Optimizing bottom time on repetitive deep dive.

[CuNanoTube]

My father and I are diving with our club on the may 24 weekend, and the dives planned (in Barcelona new york) are deep. The first day is a 120' followed by a 90', and the second day is two 105' dives. We both dive with computers, but were wondering if we could add an extra stop on our ascent to help increase bottom time one the next dive. I am not talking about a mandatory decompression stop, that is well beyond my current skill-set, but a stop to help us off-gas if that is possible. We will both be nirtox trained by the dives (not that the math is any hard) however we will not be able to clean our tanks in time for the trip. I do however have an Al40 / an Al 19 at my disposal that could be filled with nitrox.

Now, if this is a bad idea (again, not talking about mandatory decompression) tell me straight up. If there will be no benefit do the same, but if there is a way to make my next dive after the 120' more than 9 minutes (without an insane SIT, based on NAUI table for example purposes) please tell me.

Oh, the trip:
http://www.mississaugascubaclub.com/Events_files/052111-BarcelonaNY-MSC.pdf

Off topic, because someone will mention this: No, I do not have my AOW. I will however be doing this dive with my AOW instructor as he is another member of the club.

 

[elan]

What is at that depth ? If you can stay shallower , say do not go to the very bottom if it's a wreck then you can stay longer. Nitrox can increase the bottom time. If you can find a premix or fill it ata shop with a membrane syste you do not need to have it cleaned.

 

[CuNanoTube]

I thought of that, the vis according to the flyer varies between 20-50', so to see the whole ship I will have to get pretty close to the deck. But it is true, if I stayed at the top of the ship at 90' instead of 100' I would make things easier.

My local shop does have a membrane system, but he (to make an extra buck) only fills clean tanks. The operator on this trip uses pp O2 mixing to fill the tank.

 

[AfterDark]

My father and I are diving with our club on the may 24 weekend, and the dives planned (in Barcelona new york) are deep. The first day is a 120' followed by a 90', and the second day is two 105' dives. We both dive with computers, but were wondering if we could add an extra stop on our ascent to help increase bottom time one the next dive. I am not talking about a mandatory decompression stop, that is well beyond my current skill-set, but a stop to help us off-gas if that is possible. We will both be nirtox trained by the dives (not that the math is any hard) however we will not be able to clean our tanks in time for the trip. I do however have an Al40 / an Al 19 at my disposal that could be filled with nitrox.

Now, if this is a bad idea (again, not talking about mandatory decompression) tell me straight up. If there will be no benefit do the same, but if there is a way to make my next dive after the 120' more than 9 minutes (without an insane SIT, based on NAUI table for example purposes) please tell me.

Oh, the trip:
http://www.mississaugascubaclub.com/Events_files/052111-BarcelonaNY-MSC.pdf

Off topic, because someone will mention this: No, I do not have my AOW. I will however be doing this dive with my AOW instructor as he is another member of the club.

It's hard to bend the rules without risking DCS however minor. This sounds like something you should be discussing with the instructor you mentioned. This is a very serious question you ask.

 

[CuNanoTube]

Oh I know. I just wanted to do some research before approaching him. Google failed me in this regard, which is why I am posting here.

 

[DevonDiver]

I'm not quite sure what you are suggesting.

(Super-simplified explanation below... am not trying to turn this into an in-depth deco theory discussion).

If you are diving with computers, they will track the dive you have done using an algorythm to model your nitrogen absorbtion and release. Doing a multi-level dive, is a good way to optimise your dive using a dive computer. However, multi-levelling still 'adds' to your nitrogen. Unless you are actually in decompression any improvised stops would only be treated as a multi-level dive profile. Simplistically, whilst 'releasing' nitrogen loaded on the deeper portion of the dive, you would still be 'absorbing' nitrogen at the shallower level. The overall effect is more nitrogen.

