# Science question for you underwater types



## Ringo Redux (Jul 7, 2010)

Hey guys,

I'm doing a but of recreational research (stay with me, here - I'm a geek), and I ran across a question that I don't know the answer to and didn't know how to properly phrase for google. That is, I found some conflicting information and I'm looking for real world experience. So, I'll pose a scenario and see if anyone can answer.

Assume you're in 30 feet of water, breathing normally (via SCUBA). If you were to draw one deep breath off your tank (as though you intended to hold your breath, or maybe you just sighed at the underwater beauty), what amount of PSI would you draw off your tank? 

What I mean is - lets say your gauge reads an even 800 psi, you take your deep breath - what does it read after that? 

Anyone, please feel free to comment.

-R.


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## Ocean Master (May 20, 2008)

You would have to get some highly technical equipment to find that answer. You definatly would not see it on your standard SCUBA gauge.


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## amnbowser (Oct 2, 2009)

*breath*

No one can answer this. Think about what you're asking everyone has different lungs now you could average but you need to breakdown the problem further. IE age sex race good luck


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## Ringo Redux (Jul 7, 2010)

OM, I figured that was the case. But, it's still pretty telling. I assume that would mean that the amount of PSI you'd draw is relatively insignificant?

AMN - also a good point. Operating on the idea that a 'normal adult' contains anywhere from 4 to 6 liters at maxiumum lung capacity, we'll assume a 'full breath' is 5 liters.


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## FenderBender (Oct 2, 2007)

It depends on your lung capacity, and the size of your tank, for starters. 

A big gulp of air for person A is not the same is it is for person B.

The psi drawn off a tank for the same breath will not be the same on an Aluminum 80 cf tank as it would be an Aluminum 50, or a AL 100, or a steel 120, or a pony bottle...

See where I'm going with that?


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## FenderBender (Oct 2, 2007)

Another way to think about it is at 33 ft (2 atmospheres) you will draw twice as much air into your lungs as opposed to the surface, if that helps any.

Sit on the surface with a full tank, draw a bunch of equal sized deep breaths, count them until your tank is empty. Divide that number by 2, and then divide 3000 by that number. (Assuming you started with a tank at 3000 psi.)

For example, if it takes you 200 big breaths on surface to empty your tankl, it would only take 100 breaths at 33 ft. 3000/100 = 30. So you would use 30 psi. per gulp.


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## Ringo Redux (Jul 7, 2010)

FenderBender said:


> Another way to think about it is at 33 ft (2 atmospheres) you will draw twice as much air into your lungs as opposed to the surface, if that helps any.
> 
> Sit on the surface with a full tank, draw a bunch of equal sized deep breaths, count them until your tank is empty. Divide that number by 2, and then divide 3000 by that number. (Assuming you started with a tank at 3000 psi.)
> 
> For example, if it takes you 200 big breaths on surface to empty your tankl, it would only take 100 breaths at 33 ft. 3000/100 = 30. So you would use 30 psi. per gulp.


FB - this is phenomenally helpful. I was also going to do a timed test. I.e. - breathe normally for 1 minute and take the PSI Delta. Then, assume a 'deep breath' is double capacity of a normal breath, and divide accordingly. Great help so far, guys.


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## stauty trout (Jul 19, 2012)

You have to remember also that the deeper you dive the more compressed the air becomes in the tank and therefor would read a lower psi than it would at the surface... Also the temperature affects your psi (the hotter the higher psi and colder vice versa) If your wondering about how long you might have left once you hit 800psi at 30 ft before coming up you have a good while.. I use a rule of thumb of starting my ascent at no less than 500psi so you don't get in a pinch


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## FenderBender (Oct 2, 2007)

ata0002 said:


> You have to remember also that the deeper you dive the more compressed the air becomes in the tank and therefor would read a lower psi than it would at the surface...



I am going to have to disagree with you on this one...

A scuba tank is a sealed system, unlike our body (and lungs, ears, etc.)

