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Old 25th July 2010, 15:37   #1 (permalink)
LJM
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CCR Gas Consumption

Hello,

For comparative purposes, I need a to figure out gas consumption on CCR, at depths of 10m - 100m.

Any help would be greatly appreciated!

Regards,

Lawrence
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Old 25th July 2010, 16:02   #2 (permalink)
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Re: CCR Gas Consumption

Gas consumption on a CCR is nowhere near as cut and dry as on OC. For one thing, you have 2 gases to consider; Oxygen and Diluent.

Oxygen consumption during descent and the bottom portion of the dive is largely based on the metabolism of the diver. On ascent, the o2 consumption will rise, but the degree of increase is based largely on the way the diver manages the loop (e.g. the closer you stay to min loop, the less oxygen you will need to maintain a higher PO2 during ascent and deco).

On a square profile, diluent consumption is heavily front loaded. The majority of the volume used will happen during descent as the loop is compressed and the unit makes up the volume w/ dil. From that point forward, small amounts may be added for corrections, adjustments, and the small amount that is absorbed by the body. Again, the amount that is used is largely determined by the technique (e.g. minimum loop w/ good discipline around nose breathing requires much less gas than a full loop and poor discipline).

So just like everything else in rebreathers, it just isn't that simple. Good instruction and good dive logging will help you determine your own consumption rates for planning. There are really no hard and fast guidelines out there.
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Old 25th July 2010, 16:38   #3 (permalink)
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Re: CCR Gas Consumption

Thanks Ryan,

That confirms what I has 'guessed', thinking it through logically (I'm not CCR trained.

Is it possible to typical value, or order of magnitude? It's for a comparison as part of an academic paper, so is only indicative.

Thanks,

Lawrence
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Old 25th July 2010, 18:42   #4 (permalink)
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Re: CCR Gas Consumption

Well, Boyle's still applies. So if you measure the total system volume (your respiratory system + the total loop volume) you'll know the volume that will need to be replaced as you descend. So let's say for arguments sake that you have a 12L system; If you assume you start the dive with a full loop, you would need roughtly 12L of DIL for every 10M of depth you descend. You need to account for adjustments/ corrections, etc. as you go. So what you'll find is that a 2L or 3L dil tank will cover just about any dive you want to do. The reserves are what is important for bail-out (if you do it onboard), SCR, and flushing.

1L-1.5L/min for O2 is a reasonable planning rate.
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Old 26th July 2010, 14:29   #5 (permalink)
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Re: CCR Gas Consumption

As an indication

On the inspiration/vision, a typical one hour dive with max depth 45m and min loop discipline but using diluent to inflate wing I usually use
30 to 50bar oxygen on a 3 liter cylinder
60 bar diluent

~5 to 10 bar oxygen is used for cell calibration, pre-breathing and surface swimming and waiting. Included in the values above.

Most of the oxygen is used in the ascend phase to enrich the loop and decrease the deco stop. I tend to run pure oxygen @ 6m (in reality it is 0.85).
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Old 26th July 2010, 16:47   #6 (permalink)
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Re: CCR Gas Consumption

Depth dosent apear relevent except for very shalow dives


A 3 hour dive to 70m i use 50-60 bar 02 and 50-60bar Diluient out of a 3ltr

Same for a 30m dive

But on a 15m dive in mexico with a lot of up and down stuff Id use 60 bar 02 and 100bar dill.


Basicly I wont jump in and do a dive with less than 150 bar. 02 or dill


ATB

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Old 27th July 2010, 08:13   #7 (permalink)
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Re: CCR Gas Consumption

Quote: (Originally Posted by Mark Chase) View Original Post
Depth dosent apear relevent except for very shalow dives


A 3 hour dive to 70m i use 50-60 bar 02 and 50-60bar Diluient out of a 3ltr

Same for a 30m dive

But on a 15m dive in mexico with a lot of up and down stuff Id use 60 bar 02 and 100bar dill.


Basicly I wont jump in and do a dive with less than 150 bar. 02 or dill


ATB

Mark
Mark,

Don't understand the rational behind your conclusion: why would you need 150b+ of O2 if you routinely use max 60bars?

I routinely jump with 100bar ending with 50bat that gives a good safety margin of another hour.

Did I missed something?
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Old 27th July 2010, 08:49   #8 (permalink)
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Re: CCR Gas Consumption

Quote: (Originally Posted by wizebt) View Original Post
Mark,

Don't understand the rational behind your conclusion: why would you need 150b+ of O2 if you routinely use max 60bars?

I routinely jump with 100bar ending with 50bat that gives a good safety margin of another hour.

Did I missed something?
Emmanuel

Every one has an idea of their safety reserve and for me 90bar in a 3ltr is a what I settled on.

I am committing to 3hours because on avg 2hours 15mins of the three, will be deco. 150 bar leaves 90 bar in a 3 to cover accidental loss of 02 or massive extensions on deco due to a problem forcing a long stay on the bottom etc.


I have loads of spare diluent via my bailouts but all my spare 02 is in my 3ltr unless I use the 50% to try and deco out on via the unit. IF the drop tank turns up ill have 80% to use but i never rely totally on the drop tank.

That said 150 is just my idea. Oter people have diferent ideas, and thats fine.

ATB

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