Oxygen solubility and half times

[padiscubapro]

Rebreather sensors,

I am familiar with a few CCR units and can tell you how they "read" and where they are located..

The inspiration has 3 sensors located in the main gas flow with constant respiration the gas composition is fairly uniform, as a test I fitted a extra sensor right at the mouth piece and the readings from the controller and alternate sensor were always within .1 bar usually .05 bar or less, and any differances the RB sensors were usually slightly higher since the sensors are near the o2 injection point. If you monitor the controller as it injects o2 you see a rise of a few hundreds of a bar before it drops to is steady state point.

The mk15/ 15+/16 ect has its sensor within the gas flow but its a secondary path (not all the gas flows past the sensors)

The Cislunar has the sensors off the main gas flow.


All the sensor that I have checked have a rated response time of around 6 second for full scale(incedently this is the minimum time between o2 injections that the inspiration allows), for small fluctuations its basically real time..

Most RBs give you the actual sensor readings for all its cells, to maintain control most units use either a strict voting logic or combination voting/averaging logic to try and keep readings stable.

I initiate a dive with a setpoint of .7 and on decent o2 is never injected just diluent (manually) when doing a free decent to about 30m or so the PO2 is generally around .9-1.0, I am sure some people can attain readings higher than this but from experience the only times I have ever seen large spike is at the initial decent when not enough diluent was being added..

Automatic o2 additional is either done in the exhale lung (preferred) or after the scrubber and before the inhale bag, this gives some mixing time for the gasses.

 

[The Iceni]

Posted by PT on 15th March

What I mean by "enrichment"

Dennis,
You said "As external pressure is matched by internal pressure, through addition of diluent, the O2 fraction decreases but the PP/O2 increases. The endpoint arrives but at a lower PP than you supposed in your thesis. That is attributed to a lower starting point than assumed. Remember the flexible body. Thus, at 5 atmospheres the mix is 28% Nitrox and the PP/O2 is 1.4 bar."

I think we are violently in agreement!

All I am saying is that if the diluent added to maintain the working volume of the loop (at its nominal 10 litres) has ANY oxygen in it, this oxygen will be added that already in the loop thus enriching the loop contents. As you say this IS NOT by increasing its fO2 but by increasing the absolute amount of oxygen in the loop. In fact, in all likelihood the f02 will be reduced. The additional oxygen is reflected in the pp O2, which after all is all we are interested in; - A cylinder ful of air contains more oxygen than a quarter full cylinder of Nitrox 36 although the fO2 in the latter is greater.

A good example is a loop filled with air at the surface with air as diluent (S = 0.21). No matter how much air is added during the descent the fO2 in the loop will remain constant at 0.21. However, the pp O2 will increase linearly with depth - 0.21 bar per 10 metres - and will be no different from open circuit air with a pp O2 of 1.045 at 40 metres.

With a rebreather, you start at a higher value for "S".

(On the other hand if helium is used as the diluent no oxygen is ever added to the loop so the pp O2 in the loop will not change except by metabolic loss.)

To prevent a dangerous spike at the bottom of the descent with air as diluent I gather the EFFECTIVE surface pp O2 must be manipulated to below 0.5 bar;

Can I take an example of a loop filled to 50% capacity on the surface with 70% Nitrox which is taken down to 40 metres?

P is ambient pressure at depth in bar
S is the starting pp O2, which IN A FULL LOOP = 0.70/2 = 0.35 bar
p is the starting total pressure in FULL loop =1 bar/2 = 0.5 bar
fO2 is oxygen mix of diluent.
D is the pp O2 in the loop after the addition of diluent to working volume.