The pressure inside a scuba bottle is phenomenally higher than the water pressure outside it, pushing in (or farther compressing it.)

If anything, your tank is trying to bulge out, not shrink down, but it can't do either because it is sealed and built with walls of aluminum or steel.

Correct me if I am wrong in this way of thinking, this is an interesting discussion.


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## Firefishvideo (Jul 1, 2010)

At 30' you are at 2 atmospheres of pressure. I calculate my personal usage (stationary at rest) at .5 cubic ft. per minute at one atmosphere.
So at 30' I would use 1 cubic foot per minute. Assuming the tank is an 80 cubic foot tank , it would last for 80 minutes before being empty. The tank holds 3000psi so I would be using 37.5 psi per minute or .0625 psi per second. At rest I breath about 4-5 times per minute so each breath would consume 7.5 psi.
These are my SAFE, TESTED numbers by which I plan deep dives....I use a 1 cubic ft. per minute usage for bottom times and the .5 for decompression......I usually use less on both , but those # keep me alive.:thumbsup:
All of those # would go up if I were exerting myself, or at a deeper depth. Both factors effect the usage exponetially.
....also I have a reputation for not breathing underwater....so others might have higher usage.:whistling:


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## stauty trout (Jul 19, 2012)

Fender... It has to do with Boyles law.. The temperature has a major effect on the psi in the tank. Check out this site for forumals and explanations of how to calculate fluctuations in the psi in your tank when dealing with depth/temp changes http://www.freezingblue.com/iphone/flashcards/printPreview.php?fileid=1441


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## Firefishvideo (Jul 1, 2010)

Forgot to mention.....that from personal experience ......when at 300' with 300 cubic feet of gas on my back, I COULD see the needle move....and its NOT a good feeling when it does not rebound....just tick tick tick ever downward....inch: .....makes you want to go home!
....but good fun when you are safe back on the boat!!!!:thumbsup:


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## stauty trout (Jul 19, 2012)

Firefish.. That's nuts man I've never been over 150' and you can pretty much see the needle moving with every breath especially chasing/fighting grouper... I could image at twice that depth how fast it would go! What have you dove on at that depth?


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## Firefishvideo (Jul 1, 2010)

ata0002 said:


> Firefish.. That's nuts man I've never been over 150' and you can pretty much see the needle moving with every breath especially chasing/fighting grouper... I could image at twice that depth how fast it would go! What have you dove on at that depth?


dove the Ozark ..off of Destin ....180' to the crows nest....300' to the deck ....never got near the sand...its at 330-350!!! I was breathing trimix with....as memory serves....around 12% o2 and 55% Helium. Equivalent narcotic depth was about 102'....so not too badly narcd.
Awesome dive!!! Amberjack that looked as big as sharks, and some very big, very territorial grouper!!!


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## Firefishvideo (Jul 1, 2010)

amnbowser said:


> No one can answer this. Think about what you're asking everyone has different lungs now you could average but you need to breakdown the problem further. IE age sex race good luck





Firefishvideo said:


> At 30' you are at 2 atmospheres of pressure. I calculate my personal usage (stationary at rest) at .5 cubic ft. per minute at one atmosphere.
> So at 30' I would use 1 cubic foot per minute. Assuming the tank is an 80 cubic foot tank , it would last for 80 minutes before being empty. The tank holds 3000psi so I would be using 37.5 psi per minute or .0625 psi per second. At rest I breath about 4-5 times per minute so each breath would consume 7.5 psi.
> These are my SAFE, TESTED numbers by which I plan deep dives....I use a 1 cubic ft. per minute usage for bottom times and the .5 for decompression......I usually use less on both , but those # keep me alive.:thumbsup:
> All of those # would go up if I were exerting myself, or at a deeper depth. Both factors effect the usage exponetially.
> ....also I have a reputation for not breathing underwater....so others might have higher usage.:whistling:


Everyone IS different.....You have to LOG your usage, and even do "working" dives....on which you only test your usage at rest/work for a set time/depth. Then you can calculate your usage for other depths by multiplying by the atmospheres in depth ( 1 per every 33 feet ...+1 for the surface pressure)


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## FenderBender (Oct 2, 2007)

Firefishvideo said:


> dove the Ozark ..off of Destin ....180' to the crows nest....300' to the deck ....never got near the sand...its at 330-350!!! I was breathing trimix with....as memory serves....around 12% o2 and 55% Helium. Equivalent narcotic depth was about 102'....so not too badly narcd.
> Awesome dive!!! Amberjack that looked as big as sharks, and some very big, very territorial grouper!!!



