Rebreathers & buoyancy

[Iguana Don]

First of all I know little about rebreathers.
But during the dive as the rebreather is used, does it become more positive buoyant? Talking about a "closed circuit system"
It seems to me that if you constantly breath & exhale + the O2 coming from the bottle it would become more positive buoyant the longer you stayed down. Where am I missing something?

This I gotta know

Don

 

[Mario S Caner]

Don,
It all depends on the efficiency of the rig being used, depth, duration and therefore the amount of gas used.

The CO2 is scrubbed from the exhaled breath, and more O2 is added to it before it is returned to the diver, as you keep breathing you keep using the available gas supply. It is however in much smaller amounts than you would if you were using conventional open circuit scuba. Having said that however, and to answer you question, yes a diver will become more buoyant as the cylinders are drained.

Mario

 

[padiscubapro]

This is a non issue, using 3 liter cylinder @ about 180 bar (this is approx 20 cu ft)
on a typical 3 hour dive you would consume between 180 and 270 liters of o2. This would be from 1/3 to 1/2 of a tank (buoyancy change is rather small) also most of the co2 is now bound up in the scrubber, on the diluent side unless you where constantly going deeper then shallower you probably only use 10 - 30 bar of gas (150 - 400+ psi) again this is very little change.

The only time bouyancy changes is when you are going shallower(pos buoyant) and the system is forced to add extra o2 other than what you consumed, here you would decrease the breathing loop volume and once neutral the o2 added should be the o2 consumed so buoyancy will not change.. to explain how volume (therefore bouyancy is effected take the following case). If you went deeper you would have to add to the loop for breathing purposes, which would now bring your buoyancy back

set point of 1.3 ata, change the setpoint to .7ata as you breathe the volume will get smaller and smaller depending on how well you initially minimized the breathing volume, the loop will begin to get difficult to breath even though the PPO2 is ok since you are consuming o2 and no gas is beiing replaced so loop volume is decreasing.

 

[roakey]

ID brings up a interesting question, which has me wondering...

I'm an engineer (don't tell joewr) and I have a very, very narrow question to ask; let's see if I can do it in such a way that I can be clear on what I'm really asking.

First we'll simplify. A pure O2 rebreather diver (so we don't have to worry about dilutent) goes to 10 feet and sits there breathing their cylinder dry. Let's say we stop the test the moment the O2 cylinder reaches the point where it cannot inject any O2 into the loop, so the loop is *exactly* the same volume as when we started the test.

In other words I'm trying to remove loop gas volume from the equation completely.

The question is simple: Does the diver’s buoyancy change and if so, in what way? Note: I realize we’re probably talking very, very small changes here at best, but the interest is purely theoretical, given the much greater variable in the real world is the loop volume…

On one hand we’re emptying the cylinder, and because the cylinder does not change volume, the “system” (diver, rebreather and cylinder) is becoming more buoyant.

But on the other hand, the diver’s body is metabolizing O2 and turning it into CO2, which is being chemically removed by the scrubber. So for every O2 molecule removed from the cylinder, we’re depositing a CO2 molecule in the scrubber, which also does not change in volume, so the scrubber is becoming more negative. Not only is it becoming more negative, but also it’s becoming more negative at a faster rate than the cylinder because for every O2 molecule removed from the cylinder, a CO2 molecule (an extra carbon atom) is being deposited in the scrubber.

So now we have the conclusion that a rebreather diver becomes more negative over the dive.

But where does that carbon atom come from? From the diver. Since a diver is a liquid for all intents and purposes and the removal of matter *will* cause a liquid to decrease in volume, we have in addition to the decreasing buoyancy of the external system (cylinder and scrubber/loop), we have the displacement of the diver decreasing as well, as well as their mass. So the diver’s buoyancy change may be a wash.

So if the diver’s buoyancy change over the dive is a wash, along with the buoyancy loss of the cylinder/scrubber system, it seems that during a dive a rebreather diver actually loses buoyancy.

Is this correct, or am I full of it?

Roak

 

[padiscubapro]

>But on the other hand, the diver’s body is metabolizing O2 and >turning it into CO2, which is being chemically removed by the >scrubber. So for every O2 molecule removed from the cylinder, >we’re depositing a CO2 molecule in the scrubber, which also >does not change in volume, so the scrubber is becoming more >negative.

No.. typically for each liter of o2 consumed you create .9 liters of co2, the rest of the o2 is used by the body as fuel.
also the scrubber generates heat...lots of it so there is chemical being consumed, plus it generates h20 which is deposited in the canister and also recirculated.. now on a long dive especially since the diver is not getting dehydrated the diver would probably "relieve" himself/herself of excess "Pressure" on the bladder therefore making one lighter...
Basically the overall effect of buoyancy from start to end isn't worth mentioning.. on a SCR rebreather on the other hand the diver will empty the cylinder they dive with.


