CCR Trimix Simplified by Mel Clark question

Hanssing

Joe, sorry if I misunderstand, but...

Staying on loop with a dead scrubber (your example), will perhaps cause a FCO2 of 2% in one minute, but the PPCO2 is 0.2bar!
Remember that breakthrough is considered 0.005bar.
Its not the FCO2 thats relevant.
Please use correct units or I will end up very confused

Anyway the example above is fairly straight forward, if simplyfying CO2 production to O2 cunsumption. If the loop drops from say 1.3bar-O2 to 0.7bar-O2, the PPCo2 will rise from 0bar to 0.6bar!

To work out what the resulting PCO2 is during a SCR-run with a defective or partially working scrubber the SCR-equations that Freef have presented could be used, with some modifications, where VO2 etc. is replaced by VCO2 etc. And the passive bleed into the loop is not from an orifice but from your metabolism.


Do you mean with increased tidal volume, or total loop-volume?
You quote the total loop-volume, but this will not affect the steady-state PCO2, as far as I understand.
Only the tidal-volume will affect it, since this is how much gas gets exhaled out of the loop.

Could still be me that is being dense, but isnt FCO2=0.05% considered breakthrough at 10atm? Ie. PPCO2=0.005bar

Nicolai

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jradomski

You need to start with % before you can figureout the partial pressure..

also to get theses numbers you would need a totally dead scrubber.. this is highly unlikely, you would need to have the loop at is smallest volume and not flush for at least a minute.. This point is to show the worse it could get..

Sit on the couch, and count how many breaths you are taking while you are relaxed for one minute... The more you are taking the lower the loop co2 will be since it will be flushed more often.. if you start activity your breathing rate will go up so you will have even mre flushes so any increased co2 is offset by the more frequent flushes.. with light activity I'll bet many people are at least 20 breaths per minute.. so at these levels at 3 to one we would divide the level by about 7, for a 5 to one we could divide it by 4..

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

jradomski

it depends if you are just doinf refill breaths or doing partial flushes as well.. The total loop volume is generally larger because the person usually takes a bigger breath on the initial refill than on subsequent breaths.. The best is the first and last breaths are larger than "normal breaths".. The larger the breath is to the dead space the better..

What also works well is an alternating approach..
do a 4 to 1, then on the next inhale breath of the loop dont exhale it back into the loop but vent it, now you are insuring your "dead" spaces are flushed and are effectvely 2:1..

we cant easily use the scr function as you describe, we are purposedly using very small time scales and nearly full replacement..

The level at which breathing begins to increase is .02bar this level can probably be tolerated indefinately.. symptoms generally dont become noticible to about .05bar.. breakthrough for a scrubber ( .005bar) is generally defined at a point well below the symptom point (its not based on symptoms ).. BTW, in OC breathing gas the allowable % is .1% so at 10 ata grade e gas is .01bar

so .4% is just below the noticable symptom point at 10 ata..

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

paulraymaekers

practising SCR mode is also a nice thing in case you loose your O2 for some reason...


I'm a bit in between, for L2 (Mod1): in case of doubth, just OC! (always)
the higher level, the more challanging dive, I prefer to have more options

for +100m dives, we always carry team bail-out, I know I can not do a correct OC deco if for some reason I have to do the whole deco on OC AND loosing the team completely...
I could PERHAPS do a 'very risky' OC bail-out.. if I lost the team..

so in that case, plugging in my OC gases to go SCR will dramatically increase the number of options I have
in case electronic falure: SCR solves my problem
if I loose oxigen: SCR solves my problem
if my scrubber only works +/-: SCR solves my problem... a bit

flooded loop, tja, it will be a 'very risky' bail-out, I'll shoot my SMB's and hope they come and help me out :-)

paul

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Hanssing

Well the methods you describe affect the SCr-rate so its not exactly 4:1 but something else..... Its in fact lower, wich in regards to CO2 is a good thing.


I can understand that you thereby would get a saw-tooth profile in the PCO2, but otherwise the normal SCR-theory shoudl apply?

OK, using this a baseline the first thing we need to agree on is that DEPTH does not affect the loop PCO2, only the "refresh"-rate ie. SCR-ratio and the divers ventilation.

Divers should also be aware, that when bailing out to SCR-mode, after a suspected CO2-hit, and remaining on loop (say for deco) the SCR-ratio should be kept as low as possible to keep PCO2 low (logical, but if the diver has functioning electronics, one could focus on the PPO2...).

I just did a small excel-sheet for this, and the controversy is offcourse how to model the scrubber effeciency. I did the calculations, so the exhale-breath does not go into the loop. But even when doing 2:1 rate, the loop PCO2 witha 50% active scrubber rises to 0.05bar ie. up into the symptomatic area.
Care to review?

Nicolai

Disclaimer: I'm no Rebreather-expert, I'm just a newbie so dont take anyting I say to seriously.....

Attached Files

SCR_Bailout_v001.zip (3.1 KB, 15 views)

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Woohooo - I can change my rEvo!
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jradomski

I don;t have excel on this machine, only a viewer.. at what time period does it reach 0.05bar??

btw if you are calculating this the way a PSCR works being that onlt a certain percentage is dumped every breath, its not the same as dumping the entire loop every x number of breaths.. since one will always have some residual, and one (if doen correctly) starts from scratch again..

also your co2/o2 rates are for someone working not an easy swim (or deco).. the rates are much higher than would be expected by at least 50% for someone that has to swim.. even MY numbers are probably 50% high for someone just trying to deco..