I've always wondered why use trimix as opposed to heliox? Is it the high cost of helium that makes recreational guys use trimix? 

I'm not certified in trimix diving, but I worked as a commercial diver for a little over 3 years and never saw any gas with nitrogen in it other than air and the 50/50 mix we would decompress on sometimes. A 300' dive we would use 88/12, 88% helium and 12% O2. Never dove sat, but worked as a tender on an 880' job where they were breathing 98% Helium and only 2% O2!! Talk about Alvin on the Chipmunks...

I feel like heliox would be safer than trimix. Why don't recreational guys use it, other than cost?


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## Ringo Redux (Jul 7, 2010)

Happy to stimulate such great discussion. It's nice to have you folks with such wide knowledge of the subject. I'm appreciating all this. I think, for the purpose of my question, the depths would be no greater than 60 feet. So, operating in 2 atm, correct?

And, based on what you guys have said, there would be 2 answers. One for each pressure change. At least, I think that's what I'm reading.


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## stauty trout (Jul 19, 2012)

Firefish.. That's awesome! Seen some of those big aj's diving the deep rigs never shot one though! 45 pound one pulls you around enough if you don't stone it couldn't one 2-3x that size pulling on you... Bye bye gun! Haha


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## Firefishvideo (Jul 1, 2010)

Fender.....we use nitrogen because it is absorbed more slowly than helium....so on a short (non surface supplied) dive...you get a shorter deco obligation. The extra helium would cost more but would also needlessly add time in the water hanging on a deco line.

Ringo....66 feet would be 3 ATM ....you have 1 ATM at the surface. So for me it would mean 3 cubic feet per minute working or 1.5 resting.


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## SaltAddict (Jan 6, 2010)

Scott, I'm glad you chimed in. I didn't want to type all that lol. To the OP, type "respiratory minute volume" into a google search and click on the wikapedia link. This should quench the geek thirst. It is some thing you have to do as a technical diver. Having decompression obligations, it is imparative that you know how much gas you breath. I pay close attention to my gas consumption when I shoot an AJ. Mainly because if you want to know how much gas you breathe under stress... there is no better test than going toe to fin with a stud AJ. Most of my focus while tangling with an AJ is on my breathing. I use it as an exercise in breath control. Another moment I work on breath control is the first sight of a shark. No matter who you are, the body triggers an adreneline dump. Training yourself to "deal" with that adreneline dump is important to me because sharks react to panic. Just fun training things I like to do.

Thanks for kicking off this thread. Not much tech talk on this forum.

P.S. Scott would you be interested in doing the ozark again? It will probably be a year or two before I am ready. I have to complete trimix classes and get a couple shearwater predators. It is a bucket list thing for me.


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## Ringo Redux (Jul 7, 2010)

Ah yes - equations. Now we're talkin! :thumbsup:

That truly did sound as lame coming out as it did in my head. Apologies. 

I wonder - does the volume inhaled of mixed gas differ among types? That is, nitrox, trimix, o2, etc. - Sciencing aloud, here.


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## SaltAddict (Jan 6, 2010)

Interesting question to which I do not have an answer.:detective:


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## Firefishvideo (Jul 1, 2010)

Ringo Redux said:


> Ah yes - equations. Now we're talkin! :thumbsup:
> 
> That truly did sound as lame coming out as it did in my head. Apologies.
> 
> I wonder - does the volume inhaled of mixed gas differ among types? That is, nitrox, trimix, o2, etc. - Sciencing aloud, here.