D = S + ((P - p) x fO2);-

At 5 metres D = 0.35 + ((1.5 - 0.5) x 0.2) = 0.55
At 8 metres D = 0.35 + ((1.8 - 0.5) x 0.2) = 0.61
At 10 metres D = 0.35 + ((2.0 - 0.5) x 0.2) = 0.65
At 14 metres D = 0.35 + ((2.4 - 0.5) x 0.2) = 0.73
At 16 metres D = 0.35 + ((2.6 - 0.5) x 0.2) = 0.77
At 20 metres D = 0.35 + ((3.0 - 0.5) x 0.2) = 0.85
At 25 metres D = 0.35 + ((3.5 - 0.5) x 0.2) = 0.95
At 30 metres D = 0.35 + ((4.0 - 0.5) x 0.2) = 1.05
At 35 metres D = 0.35 + ((4.5 - 0.5) x 0.2) = 1.15
At 40 metres D = 0.35 + ((5.0 - 0.5) x 0.2) = 1.25 bar

A linear increase of 0.2 bar per 10 metres, simply because of the added air.

Once on the bottom "S" becomes 1.3 - the set point - so IN THEORY there are likely to be significant spikes with any further descent subesquently during the dive. This is what I am trying to understand and prevent.

Does this make sense?

Does this make sense, Padiscubapro person?

If so, can you explain how you can start your dive with a set point of 0.7, a loop fuller than 75% and use air as the diluent yet still get a pp O2 of less than 1 bar at the bottom of the descent (Metabolism only accounts for 0.1 bar per minute.)

Where does the oxygen go?

 

[padiscubapro]

First of the 75% presumption is pretty poor.. I don't know any ccr diver that would purposely have the bags that full.. The goal is to have just enough gas to take a full deep breath maybe a slight bit more.. my breathing bags are basically flat on the surface, I'll take a deep inhale then submerge, and exhle after I'm down a few feet. You have to remeber diving on a CCR is opposite that of OC, since breathing bags expand much more easily than your chest, you will become more positive when exhaling and slightly more negative on inhale.. I would guess between dead are spaces and lung volume it couldn't be much more than 7 liters at best total volume.. If I had bags 75% full that would be about 20 lbs of lift...
I haven't measured the actual lift but I have heard figures ranging from 26-30lbs of lift for full bags (on the inspiration) I'm rounding a bit but from this we can assume the maximum bag volume is about 13 liters plus gas in scrubber, hoses and lungs.
If I get a chance I'll measure and calculate the volume of the mouthpiece, hose, and free space in canister.

I don't need to carry an additional 20lbs of lead around my waist.

I dont use that rapid a descent, I go around 10m or so a minute so I guess I use 3l of o2 on decent to 30m.

 

[padiscubapro]

here are some #s I'm running by mental calculation, I can't find a pen and this computer doesn't have calculator installed...

assuming 7l loop, 70% is approx 5l of o2- I'll consume 3l so there is 2l left after decent.. plus 2 l of nitrogen from original mix.. this will make up 25% of the new mix(1l).. so .5l of o2 plus about 1.25l of o2 for the air diluent 6l@.21 for 1.75l of o2/7l total volume is a 25% mix x 4atm = 1.0 Po2 my numbers should be pretty close...


If I'm swimming my decent is faster but I'm consuming more o2 so its pretty much the same..


Joe

 

[The Iceni]

Hi again Joe,

I agree with your figures but I still have some difficulty.

In fact the loop consists of lung plus counterlung, scrubber and tubing.

1) Total lung volume in an average, fit healthy man can exceed 5 litres. I have assumed 5 litres.

2) The counterlung assembly can vary from nil to 5 litrs with the inspiration, plus scrubber and tubes (which like the anatomical lungs cannot be reduced in volume to zero).

In my descent I have assumed 5 plus 5 with an empty counterlung assembly at the start of the descent and your figures agree with mine but I understod that the comfortable loop volume at depth will be about 5 litres (I agree it may be less).

So if the scrubber and tubes plus lung is 6 litres and the full counterlung is 2 litrs, an emply counterlung will produce 75% of full loop volume!

However, what concerns me is what happens after the initial descent when the set point is changed to 1.3 and air is the diluent?

