Tank sizes, DCI, and how they relate...

[CelticRavenVA]

Sort of a broad topic, but my question is pretty simple. I am a fairly new diver and not the best at managing gas consumption yet. While I am working on that and seeing noticeable imporvements every dive, I am looking to get a cylinder for some of the local diving as a matter of convience.

The question, AL80? AL100? Does the size of the cylinder or amount of air you actually breath have any bearing on forms of DCI? Or is it only how long you breath it and it cooks out at the same rate no mater how many breaths you actually take?

I don't burn through an AL80 super fast, I have no problems staying down for 45+ min. I usually am ending up on the boat while the more experienced divers (people I tend to dive with) are still at 1300psi, while I am at 500. Usually alot of this gas is burned right at the surface while getting back on the boat or at my safety stop.

Anyway, all gas consumption questions aside can someone answer the above for me and clear that up?

 

[DandyDon]

Hey C-R!

I'm a bit of a air-hog, but many newbies got thru it a lot faster than I. This gets better with practice and training.

When I'm diving with a newbie, I like to try to get them a 100 while I'm diving 80; many rental shops have some. Always ask. Anyway, not only can the newbie last longer, so we can both stay down longer, but feeling more comfortable about the air problem - he starts to relax, slow down, and burn less.

As far as the DCI qustion, while you're waiting on a more professional answer: It doesn't directly matter whether you're on a 60 foot tank or a 120. Nitrogen loading comes time at depth. If you do a sqaure profile dive to 60 feet for 60 minutes, you're beyond Padi suggestions, whether you breahted 45 feet or 100. The only way a 120 could add to this is if you ignored your dive plan and computer and stayed down longer 'cause you had the air.

Hope that helps some...

 

[CelticRavenVA]

Thats kinda what I thought and I follow what my computer is telling very closely. I just had to make sure....

 

[Dr Deco]

There is not any relationship between tank size and DCS since dissolved nitrogen uptake is governed by blood flow (perfusion of the tissues). Tank size will relate to how long one can stay submerged, naturally.

Dr Deco

 

[Wreckie]

There is not any relationship between tank size and DCS since dissolved nitrogen uptake is governed by blood flow (perfusion of the tissues). Tank size will relate to how long one can stay submerged, naturally.

Dr Deco

That being said air intergrated computers will penalise you for breathing fast. This is not due to the amount you breath but the computer assumes you are working hard, therefore blood flow increases and you ongas quicker..... course you could just be new and breathing a little quick but the lump of silicon dont know that!

On an historical side note early divers got bent less, why? they had smaller, lower pressure cylinders... they couldnt stay down long enough to run up large deco obligations.

Mike

 

[Scubaroo]

Thats kinda what I thought and I follow what my computer is telling very closely. I just had to make sure....

When you say you follow you computer very closely, do you mean that you're "riding" it by only rising when it tells you to do so, ie when it counts down to 0 minutes remaining at that depth? Or that you just keep an eye on it and make sure you don't violate NDLs? The computer is just providing a best guess algorithm developed from observations - decompression theory is still a fuzzy science, and you can get bent even if your computer tells you that you're okay, especially over multiple dives, if you push the boundaries all of the time.

 

[DBailey]

Thats kinda what I thought and I follow what my computer is telling very closely. I just had to make sure....

Don't let your dive computer do your dive planning for you. How does the saying go?

Plan the dive.
Dive the plan.

So you should be able to plan on the surface how long you can safely stay in the water at depth. Don't jump in the water and say "I'll stay down as long as my computer will let me".

 

[CelticRavenVA]

I stay at least 5 min from NDL at minimum. I perfer more then that, since I am a high risk person. (Or at least what I consider high risk) I am in good shape and work out, however I smoke and drink. (Not before dives) If I am diving I don't smoke that day before the dive, and I wait at least 2 hours once we finish our last dive to have a smoke or a drink.

Wow, did that make me sound like a lush or what??? I'm not really as bad as that sounds...

Going off to ponder my life....

When you say you follow you computer very closely, do you mean that you're "riding" it by only rising when it tells you to do so, ie when it counts down to 0 minutes remaining at that depth? Or that you just keep an eye on it and make sure you don't violate NDLs? The computer is just providing a best guess algorithm developed from observations - decompression theory is still a fuzzy science, and you can get bent even if your computer tells you that you're okay, especially over multiple dives, if you push the boundaries all of the time.

 

[CelticRavenVA]

As for dive planning, I actually double check everything. I check what my computer belives, then I check it against dive tables, then I double check our written plan and verify it totally with dive tables. Then I before entry I recheck the computer to make sure I have plenty of time. The computer is a sunto cobra, air integrated, which I understand is on the conservitive side to some degree's.

I am pretty paranoid about getting bent and am as careful as I can be about the risks involved.

 

[DepartureDiver]

As everyone has said, DCI is related to time and not breathing rate. For some other comments, my favorite size tank to buy is a steel 104 or 112 (about the same size as an Al 80 ... just fatter). With steel you don't need as much on your belt and it is LIGHTER out of the water than al tanks (it is stronger and needs less metal to hold the gas). Also, depending on your depth, you should be at your safety stops with more than 500 psi (surfacing psi doesn't matter). Think about it. If doing a deep dive to 100'+ and then your buddy needs to share, you need enough to get both you and your buddy through a proper slow ascent plus through the safety stops ... another reason that larger tanks are nice. Lastly, think of the time to stay within the no-dec limits as a percentage. In other words, 5 minutes under at 120' is nice, but not much at 60'. Enjoy your new sport!