The volume does not change, but at deeper depths you notice that the helium mixes are easier to breath.....Helium is a smaller molecule , and tends to be a bit "thinner".....thus easier to breath than a "thicker" air/nitrogen mix.


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## Ringo Redux (Jul 7, 2010)

FF - I don't doubt what you say (as you're clearly more experienced than I am), but - interesting how that's a conundrum. A smaller molecule would imply that you get more molecules of the gas per aspiration. Curiouser and curiouser...


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## Firefishvideo (Jul 1, 2010)

SaltAddict said:


> P.S. Scott would you be interested in doing the ozark again? It will probably be a year or two before I am ready. I have to complete trimix classes and get a couple shearwater predators. It is a bucket list thing for me.


 I'd do it again...I was just out of Adv. trimix class when I did it...and I really couldnt have gotten the most out of the dive. I have some other spots that I would love to dive....but the Ozark is a monster of a simi-local dive.....I just don't want to have to pay the $1000's of dollars to ride the MV Spree charter. I dove it from the H2O below... The split charter was about $200 each.. ....now - most of the local captains won't even consider taking you there. Too much risk with not that much reward for them.


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## Firefishvideo (Jul 1, 2010)

Ringo Redux said:


> FF - I don't doubt what you say (as you're clearly more experienced than I am), but - interesting how that's a conundrum. A smaller molecule would imply that you get more molecules of the gas per aspiration. Curiouser and curiouser...


Yep, you would be right!...more molecules....but they still occupy the same volume. Now you are needing to calculate density via MOLES..... and that is where I draw the line!
http://en.wikipedia.org/wiki/Mole_(unit) ...here you go.....you are on your own on that one! I'm too far out of Molecular Chemistry to remember how to do that.....or want to....I'm going diving.


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## SaltAddict (Jan 6, 2010)

Good deal Scott. I'm sure Whackum Stackum would do it with us. I know a couple other guys (atleast 3) that want to do it as well. I have to do both trimix classes, but once I get started on them, I will get them knocked out as quick as the courses will allow. I would be very interested to do some deeper dives once training is completed. 

I may even have a boat we can charter. I have to talk with a friend (who would probably be one of the divers) and see.


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## Ringo Redux (Jul 7, 2010)

Firefishvideo said:


> Yep, you would be right!...more molecules....but they still occupy the same volume. Now you are needing to calculate density via MOLES..... and that is where I draw the line!
> http://en.wikipedia.org/wiki/Mole_(unit) ...here you go.....you are on your own on that one! I'm too far out of Molecular Chemistry to remember how to do that.....or want to....I'm going diving.


I thought about that last night when I got in the shower. I was standing there and went, 'Man, that was really an idiotic thing to say.' Geometry came back to me a little late. :whistling:


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## Mudigger (Aug 5, 2011)

Let's start with the definition of a sigh. Sighs are an integral part of pulmonary physiology. So much so that ventilators have sigh settings on them. 

"Definition of sighs (how to define sighs)
Sighing is defined as an involuntary inhalation (inspiration) that is 1.5-2 times greater than the usual tidal volume. Some studies suggest that a sigh should be defined as 1.5 times increase in the personal usual normal tidal volume (Leiner & Abramowitz, 1958). Some other authors suggested another definition of a sigh: it should be at least as twice as deep as the person's average inhalation (Wilhelm et al, 2001; Caughey et al, 1943)."

So, many of you are correct. It depends on a person's average tidal volume (or regular volume of a routine breath). Then you must calculate the pressure deferential associatted with that volume at a given depth and temperature and determine if you want to know the actual gauge pressure or the absolute pressure. 

Do we really want to calculate this. It, actually, will be negligible and likely cannot be measured on standard gauges. 

Yes, I'm a dive medicine physician. Trained by NOAA and I work with commercial divers in the Gulf. 

OK, enough already. I'm going have a beer.