>Is this correct, or am I full of it?
you decide!

Roak [/B][/QUOTE]

 

[roakey]

Originally posted by padiscubapro
No.. typically for each liter of o2 consumed you create .9 liters of co2, the rest of the o2 is used by the body as fuel.
also the scrubber generates heat...lots of it so there is chemical being consumed, plus it generates h20 which is deposited in the canister and also recirculated.. now on a long dive especially since the diver is not getting dehydrated the diver would probably "relieve" himself/herself of excess "Pressure" on the bladder therefore making one lighter...
Basically the overall effect of buoyancy from start to end isn't worth mentioning.. on a SCR rebreather on the other hand the diver will empty the cylinder they dive with.

The heat's moot, matter is neither being created or destroyed, so the weight's a closed system. The "chemical consumed" is turned into other chemical(s) with the same mass as it started with, heat or not.

I'm a bit confused at your statement about 1l of O2 yields .9l of CO2 + "the rest is used"

If I remember my biology and do lots of rounding to simplify, 1.0l of pure O2 inspired will result in 0.9l of O2 expired + 0.1l of CO2 expired (the whole theory behind the efficiency of a rebreather) is this what you're saying? If so, the 0.9l of O2 goes through the scrubber untouched and the scrubber eventually absorbs the 0.1l of CO2. So you're still picking up that extra carbon atom.

Urination doesn’t enter into the picture, I'm trying to balance the chemical reaction and see its result in a mass change.

Roak

 

[padiscubapro]

Originally posted by roakey

The heat's moot, matter is neither being created or destroyed, so the weight's a closed system. The "chemical consumed" is turned into other chemical(s) with the same mass as it started with, heat or not.

I'm a bit confused at your statement about 1l of O2 yields .9l of CO2 + "the rest is used"

If I remember my biology and do lots of rounding to simplify, 1.0l of pure O2 inspired will result in 0.9l of O2 expired + 0.1l of CO2 expired (the whole theory behind the efficiency of a rebreather) is this what you're saying? If so, the 0.9l of O2 goes through the scrubber untouched and the scrubber eventually absorbs the 0.1l of CO2. So you're still picking up that extra carbon atom.

Urination doesn’t enter into the picture, I'm trying to balance the chemical reaction and see its result in a mass change.

Roak

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

here is the exact equations of the scrubbers

first step c02 mixes with water vapor forming carbonic acid (thats why pre breathing the loop is important and why the scrubber chemical has moisture to start with)

c02+h20 -> h2co3

this compound is pased over a base compound

H2CO3 + 2 NaOH -> NaCO3 + 2H20 + heat
(carbonic acid ) + sodium hydroxide -> sodium carbonate + water + heat

the final process is

Ca(OH)2 + NaCO3 -> CaCO3 + 2NaOh

calcium hydroxide + sodium carbonate -> calcium carbonate + sodium hydroxide

The typical chemical composition is
94% calcium hydroxide, 4 % sodium hydroxide, 1% potassium hydroxide and silica to act as a binding agent

I hope this helps..

 

[roakey]

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 (displacement) of the system and there is no change in mass of the system, therefore there is no change in the buoyancy of the system. The term “closed” has another meaning here!

For an SCR at a fixed depth, the buoyancy change only amounts to the mass of the gas vented.

Does that make sense, Iguana Don? Keep in mind that ascents and descents will change your buoyancy, but that follows the same rules as you learned in OW.

Roak

 

[jepuskar]

Here is one of the first threads started in this section over 3 years ago...even back then..PADISCUBAPRO was dishing out the knowledge...

What a great guy..

 

[RebreatherDave]

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

here is the exact equations of the scrubbers

first step c02 mixes with water vapor forming carbonic acid (thats why pre breathing the loop is important and why the scrubber chemical has moisture to start with)

c02+h20 -> h2co3

this compound is pased over a base compound

H2CO3 + 2 NaOH -> NaCO3 + 2H20 + heat
(carbonic acid ) + sodium hydroxide -> sodium carbonate + water + heat

the final process is

Ca(OH)2 + NaCO3 -> CaCO3 + 2NaOh

calcium hydroxide + sodium carbonate -> calcium carbonate + sodium hydroxide

The typical chemical composition is
94% calcium hydroxide, 4 % sodium hydroxide, 1% potassium hydroxide and silica to act as a binding agent

I hope this helps..

Yeah, what he said......