Rebreathers & buoyancy

[DepthCharge]

for every 1 liter or o2 you consume .9 liters of it ends up as co2 (which is absorbed by the canister) the remaining .1 is used by your body in normal metabolism.
consume doesnot equal breathed.. on each breath of "air" at the surface you exhale about 4%co2 (actually a little less)17%o2 and remainder nitrogen and misc gases (plus moisture), a rebreather works by scrubbing this c02 out the remaing 17% is now (you didn't consume this) available (a ccr rebreather will maintain a specific po2). The gas efficiency comes about by not wasting this unused o2

Normally I agree with alot of what padiscuba pro says.............however on this, what I'm understand he's saying (and I might not be understanding correctly), is not accurate. O2 is used to generate energy within the body. It does this by combining (burning) carbohydrates. The most common one is sugar, a 6 carbon carb. Thru the Krebs Cycle O2 is added to the sugar, generating CO2. The energy produced by this reaction forms ATP which is where the cells of your body gets it's energy from. The net result from the Krebs Cycle is 6 molecules of CO2+ (IIRC) 1 molecule of H2O. This water is what accounts for the difference in the O2/CO2 ratio.

 

[padiscubapro]

for every 1 liter or o2 you consume .9 liters of it ends up as co2 (which is absorbed by the canister) the remaining .1 is used by your body in normal metabolism.
consume doesnot equal breathed.. on each breath of "air" at the surface you exhale about 4%co2 (actually a little less)17%o2 and remainder nitrogen and misc gases (plus moisture), a rebreather works by scrubbing this c02 out the remaing 17% is now (you didn't consume this) available (a ccr rebreather will maintain a specific po2). The gas efficiency comes about by not wasting this unused o2

Normally I agree with alot of what padiscuba pro says.............however on this, what I'm understand he's saying (and I might not be understanding correctly), is not accurate. O2 is used to generate energy within the body. It does this by combining (burning) carbohydrates. The most common one is sugar, a 6 carbon carb. Thru the Krebs Cycle O2 is added to the sugar, generating CO2. The energy produced by this reaction forms ATP which is where the cells of your body gets it's energy from. The net result from the Krebs Cycle is 6 molecules of CO2+ (IIRC) 1 molecule of H2O. This water is what accounts for the difference in the O2/CO2 ratio.

the .9 co2/liter of oxygen is a compromise based on "normal" diets, and is the value use by the US navy (Schreiner used 0.8, Buhlmann used 1.0) for RQ (respiratory quotient)
If you are burning strictly carbs the value is higher if you are burning protein the value is lower.. People on high carb diets produce much more CO2 than those on high protein diets.. Carbs are easily converted while protein is not and has other byproducts.
The previous reply was not totally compete in that yes water is part of metabolism but again it depends on what your are burning as a fuel.

I think I've gotten better over the years trying to be more complete with my answers, but the answer was well enough for practicality (showing there isn;t a 1:1 correspondence).

also the values are based on liters of gas not molecules so remember to take this into account when figuring out the balancing of the equations.. remember 1 mole of ANY gas is 22.414 liters @STP. (chemistry 101 )

Reason for edit: vevese protein and carbs

 

[MikeR]

This has me a little intrigued. I am into powerlifting. My diet includes about 350-400 grams of protein, with low carbs and fat. On the physical side this works very well. I was not aware of the difference in co2 production on high protein diets. Very interesting. I definately need to do a little research.

As for buoyancy. Like padi said- ascents are the only change that nets a positive bouyancy shift.

 

[teksimple]

Nevermind, the answer has been in front of me all along!
For CCR and O2 rebreathers at fixed depth there is no change in volume
Roak

I will have to say that Roakey, you were right the first time with your proposal. Here is why:

OK, everyone, the oxidation and Krebs cycle equations are an interesting diversion, but the buoyancy equation we use here uses only two factors. Remember Archimedes principle.

More on the equation here: http://www.physics.smu.edu/~scalise/mechmanual/archimedes/lab.html

An object is buoyed up by a force equal to the weight of liquid the object displaces.

The two factors to consider are:
1. TOTAL WEIGHT (or MASS, since distance to the center of the Earth remains static)
2. TOTAL VOLUME

Roakey is CORRECT in saying that it is a closed system, and no mass (weight) is being added. So that part of the equation does not change. If we ask the question proposed by Roakey that no more oxygen is added to the system, we see the following:

If you use up the oxygen (gas) in the collapsible counterlung(s), you decrease the volume of the system..

THE TOTAL VOLUME OF THE SYSTEM DECREASES, THUS THE REBREATHER DIVER WILL EXPERIENCE A MEASURABLE LOSS OF BUOYANCY.