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## Mudigger (Aug 5, 2011)

One more point, as I open this beer. Taking a breath as when you inhale maximally is more than the volume of a sigh. That is defined as "total lung volume" and can be measured in a pulmonary lab at most hospitals.


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## SaltAddict (Jan 6, 2010)

I don't recall ever intaking a sigh underwater. Now when my spear pulls and I lose a fish, I draw maximum lung capacity. However, this is so that the prolific string of obscenities I have linked together can fly through my regulator with the smoothest of fluidity.


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## WhackUmStackUm (Jan 31, 2010)

As a physicist (reformed) and deep diver I would break the problem into two parts, as Scott did:
Calculate the volume of gas removed from the tank per breath.
Given the tank's volume, calculate the effect on PSI for the particular tank you are using.
For those interested in getting into deep diving, calculating the volume of gas that you will consume, is a critical part of your pre-dive planning.

When diving a rebreather, we do a similar calculation to determine the amount of bailout (backup open circuit) gas that we need to carry. 

When using a rebreather, the amount of "normal" gas you will use (the gas on your back) is calculated using a different method. However, this process may be a topic for another thread at some point.


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## WhackUmStackUm (Jan 31, 2010)

SaltAddict said:


> Good deal Scott. I'm sure Whackum Stackum would do it with us. I know a couple other guys (atleast 3) that want to do it as well. I have to do both trimix classes, but once I get started on them, I will get them knocked out as quick as the courses will allow. I would be very interested to do some deeper dives once training is completed.
> 
> I may even have a boat we can charter. I have to talk with a friend (who would probably be one of the divers) and see.


Yes, my wife Anna and I would like to dive the Ozark.


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## FenderBender (Oct 2, 2007)

If you guys want to get an ozark trip together, I'd be happy to take you on my boat, I'll captain while y'all dive it and dive some shallower spots myself on the way back in. Let me know if you're interested.


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## No Excuses (Apr 7, 2012)

Doesn't your body become less efficient at processing oxygen at higher pressures (if I remember correctly from AP Chem). Or would the increased amount of oxygen molecules you get each breath as you go deeper cancel out that effect? Just a thought.

C6H12O6 + 6O2 --> 6CO2 + 6H2O + energy
As pressure increases, the amount of oxygen needed to sustain the reaction should increase right? That or my notes are wrong.


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## SaltAddict (Jan 6, 2010)

Oxygen toxicity is a major concern at depth. However, I have NO IDEA what that equation even means lol.


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## MathGeek (Mar 31, 2012)

A relatively accurate answer can be computed with the ideal gas law if and only if you know the typical deep breath volume of the diver.

PV = nRT

You would first compute the number of moles in the deep breath

n = PV/(RT)

where P, V, R, and T indicate the pressure, volume, and temperature of the gas inhaled into the divers lungs. You need to take some care with the units so that T is in Kelvin, and V and P are in the same units as reflected in the units of the ideal gas constant, R.

You also need to compute the initial number of moles in the tank

n = PV/(RT)

where P, V, and T are the pressure, volume, and temperature of the gas in the tank.

The final number of moles in the tank is the initial number of moles minus the number of moles in the diver's deep breath. Once you have the final number of moles in the tank, you can compute the final tank pressure as

P = nRT/V,

where n is the final number of moles, T is the tank temperature, and V is the tank volume. You can also reason proportionally. In other words, if a deep breath takes 0.05% of the gas from the tank (in moles) then the reduction in pressure will also be 0.05%.


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## Firefishvideo (Jul 1, 2010)

MathGeek said:


> A relatively accurate answer can be computed with the ideal gas law if and only if you know the typical deep breath volume of the diver.
> 
> PV = nRT
> 
> ...


