), which would consume 0.05 moles per minute. It definitely isn't out of the question to have to work at 10 METS - maybe fighting current or surge, for example - which would consume 0.1 moles of oxygen per minute.

Assuming total 6L lungs (Lung Capacity and Aging) and a 6L loop volume (probably low O2ptima Cm), at setpoint 0.7 the system contains roughly 0.33 moles of oxygen independent of depth*. Without oxygen addition, you could burn through that in just over three minutes fighting that current, and you'll definitely be unconscious from hypoxia before running out completely.

At setpoint 1.3, the system contains 0.62 moles of oxygen - so under the same conditions you get about six minutes before running out.

*don't believe this?
At 10m with setpoint 0.7, the loop in total contains 12l volume x 2 ATA x 0.04 moles = 0.96 moles of gas. To give PO2 of 0.7 at 2 ATA, the FO2 is 35%, so 35% of that 0.96 moles is 0.33 moles.
at 100m with setpoint 0.7, the loop in total contains 12l x 11 ATA x 0.04 moles = 5.28 moles. FO2 is 6.4%, so 6.4% of that 5.28 moles is 0.33 moles.

 

[L13]

Yeah you will. If you start at a given PO2 and add no O2, the loop PO2 will drop at the same rate due to metabolism whatever depth you are at, assuming the same workrate etc etc. There is plenty of confusion about the difference between PO2 and PPO2/FO2 in this thread. Your (functioning!) CCR varies the FO2 to give you a constant PO2 at any depth, and it is PO2 that is metabolically important. For a 1.3 setpoint with air dil, at 10m you have "Nitrox 65" in the loop, while at 30m you have "Nitrox 32.5"; the total number of molecules of O2 in the loop remains the same, and you metabolise the same number of molecules per minute.

Thank you @huwporter ! I was starting to think that partial pressure had a different definition in CCR diving than in regular OC diving.

2. PO2 dropping more rapidly from the same distance of ascent when shallow compared to deep due to faster % ambient pressure change.

I am starting to think that a lot of the "shallow is dangerous", is actually an over simplification of "start of dive is dangerous" and "ascent to shallow is dangerous." There may still be some "shallow is dangerous" that hasn't been explained yet.

SCRs definitely have added risk shallow compared to deep, but that is a different discussion.

I guess that "different discussion" is what I was asking for. Please?


I know I lack the experience, the lessons learned, and some of the knowledge of more advanced divers. I am here to gain the second two (i'll have to do my own diving to gain the first). However, when they present obviously wrong info, those more advanced divers make it hard respect and trust them for the otherwise good knowledge and lessons learned that they have.

When 20% of what I read I know to be correct, and 20% I know to be wrong, what should I think about the veracity of the 60% I don't know? How much of it is dogma and how much is based on fact? Is half of it wrong too?