Could I ask you to redo you calculation for a further descent after the set point change from 30 m to 40 M with air diluent and a 10 litre loop?

I used the following formula

D = S + ((P - p) x fO2)

My concerns are that ANY futher descent at depth following the set point change must give rise to an increase in the loop pp O2, which cannot be used by metabolism.

I think I may have been in error in concluding that the relative volumes of lung plus, scrubber, plus tubing and an inflated counterlung are about the same. I now realise this may not be the case but do not think it changes my hypothesis at all. Air must be added to the loop for any increase in depth.

By the way. There is no buoyancy change during the respiratory cycle of a rebrather! The gas is simply moved from lung to counterlung and back again.

Kind Regards,

 

[padiscubapro]

I'll look over your pose later but heres a thought..

I don't have to run the calculations.. I can tell you from experience, you will definately spike the po2. After stabilizing the loop at 1.3 and having a confortable loop volume, you'll always be above your 1.3 setpoint. adding diluent only lowers the po2 slightly (from observation not enough to counter the rise in po2) and if you don't match the rate of decent with added diluent the damn controller injects o2.. essentially the deeper you get you have to start staggering the decents.. decend a little, add some diluent... breathe it down again... decend, add diluent ect
what you must do is lower the setpoint for the next decent add diluent and start a slower decent.. since even the po2 of air is fairly high at this point decents must get proportiaonally slower..
When I get a chance I'll run the numbers anyway.. but for anything to be valid you have to either maintain a po2 slightly above setpoint or lower the setpoint otherwise the controller will make nonsense out of the results.
What I would do, instead of having to spend time to breath down the mix, I'd flush with diluent, bring my PO2 down to .84 and continue decending from there knowing that as I go deeper my po2 will contilually rise.. this is really the only way to do a decent in a reasonable timeframe.. this way I can get to say 70m with a total decent time in the same frame of going to 30 or 40 meters (3 minutes or so) BTW thats the absolute limit I'll use an air diluent and the conditions must be perfect.. otherwise I'm on a trimix base diluent and I can decent even faster since my Fo2 is probably on around 16% or less...

 

[The Iceni]

You are the first rebreather diver who has confirmed my concerns and, as you suggest, the detailed maths is somewhat irrelevant.

As I said at the beginning of this thread I was flamed by a number of British rebreather divers who accused me of being alarmist.

You may have read my last post before I edited it. I have since realised it matters not a jot what the relative volumes of lung, scrubber, tubing and counterlung are, as they are effectively one compressible bag and to maintain a comfortable working volume sufficient diluent must be added to the loop at ambient pressure. If the diluent contains ANY oxygen this is added to that already present.

As you rightly imply there is a problem with diluent flushes at depth when air is used, simply because it contains so much oxygen and a Trimix diluent is indicated.

It is quite impossible to reduce the pp O2 in the loop to less that the pp O2 of the diluent at any depth. At 30 metres the pp O2 of an air diluent is 0.84 bar. At 40 metres it is 1.05 bar. So if a diver hopes to reduce his pp O2 below 0.84 bar at depths greater than 30 metres he will not be able to do so. The only thing a flush will do is to move the loop pp O2 towards the diluent's pp O2.

I know of at least one rebreather diver, highly critical of my observations, who considers rebreathers are safe to use down to the "MOD" of the diluent. I do not believe this to be the case at all for the above reasons.

Air is safe to use to 56 metres open circuit where it has a pp O2 of 1.4 bar. As I am sure you know, any air diluent flush at that depth will simply move the pp O2 in the loop towards 1.4 bar, and cannot reduce it to below a set point of 1.3.

Together with the other observations you have made I fear the manufacturers of certain rebreather units may have a problem they have not adequately addressed, at least in respect of the education of rebreather divers.

I wonder if some are inadvertently exceeding maximum safe depths because of a fundmantal lack of understanding of the dangers they face?