You guys are working at this from the wrong end. As I already explained, you have to find your usage over time (with in water testing), and divide it by the time/depth (in atmospheres)...then you can figure your cubic ft per minutes usage. Once you have that you can use it to calculate any tank,any depth. You can get more technical, but probably not any closer to the real value.
If you try calculating from the one breath end of the problem you will induce too much error for the value to be of any use when you multiply it by any amount of time or depth.....plus unless you measure your lung volume, you are just guessing.
The tank temp makes no difference, because it will not change that much once you get in the water and check your gauge.
Technical divers are using the method I described to do very long , very deep dives....and they can tell you what their pressures will be in each tank at ANY point in the dive, BEFORE they do the dive. Its not that hard.:thumbsup:
PS: I was joking when I suggested using MOLES....you are working with volume vs pressure, the # of molecules does not matter.


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## El Kabong (May 9, 2012)

+1 for what Whack and Fire are telling you guys. There won't be any significant difference between breathing a tank with 21% oxygen vs. breathing a tank with 32% oxygen (assuming we're not talking about adding helium to the mix as well).

Calculating a "deep breath" is probably also a waste of time. If anything, most divers take shallow breaths.

You do have to average your sac rate over time and dive type. I usually have a sac in the .4 to .5 range when doing shore dives, .5 to .7 spearfishing, and .7 to .8 when doing technical dives. That's quite a difference! You only figure this out over time, there's no quick and easy method.


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## SHunter (Jun 19, 2009)

No Excuses said:


> Doesn't your body become less efficient at processing oxygen at higher pressures (if I remember correctly from AP Chem). Or would the increased amount of oxygen molecules you get each breath as you go deeper cancel out that effect? Just a thought.
> 
> C6H12O6 + 6O2 --> 6CO2 + 6H2O + energy
> As pressure increases, the amount of oxygen needed to sustain the reaction should increase right? That or my notes are wrong.


Your formula is basic cellular respiration. I don't think that the number of atoms would change under pressure. The mechanism to produce energy has to continue or you die so why/how would the numbers change? Humans can for a short time anaerobically build up lactic acid for energy. If you do not have enough hemoglobin in the red blood cells then there is too much oxygen with nothing to attach which becomes a problem. I'm guessing that someone like Firefish is in great shape and has plenty of hemoglobin to keep the oxygen circulating efficiently. This is a generalization but I have noticed that young women seem to have good oxygen consumption when diving. Some of the gals we dove with were the first to go in the water and last to go out. 

Ringo-To get back to the original question, I agree with one post on how to estimate but to come up with a number for everyone should be different according to the individual's physiology. And, I agree that this is an interesting topic. Thanks for starting the thread. Any internet research to see if you can find the answer?


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## El Kabong (May 9, 2012)

Remember the deeper you dive, the higher the ppo2 of whatever gas you are breathing. Technical divers routinely dive with hypoxic breathing gasses (less than 21%O2). Air (about 21%O2) begins to get dangerously toxic at around 180ft. So you want to have a bottom gas with less oxygen than air at certain points.


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## SHunter (Jun 19, 2009)

Firefishvideo said:


> You guys are working at this from the wrong end. As I already explained, you have to find your usage over time (with in water testing), and divide it by the time/depth (in atmospheres)...then you can figure your cubic ft per minutes usage. Once you have that you can use it to calculate any tank,any depth. You can get more technical, but probably not any closer to the real value.
> If you try calculating from the one breath end of the problem you will induce too much error for the value to be of any use when you multiply it by any amount of time or depth.....plus unless you measure your lung volume, you are just guessing.
> The tank temp makes no difference, because it will not change that much once you get in the water and check your gauge.
> 
> I think that this is a reasonable explanation on how to determine usage for an individual.


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## markhsaltz (Aug 2, 2010)

you obviously need to get a job..Way too much time on your hands thinking of that..Maybe one day, I'll get enough time to think of such profound questions..thanks for the amusement...


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## markhsaltz (Aug 2, 2010)

ya'll need to remember that as you go deeper the water will cool the air. for every 1 degree in temp change you either lose or gain 5 psi..throw that into the equation..Why not just use an integrated dive computer that can/ will calculate this for you?...hummmmmmmm....


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## eodryan (Mar 4, 2009)

For a super technical answer try: http://www.reddit.com/r/askscience/


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