SAC versus depth

[TSandM]

Mr. Raine, first off, I'd like to offer that rjack dives a LOT. In fact, he sometimes dives with me.

Second, I'll just talk about my experiences here. Yes, I put more gas in my wing to go down to 100 feet than to 20. But if I'm wandering around the Edmonds Underwater Park at 20 to 30 feet, going up to look at the tops of the structures and then back down to look at the bottoms, I'm going to end up venting my dry suit and then putting air back in it a lot of times. Whether that ends up being more than the total amount I put in the wing at depth, I don't know.

But I can't come up with any OTHER explanation for why somebody would have higher gas consumption in shallow water.

 

[rjack321]

Second, I'll just talk about my experiences here. Yes, I put more gas in my wing to go down to 100 feet than to 20. But if I'm wandering around the Edmonds Underwater Park at 20 to 30 feet, going up to look at the tops of the structures and then back down to look at the bottoms, I'm going to end up venting my dry suit and then putting air back in it a lot of times. Whether that ends up being more than the total amount I put in the wing at depth, I don't know.

But I can't come up with any OTHER explanation for why somebody would have higher gas consumption in shallow water.

That's exactly my point. Proportionally and cumulatively it adds up. The same structure in deeper water and you wouldn't be venting and adding at all. A smidge of breath control here and there and you'd be fine.

And I haven't heard a good competing explanation from Mr Raine either since the OP's calculations and experience are the opposite of his theories.

PS I wish I could dive more, I should pass 450 in MX

 

[mikerault]

Let's do a thought experiment. What happens as we descend to our lung volume....nothing, it stays constant since our regulators are equalizing the pressure as we descend. However, what is changing is the percentage of O2 our lungs are being exposed to. The actual number of O2 molecules in our lungs increases so theoretically we may unconciously breath shallower at deeper depths while still getting the same, or more O2 into our lungs. This is why we have to reduce the concentration of O2 below that normally present in air if we go really deep. Thus SAC may decrease with depth assuming the same level of effort is being expended.

Just a laymens thoughts...

Mike

 

[TSandM]

It's a good thought, Mike, but it's not true.

The actual volume per minute that you have to move through your lungs is not dependent on the amount of oxygen in what you're breathing. You move the volume per minute that you have to to keep your CO2 level normal. Regardless of the density of what you are breathing, the volume has to be the same. So you won't see any decrease in the minute ventilation with depth.

 

[Peo]

Sorry, this just doesn't fly.

Look at the math here...

If you're perfectly neutral with no gas in wing or suit, there is no buoyancy swing as you changes depth. This is obviously not the case, especially not so in the early part of the dive, so let's say that you are 4 pounds negative if you burp all gas.

To lift 4 pounds, you need to have 0.07 cu ft of gas in your wing/drysuit.

If you do a depth swing from 30 ft to 20 ft, and you are TOTALLY incapable of compensating ANY of that using your lungs, you'll have to burp out (.07*2/1.7)-.07)=0.012 cu ft of "water volume", which would require 0.024 cu ft of gas from your tank since you're at ~two atmospheres.

Repeat that 50 times during a dive, and you'll have used up 1.2 cu ft of gas doing these swings. That's a bit over 1% of a full AL80! You won't notice this in your SAC.

Do it 500 times, and you will start noticing it, but that'd be a pretty intense dive. ;-)

No, there are surely some other factors playing in here.

I've also been surprised how (relatively speaking) much gas I use up in the early phase of the dive.

One reason is water temperature.

Looking at the Edmonds Underwater Park example again; summer is the worst case.

It's 78 degrees outside. Your tank is at 3500. You hit the water, which is still 50 degrees. Once the tank and the gas in the tank has changed it's temperature, you're tank pressure has dropped from 3500 to 283*3500/298=3323 psi (1), without hitting the purge button, filling your wing or your drysuit, or taking a single breath. Just the temperature change. Guess what that does for a SAC calculation if you're diving a set of double E8-130s for a short shallow dive. ;-)

Other factors surely play in too. Perhaps the shallow dive is higher intensity than the deep one. Perhaps the diver is paying more attention to keeping his breathing slow on a deep dive than a shallow one. Depending on the dive, sometimes my breathing doesn't really go into "dive mode" until I'm on the bottom, unless I'm explicitly thinking about it.

