Air Consumption Question

toefa · Jul 9, 2009

Hi Guys,

Just trying to get my head around the way pressure effects our air consumption and thought i would make sure i am grasping this correctly..

With my normal tank. If as an average, i go through the tank on the surface (0 feet/metres) in 60 minutes, does that mean when i descend to 10 metres where the pressure is 2 ata, i will consume my air on average, 2x faster - so if i could somehow start a dive at 10 metres, i would have a bottom time of 30 minutes?

Then at 50 metres or 6 ata my air would last 10 minutes - again not factoring the ascent descent?

If this is right, i do understand that i need to factor in time for ascending and descending as well as safety or deco obligations, but i just wanted to make sure i am well aware of the maths to do with pressure and air consumption.

Thanks,
Toefa

Thalassamania · Jul 9, 2009

Right. You've got the ATA thing that messes most people up. Here's a hint for ascent and descent calculations: use the time of the ascent/descent and pretend that you spent that time in the middle, for example if you're going down to 50 meters and that takes you two minutes, figure it as two minutes at 25 meters.

ae3753 · Jul 9, 2009

Here's NWGratefulDiver's Scuba Gas Management article. Worth reading, understanding, and using.

toefa · Jul 10, 2009

OK, thanks guys!

So now i just have to work out my SAC and i will be able to make basic dive plans that include gas management.

To work out SAC, i need to go back to my log and work out my consumption by working out what my consumption would have been at the surface (that is multiplying my consumption by the pressure)? Is that essentially it? How many dives should i base this off?

Kevrumbo · Jul 10, 2009

Use 22 litres/min*ATA as a baseline SAC rate example (your own nominal SAC rate might be better, hopefully not worse than this after a few trials). Then using a 11 litre/bar tank (same as an AL80), this translates to 2 bar/min*ATA rate on your SPG (that's 22 divided-by 11 equals 2 bar/min*ATA).

Therefore after 10min nominal breathing on the surface (1ATA), you would expect to use 20bar of gas --that is: (2bar/min*ATA)(10min)(1ATA) equals 20bar used from your 11 L Tank.

Or instead, after 10min nominal breathing at 10meters depth (2ATA), you would use 40bar of gas.
Or after 10min at 20meters (3ATA), you would use expect to use 60bar of gas etc.

The skill to develop during the dive is to get a feeling for how much gas you're using up from your tank in 10min intervals, even before checking and confirming the number in bar with your SPG. . .

Peter Guy · Jul 10, 2009

toefa -- remember, Air Supply Management Planning is just that, planning. Once you have done your dive, you can look at what you actually did and then compare it to your plan. After a while (a very vague term) you'll get a feel for how much you breathe on what type of dive -- but remember, EVERY dive is its own dive.

Here's a "Cheat Sheet" I created and some people have found it helpful (and, btw, I stole as much as I could from NWGratefulDiver and Lamont -- both friends of mine).

Cheat Sheet

toefa · Jul 10, 2009

Thanks Kevrumbo, that's a great help! So i can use that as a conservative baseline which will work out as 2 bar per minute x the pressure i am at. Or use my own SAC which could only be better - that is, shouldn't be worse.

toefa · Jul 10, 2009

Thanks Peter Guy. Yeah i do understand that i should only use it as planning and that i need to adapt my diving to what is actually occurring on the dive. I just really wanted to understand what is going on as i go down.

That cheat sheet is very good, i will print that out and read over it a few times!

fisheater · Jul 10, 2009

Depending upon the brand/model, your computer may compute your SAC for you. Often, you'll have to download the data to get the SAC computation.

Kevrumbo · Jul 13, 2009

toefa:
Thanks Kevrumbo, that's a great help! So i can use that as a conservative baseline which will work out as 2 bar per minute x the pressure i am at. Or use my own SAC which could only be better - that is, shouldn't be worse.

Toefa, you can make it more simple if you use twin manifolded 11 Litre tanks (22 litres per bar) --so your conservative baseline becomes 1bar/min*ATA: now all you have to do is know your time and depth in ATA, multiply the two and you'll know how much in bar you've used up in that time interval. Much easier to do in your head (but much harder on your back because of hauling around the twin 11 L tanks :lotsalove:

).

My nominal cold water SAC rate in bar for diving here in Southern California happens to be 1bar/min*ATA --which is why I use twin 11 L tanks for solo diving-- so my gas planning and figuring consumption on-the-fly is so much more easier especially using the metric system. In warmer tropical waters when technical diving, my SAC rate is 25% to 30% better so I use a value of 0.7bar/min*ATA with twin 11L tanks (or just figure 7bar per 10minute interval multiplied by the depth in pressure ATA).

Air Consumption Question

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toefa

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Thalassamania

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ae3753

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toefa

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Kevrumbo

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Peter Guy

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toefa

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