Speed and gas consumption

[Doc Harry]

What is the relationship between speed and gas consumption?

This is an interesting question that has some major significance to divers in a overhead environment.

If you were low of gas (for whatever reason) and weren't sure you could make it to the exit, how would you proceed? Slowly? Normally? Quickly?

The real question that needs to be answered is this one:

At what speed can you cover the most distance per cubic feet of air? In other words, at what speed do you get the most "miles per gallon?"

Boys and girls, if you haven't recorded some speed/gas trials, then you really should. The results are very interesting.

1. Set up a fairly straight measured course with guideline at a relatively fixed depth. The course should be rather long because you lose some time turning around. A longer course also lets you get into a "steady state."

2. Swim some laps (a few hundred feet) at a fixed speed ("fixed speed" is subjective). Record your starting & ending gas and time, and the distance traveled.

3. Do several trials, each at a different pace. One trial should be a very slow, leisurely pace. Other trials at a slow pace, normal pace, rushed pace and maximal pace.

4. Be sure to rest adequately between trials so that you're not still out-of-breath from the previous trial.

5. Repeat the above trials using different kicks (flutter, mod flutter, frog, mod frog, etc.)

Then take the results home and review the results.

1. Calculate your speed (in feet per minute) for each speed trail. This will give you a rough idea of how fast you swim, especially if you were wearing the same kit you use in the overhead environment (i.e., doubles, stage, can light, dry suit, etc.) For instance, I swim at about 50 feet per minute doing a frog/modified frog kick at a "normal" pace in a no-flow cave in a dry suit with doubles.

2. Calculate:

x = speed
y = (cubic feet consumed/500 feet distance traveled)


Plot x versus y on some graph paper

What you will see is that y increases exponentially with x. In other words, you can cover the most distance for the least amount of gas if you kick rather slowly. This is because, in water, drag (and thus gas consumption) quadruples with the doubling of the speed.

Has anyone tried this sort of test before? Unfortunately, I cannot find my data at this time. Does anyone else have some data that they can post?

You need to find your optimum speed and know your speed parameters. You need to convince yourself that when you're low on air, the best way to maximize your chance of survival is to proceed at a leisurely pace, not a rushed pace, to optimize gas consumption.

 

[Mike Boswell]

Hi Harry,

I think this is a great topic (at least for us nerdy engineer types) and timely for me.

Yesterday I was looking at two series of warm-water dives I have done, trying to make sense of the Suunto calculations of SAC rates. My Suunto SAC's tended to vary from 0.33 to 0.43 CuFt/Min in both series, with the drift dives (Coz) averaging 0.36 and the swimming dives (Bon) averaging 0.38.

Interestingly, from the profiles it was clear that I used the most air when doing sawtooth patterns up and down over topgraphy. This despite of, or maybe because of, the fact that I control my elevation with lung volume.

My own SAC calculations using the Suunto average depth and air volume used tend to be 6 -12 percent higher than the Suunto numbers, but when I just use the numbers from the bottom part of the profile the Suunto numbers are correct.

For those with computers that record the data, it would be easy to perform your swimming tests and do the SAC calculations using the profile numbers from your computer.

 

[dumpsterDiver]

Why not record and plot distance versus gas consumption for a fixed time period that are performed at say 3 different perceived exertion levels? There might be some subjectivity in determination of exertion level, but this would occur during the real emergency anyway.

Also, it would probably be dangerous, but in a life or death situation, would you not try to skip breath? Am athlete can handle elevated carbon dioxide levels, especially for relatively short periods of time. I've never quantitatively measured it, but if I forced myself to take very slow, large inhalations and exhalations and to hold my breath during the inhalation, I think I can conserve air for some finite period of time, before the carbon dioxide level gets too high.

 

[TSandM]

In extremis, I'd do about anything to stretch my gas supply! Skip breathing will do it, at a real cost, but that cost might be lower than the alternative.

This despite of, or maybe because of, the fact that I control my elevation with lung volume.

Because of. I learned this from my dear friend KMD. When I was complaining that running a reel made me suck gas, he observed that I would come in to a tie off and not take the time to adjust my wing to be neutral, but maintain neutrality with my breath. He suggested I not do that, and it was really surprising to me how much difference that made. I knew, if I thought about it, that breathing off the top or bottom of your lungs, because it reduces tidal volume, is less efficient, but I really didn't realize how much impact it has.

But it takes a very delicate and practiced hand on the wing to get it right all the time, and breath control for ordinary dives works just fine. It's only when you're cave diving, and trying to save every psi because it means you'll get to see more cave, that changing the technique is really worth while, I think.

 

[Mike Boswell]

In extremis, I'd do about anything to stretch my gas supply! Skip breathing will do it, at a real cost, but that cost might be lower than the alternative.