 

[padiscubapro]

It is best to have a diluent that is flushable at your working depth so you can reduce PO2 quickly and its safe to do a bag flush if you suspect CO2 or have to verify sensor integrity.. If you understand your limitations you can dive deeper than the mod, if you adjust your setpont and burn off the excess o2.. You must now realize what will happen when you add diluent or do a flush.. Like I said earlier I have used "Air" dilent to around 70m the my PO2 in the loop was only 1.3 (therefore my narcosis level was higher than that of an OC diver), I was quite aware of the consequences and what I would be facing upon a loop flush (a PO2 of 1.6+). at this depth assuming 7 liters loop volume, I'd have to consume approximately .3l of o2 so even at rest its no longer than 30 seconds of high exposure.. For recreational depths this really isn't that big an issue, as long as you control your decents.

 

[padiscubapro]

"By the way. There is no boutancy change during the respiratory cycle of a rebrather! The gas is simply moved from lung to counterlung and back again. "


This is not quite true.. there is a SLIGHT change and its opposite to whats expected.... exhaling into the lungs gives you SLIGHTLY more buoyancy since the counterlungs expand more than your chest expands on an inhale.. so inhaling REDUCES your buoyancy slightly.. If you are perfectly neutral holding a normal breath, you will feell slighlty positive on a full exhalation... I actually use this to benefit at the surface(but in opposite), I'll inhale as fully as I can to help the decent then exhale after Im a meter or so below the surface since I rather use as little weight as possible.. I don't want to carry additionall lead for negative buoyancy for the first 1 or 2 meters...


50M for air diluent isn't really that bad because a flush would yield 1.26, decending from 40 to 50 as long as its not very rapid wouldn't spike the PO2 out to a dangerous level, but as I said earlier its better to lower the setpoint and breath down the PO2 before decending further since at 50m it would bring you quite neear your setpoint..
I only teach an ANDI rebreather program, which really hasn;'t stongly supported the inpiration until recently... Talking with other RB instructors from other agencies the various training methodologies vary tremendously.
From personal observation diving every day with a high PO2, I (others I have talked to said the same) seem to build a tolerance for higher PO2s.. for example on deep dives I always flush the loop at 6m with oxygen to get to a 1.6, at the start of the dive sequences, I'll typically have the o2 induced "snot" after a dive... as the dive sequence progress the effect become less and less by the end of a trip I generally don't seem to have any side effects from the high PO2.. this seems to disappear after a few days of non diving and has to start over..

Going by NOAA tables Its not uncommon for me to double the allowable 24 hour exposue limits (I try not breaking the single exposure limits) or more... I don't advocate other do this, since I willing accept any rishs and try to limit my chances of problems by maxing the surface interval.. My min surface interval is usually at least 90 mins preferably 120 or more, unless the first exposure was short then I'd allow 60 minutes

 

[The Iceni]

I said
"By the way. There is no buoyancy change during the respiratory cycle of a rebreather! The gas is simply moved from lung to counterlung and back again. "

To which Joe replied
This is not quite true.. there is a SLIGHT change and its opposite to whats expected.... exhaling into the lungs gives you SLIGHTLY more buoyancy since the counterlungs expand more than your chest expands on an inhale.. so inhaling REDUCES your buoyancy slightly.

Does this only happen in the prone position and close to the surface, I wonder? If so, do you think this because the counterlung is a few inches shallower than the lungs, and the gas in it at a lower pressure than that in the anatomical lung and therefore at an increased volume and greater buoyancy?

Does this also mean that the prone rebreather diver has to do some work to inhale because he is "pulling" this gas down to deeper pressure?

Certainly this is much less than in OC diving since in open circuit a tidal volume of 1 litre leads to a change in buoyancy of one kliogram, which must be added to the work of breathing. (I suspect this is why the venturi assisted regulators are so popular.)

Certainly the old twin-hose regulators could be made to free-flow simply by lifting the mouthpiece above the level of the regulator. However, if this is the explanation in rebreather divers we are talking in grams, not Kilograms!

Any ideas as to why this happens in CC, Joe?