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Joe Radomski
CCR Trimix Instructor Trainer
ANDI Instructor Trainer Director #10

All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.

Dave Sutton

Joe Radomski is doing a great job with the math.... so keep at it.
Mark Nix is as sage as always... PRACTICE makes PERFECT.

Here's my addition:

From the standpoint of a math-challenged diver... here's an empirical view of the entire issue, seen from the standpoint of an open-sea wreck diver. Other environments may be entirely different.

Scenario: I'm in 75 meters of water, 10 minutes of swimming away from the anchor line. More or less at my 'turn point' to start back to the line. I hear a gurgle... and begin having anxiety issues. Based on experience, I suspect a loop flood of some sort (gurgles) and a hypercapnia event beginning (anxiety). So.... time to bail.

I have a bunch of problems to deal with, brokn town into tactics and strategy: Tactics: Keep *something breathable* in your mouth so you don't drown. Strategy: Get into a situation where DCS is not going to kill you.

Break it down into time slices:

(1) STAY ALIVE for the 3 second to 3 minute period (Breath ANYTHING!!)

(2) STAY ALIVE for the 3 minute to 30 minute period (Get an EASY solution running)

(3) STAY ALIVE fore the 30 minute to 3 hour period (Explore BETTER solutions than the easy one...)


To solve (1) I need to deal with my own immediate physiological requirements. The loop is flooded... maybe. Dunno how badly. My solution is going to be to turn the BOV to "OC" and to rest, reduce metabolism, and try to get my act together. Usually this takes a minute... 10-20 breaths *minimum* until heartrate and respiration rates are back to normal.

OK... here I sit on the bottom semi-OK but *far* from my deco line. What next?

Next: Solve Act (2): I NEED to establish a deco stop system. I need to shoot my SMB and get a line to the surface. HJow long does this really take? It depends on how you are rigged... if you have gotten your act together *before you went diving*, you have an emergency reel with a self-inflating SMB already attached, snapped where you have access to it in the 20 second to 1 minute timeframe. To remove it, settle down on the bottom, shoot the bag, and tie off the line is going to take both hands, and 2-3 minutes. Do you want to *get it done* or do you want to dedicate half of your effort to that, and the other half of your effort to trying to fix your rig? I'm going to be on the BOV thru all of this too. I want to get OFF THE BOTTOM. Note here that we do not do free-hang decos, so if you have chase boats and all of that: Good for you. We don't have them as a norm so we tie-off our SMB's to the wreck so we can stay on-station as we hang.

Anyway, now that I have a line to the surface, I'm going to begin ascent... probably still on OC. I'll be task-saturated... zero doubt. Hanging in a current on a thin line instead of the substantial anchor line that I'm used to. Bouyancy control, depth-keeping, trying to get onto my bailout deco, etc... I'm gonna be damned busy. Blow off the deep stops? Maybe, depending on my gas situation. Still probably OC at this point, but I'm likely getting "concerned" about gas..... so I'm thinking "maybe the loop is still available..."


OK, now I'm at my 30 meter stop... at last. I have several minutes here and I'm finally getting a little relaxed. Time to "consider" act (3). I'm still on the BOV, so going back onto the loop for a little "investigation of it's capabilities" might now be in order. This is where I'm going to see if I can go back onto the loop for a SCC (or maybe full CCR) return to the system. If it works? I'm golden. If not? Well... I'm in an OK situation for deco anyhow.



So, this is one strategy... just one. It's doable, based on both math and experience. Just *think about it*. And to make it work, you need to PRACTICE IT as so properly pointed out by Mark Nix. Think shooting your SMB on a reel from deep water is easy? It can be a piece of cake, or a nightmare... when was the last time you practiced it?


Fate favors the prepared.


Dave


.

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Mikel-Deko

Hi,
Just a question because my math are not so good.

Said you bail in 75mtrs to a Tx 16/60. There is not a problem in the loop so you decide to go SCR.
You have (or shought have) a tank with a Ean 50.
Once you are in 21 mtrs stop, you can change your diluyent from TX to Ean.
So if you have a 50% of O2 comming every 3-4 breath to the loop, the % of O2 will be hight, so your desaturation will be very good even in SCR and in a scenario that this EAN is to short you can make 6-8 breath every flush.

Or maybe i'm absolutely wrong...

Best
Mikel

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Mikel-Deko
Basque Country
www.olatu.net

"DIR is the nanny state for divers who cannot think on their own" Ron Micjan

Hanssing

I hope we can all agree that SCR for situations involving loss of O2 or loss of electronics is a tool to get you out, primarily reducing stress concerning amount of OC-gas available for you exit. Also these situations are less immediately lifethreatening, and perhaps even less likely to occur.

However CO2-hits and Flooded loops are immediate dangers, with less time to react, and less change of SCR helping....
As far as I can tell a CO2-hit due to a failling scrubber is a dicey thing to run SCR-on. Only if really pressed for gas, and well in control, does it make sense, IMVHO.

Still, time to get some offboard connectors and practice...

Nicolai

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Woohooo - I can change my rEvo!
Its going to be bitchin' tricked out piece of gear

diverklondike

This is not directed at you personally or your post but in general I would suggest proper training versus thinking that you can learn what you need to know from a book...

Just my two cents...

M