In short, adding stops may be a prudent measure, but this is because it creates a slow and controlled ascent rate from the dive (lower risk of immediate DCI). It won't however benefit any repetitive dives you might make. If you are under-water, you are absorbing nitrogen.

As you've already identified, effective use of nitrox and multi-level dive profiles (to reduce overall nitrogen absorbtion, rather than for off-gassing) will have the biggest benefits for repetitive dives.... coupled with intelligent planning of surface intervals (which is the single biggest factor on extending bottom time on repetitive dives).

 

[CuNanoTube]

I'm not quite sure what you are suggesting.

(Super-simplified explanation below... am not trying to turn this into an in-depth deco theory discussion).

If you are diving with computers, they will track the dive you have done using an algorythm to model your nitrogen absorbtion and release. Doing a multi-level dive, is a good way to optimise your dive using a dive computer. However, multi-levelling still 'adds' to your nitrogen. Unless you are actually in decompression any improvised stops would only be treated as a multi-level dive profile. Simplistically, whilst 'releasing' nitrogen loaded on the deeper portion of the dive, you would still be 'absorbing' nitrogen at the shallower level. The overall effect is more nitrogen.

In short, adding stops may be a prudent measure, but this is because it creates a slow and controlled ascent rate from the dive (lower risk of immediate DCI). It won't however benefit any repetitive dives you might make. If you are under-water, you are absorbing nitrogen.

As you've already identified, effective use of nitrox and multi-level dive profiles (to reduce overall nitrogen absorbtion, rather than for off-gassing) will have the biggest benefits for repetitive dives.... coupled with intelligent planning of surface intervals (which is the single biggest factor on extending bottom time on repetitive dives).

Thanks, that is what I thought actually. I was actually thinking about decompression theory, but realized (after posting) that to have any benefit I would need tech gas mixes. (I have an e-copy of Bove and Davis diving medicine)

 

[Dack]

Sounds like you know where the limits are, and want to push them...slightly.

For dives at 120 feet EAN32 is beyond of the 1.4 partial pressure safety zone (~106ft), so Nitrox is out of the question for the first dive - but OK for the other 3.

Since you brought it up... Are the dives part of the AOW class? Typically first AOW dive is just to 100 feet, to get your feet wet and focus on just the depth, and not combined with something like a wreck dive or maximum bottom time. Keep it simple, don't try to do too many things at once, work up to it.

It sounds like an awesome series of dives, but you might focus on simply doing a deep dive first, a few more to get comfortable, and then start looking at extending bottom time on some future dives.

Even if you were AOW cert you really can't extend your bottom time beyond your DC limts without wandering into the territory of mandatory decompression stops. You can, however, use a very gradual and slow ascent to keep you closer to the bottom and work up to extend overall dive time. But that takes practice.

- Dack

 

[DevonDiver]

It still wouldn't really benefit you for repetitive dives. Using oxygen rich gasses for decompression is all about reducing you from a point of super-saturation (deco state) and getting you to the surface without inducing DCI. Unless you were breathing pure O2, you'd still be absorbing some nitrogen while decompressing.

You could have some fun downloading trial/shareware deco planning software and try planning your dives on that using different gas mixes, just for experimentation sake.

Here's some links for deco software: Decompression Software

More links for decompression theory articles: Scuba Resources

 

[AfterDark]

How much nitrogen do we absorb above 33fsw? There are't any NDL above 33’ that I know of, which indicates to me that the amount we absorb must be very small.

Adding stops above 33’ may help. Breathing O2 in and out of water may also help.
The thing is without it having been done the way you would do it, there is no way to know the result. Reason and logic doesn't always sync with physics.

Anytime we dive outside known parameters we are taking a risk. What you ask is beyond the rec. diving parameters. I’ve done things outside established limits that I’ve gotten away with, whether it was luck or not I couldn’t say. I will decline to mention those things here because I’ll risk my a$$ for my reasons, but not someone else’s.