Because of. I learned this from my dear friend KMD. When I was complaining that running a reel made me suck gas, he observed that I would come in to a tie off and not take the time to adjust my wing to be neutral, but maintain neutrality with my breath. He suggested I not do that, and it was really surprising to me how much difference that made. I knew, if I thought about it, that breathing off the top or bottom of your lungs, because it reduces tidal volume, is less efficient, but I really didn't realize how much impact it has.

But it takes a very delicate and practiced hand on the wing to get it right all the time, and breath control for ordinary dives works just fine. It's only when you're cave diving, and trying to save every psi because it means you'll get to see more cave, that changing the technique is really worth while, I think.

I believe you are right. In warm water I don't wear a wetsuit and I don't put ANY air in the wing at all, at any depth, because with a 6-lb SS BP and 4 lbs of lead I am weighted and trimmed fairly well. Thinking about what you are saying, I suppose that I am also using my lung buoyancy to compensate for the 6 lbs of air in the tank, so in theory I cannot be breathing in a ideal way.

Darn, I guess I need a travel wing!

 

[TSandM]

Well, if you are not wearing a wetsuit, and you are carrying an Al80, all the compensation you need to do is for the not quite five pounds of air you expect to exhaust into the water during your dive. I'm quite sure a normal-sized adult male can do this pretty easily, but I suspect if I tried, I'd be pretty uncomfortable after a while.

I think you'd find a little air in the wing would make for a more relaxing dive. I put a little in mine on descent, and then just bleed a little out from time to time, as the dive profile curves upward and the gas supply curves downward. Using my breath for minor adjustments like moving over coral heads doesn't go through enough gas to matter.

 

[Mike Boswell]

I have no idea how much buoyancy change a person's lungs can produce. Ten pounds? Can anybody give some examples or ranges of typical lung volumes? How could I measure mine?

 

[Bubbletrubble]

I have no idea how much buoyancy change a person's lungs can produce. Ten pounds? Can anybody give some examples or ranges of typical lung volumes? How could I measure mine?

@Mike Boswell: I believe that the info you seek is on the Wikipedia "Lung Volumes" webpage.
For normal respiration, pay attention to Tidal Volume (500 mL).
For "forced" respiration (maximal inhalation followed by maximal exhalation), you are interested in the difference between Total Lung Capacity and Expiratory Reserve Volume = 6 L - 1.2 L = 4.8 L
For each respiratory cycle, the gas coming out of the tank will depend on the volume of the inhalation and the ambient pressure at which the inhalation occurs.
For buoyancy calculations, a ballpark conversion is 1 L of air = 2.2 lbs. of positive buoyancy (of course, this will vary with the specific gravity of the water).

Along the same lines of what TSandM wrote, your gas consumption is not going to be most efficient if you are altering your lung volume to compensate for buoyancy. Use your BCD for that. Whenever I read about people who, while wearing minimal exposure protection, claim that they only use their lungs to adjust buoyancy (as if it's a state of dive nirvana), I wonder if they've ever considered this. Under certain conditions (AL80 with minimal exposure protection), most of us could adjust for buoyancy changes over the course of a dive with our lungs alone...but why do so when we have a perfectly good tool called a BCD?

FWIW, I think that your gas consumption is increased during sawtooth dive profiles for one or both of the following reasons: (1) the additional gas you use for more frequent ear equalization and (2) more frequent kicking you might be using to alter your position in the water column.

 

[TSandM]

Bubbletrubble, that goes along with what I have read, which is that the maximum buoyancy adjustment that can be done with the lungs is in the range of 10 lbs. But of course, that number is dependent on diver size. A 5', 100 lb woman is not going to have the buoyancy resources that a 220 lb ex-football player will have with his breathing. I've talked to a number of female diving friends about this, and we all agree that the guys can initiate an ascent much faster than we can, simply by inhaling.

 

[DA Aquamaster]

Common sense comes into play here as well. For example if I am going to drop 5' to go under a duck under and then come back up to the same level I started at, compensating with lung volume makes more sense. The same applies for a brief ascent over a sand bank, etc.

Often you can just exhale and hold the exhale for a few second to start the ascent and then let inertia take it the rest of the way, then inhale normally and ride back up on the other side of the duck under with no need to add and then immediately dump gas from the wing.

However if you have to maintain the change in buoyancy more than a breath or two, it is more efficient to make the change using gas in the wing. If it is seriosuly disupting your breathing pattern or requiring you to take very small and frequent breaths, it is probably counter productive.

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Flow also makes a difference in terms of your most efficient swimming speed. In general, if flying into a headwind a slightly higher cruise speed is more efficient in terms of time and gas, even if it burns more fuel as you spend less time in total in the headwind and are blown les distance "backward". The same thing will apply when swimming against flow in a cave. Something to consider if you are exiting a siphon.