If, however, the electronics (or diver flying manual) adds the correct amount of gas to the system as the diver uses it at a constant depth, there will be no net effect on buoyancy.

Because, assuming an ideal rebreather that does not leak, etc, we see that

NO CHANGE IN MASS (WEIGHT) ON THE CLOSED SYSTEM
NO CHANGE IN VOLUME ON THE CLOSED SYSTEM
THUS NO CHANGE IN BUOYANT FORCE

 

[padiscubapro]

This has me a little intrigued. I am into powerlifting. My diet includes about 350-400 grams of protein, with low carbs and fat. On the physical side this works very well. I was not aware of the difference in co2 production on high protein diets. Very interesting. I definately need to do a little research.

As for buoyancy. Like padi said- ascents are the only change that nets a positive bouyancy shift.

The lowest CO2 production is when the body uses fats for energy.. the often quoted value is an RQ of ~.7

Here is a link for fat vs glucose
check my previous post.. My brain was ahead of my typing I reversed Carbs and protein..

http://www.tiem.utk.edu/~gross/bioed/webmodules/respiratoryquotient.html

 

[teksimple]

The lowest CO2 production is when the body uses fats for energy.. the often quoted value is an RQ of ~.7

Here is a link for fat vs glucose
check my previous post.. My brain was ahead of my typing I reversed Carbs and protein..

http://www.tiem.utk.edu/~gross/bioed/webmodules/respiratoryquotient.html

Awesome info, Padiscubapro!!! I take it that I should be eating lots of olive oil before my dives to make my scrubber last longer?

 

[O2BBubbleFree]

Awesome info, Padiscubapro!!! I take it that I should be eating lots of olive oil before my dives to make my scrubber last longer?

'Course, since olive oil is a pretty good laxitive, it might not be the scrubber that limits your dive...

 

[Life-Is-Good-Diver]

wow, i read every thread, word for word, great thread, thanks padiscubepro for the detailed threads !

 

[DepthCharge]

Hi Joe,
I guess what thru me off is when you said:

for every 1 liter or o2 you consume .9 liters of it ends up as co2 (which is absorbed by the canister) the remaining .1 is used by your body in normal metabolism.

What I'm saying is that the entire 1 liter of O2 is used for "normal" (is there such a thing :06: ) metabolism. Not just 0.1 L. The CO2 is one of the by-products of metabolism, the other being H20.
And yes there are other by-products of metabolism from proteins.........you have to get rid of the nitrogen somehow (nitrates in urine), but from carbs and fatty acids, it's pretty much just CO2 & H2O + energy.
Another of the problems with diets high in fat & proteins (aside from the increased CO2 production) is the production of ketones and the resultant keto-acidosis and effects on blood pH and the subsequent shifts (however slight) in the oxygen dissociation curve. While your body can compensate for this in the "normal" setting, it'd be interesting to see if there is any reseach on this in the hyperbaric setting. This maybe a non-issue, I have no idea. But it maybe of concern for people on Atkins type diets
Interesting discussion, thanks.

 

[padiscubapro]

Hi Joe,
I guess what thru me off is when you said:


What I'm saying is that the entire 1 liter of O2 is used for "normal" (is there such a thing :06: ) metabolism. Not just 0.1 L. The CO2 is one of the by-products of metabolism, the other being H20.
And yes there are other by-products of metabolism from proteins.........you have to get rid of the nitrogen somehow (nitrates in urine), but from carbs and fatty acids, it's pretty much just CO2 & H2O + energy.
Another of the problems with diets high in fat & proteins (aside from the increased CO2 production) is the production of ketones and the resultant keto-acidosis and effects on blood pH and the subsequent shifts (however slight) in the oxygen dissociation curve. While your body can compensate for this in the "normal" setting, it'd be interesting to see if there is any reseach on this in the hyperbaric setting. This maybe a non-issue, I have no idea. But it maybe of concern for people on Atkins type diets
Interesting discussion, thanks.

its always nice to bring back some old threads.. It just shows how relevant the info is..
Some times my posts are too complete..others like the above were a little misleading, but I guess over the years I am getting better at it.. I just have to pay a bit more attention and read my posts afterwards especially when I do them early in the morning...

The interesting thing about burning proteins is how much the RQ varies depending on the byproducts.. urea, ammonia, ect.. some have values very close to that of fat ~.7 while others approach carbs at a 1.0..

When I teach my RB classes I try and bring this type of info into the class to get the students thinking.. I try to advocate a balanced diet.. the Carbs are important since they are easy to covert to energy hence the high RQ, while proteins make the body work harder to produce energy, Even in relatively warm water your body has to work to try and maintain its temp, on long dives the carbs can really help...
From personal observation, I know when I load up on carbs for BIG dives I feel much stronger towards the end of the dive.