If you're using up a lot of gas doing depth changes, you should start paying attention to how you breath as you're doing those changes. A single breath at a depth of 30 ft uses up about as much gas as twenty of those BC adjustments you were talking about.

It's late. Any errors in the math are purely my fault. It's been a long week.

(1) Derived from the ideal gas law P*V=n*R*T. 50 Fahrenheit => 283 Kelvin, 78 Fahrenheit = 298 Kelvin

 

[Peo]

Mr. Raine, first off, I'd like to offer that rjack dives a LOT. In fact, he sometimes dives with me.

Second, I'll just talk about my experiences here. Yes, I put more gas in my wing to go down to 100 feet than to 20. But if I'm wandering around the Edmonds Underwater Park at 20 to 30 feet, going up to look at the tops of the structures and then back down to look at the bottoms, I'm going to end up venting my dry suit and then putting air back in it a lot of times. Whether that ends up being more than the total amount I put in the wing at depth, I don't know.

But I can't come up with any OTHER explanation for why somebody would have higher gas consumption in shallow water.

If you go to 100 ft, and again 4 lbs heavy, you'll fill your wing with 4*.07 cu ft=.28 cu ft of gas. That's the same gas amount as doing that EUP up-and-down thingie almost twelve times.

 

[Peo]

When I look at a plot of SAC verses depth for a profile of 85 dives for myself I get a decrease in SAC with depth. Does this make sense? Other divers are telling me that SAC for a given exertion rate should be a constant regardless of depth, but is that really true?

To answer your question, we need to know more about your dives, and about how exactly you are measing your SAC, and what water temperatures you've been diving in.

Can you post two or three typical plots, and tell us the conditions of the dives, which gear you were using, and exactly how the plots were generated.

 

[Hannavas]

... as we descend... what is changing is the percentage of O2 our lungs are being exposed to....

Actually, Gas Diffusion and Kinetic Theory of Gases would assert that the volume percentage of O2 (and that of each of the other gases) in the gas inhaled and delivered to the lungs remains constant with varying depth. For example, if air is used, the O2 % in the air inhaled remains constant at 21% at depth, as on the surface. What is increased at depth is the partial pressure of oxygen ppO2 as it is delivered to the lungs (as well as the partial pressure of each of the other gases in the air being inhaled) due to the increase in the total system pressure, per Dalton’s Law (ppO2 = %vol O2 x total pressure, absolute).

... This is why we have to reduce the concentration of O2 below that normally present in air if we go really deep.

The objective here is to avoid oxygen toxicity. Oxygen toxicity is a function of partial pressure of oxygen and exposure time. Because the total pressure exerted on the body is determined by the depth, the remaining lever (degree of freedom) to limit the ppO2 is the type of gas used. A gas mixture with low(er) O2 % can be used to go &#8220;deep&#8221; while keeping the ppO2 from exceeding O2 toxicity level (this level varies but cited by some sources as above 1.6 ata ppO2). Upon ascent to shallower depths, however, the diver must switch to another gas source richer in O2 to prevent hypoxia (< 0.14 ata ppO2 per some sources).

Disclaimer: The information above is meant only for the exchange/sharing of knowledge and should not be construed beyond its intended purpose. It is not a boasting of knowledge or experience.

 

[mikerault]

These dives were in 3mm, 5mm, 7mm, none, drysuit all with AL80's with anywhere from 11 to 24 pounds of lead in saltwater and fresh water using vest, back inflate and BP/W.

That is what I am saying, I have taken these dives and used the average calculation form my computer (MAREs Genius first, Cochran Commander next then finally the Citizen Cyber Aqualand NX.) All of my dive profiles are on www.rgi-consult.com .

Mike

 

[mikerault]

Back to one of my original questions, how many of you have taken your MAX and Average SAC rates and plotted them according to depth? I am getting a lot of "When I calculate" and "Last time I calculated" but none saying I have plotted mine and don't see it.

Mike