Bottom Time, beginner question

[rick00001967]

2 atmospheres (10 mtrs / 33 feet) is the limit you are looking for. "theoretically" you can stay indefinitely at that depth with no decompression obligation. as pointed out above, you still need to control ascent rate and breath continuously.
i would also add that if flying anytime after breathing compressed gas (diving in the pool for example) you may want to consider making sure you have at least 12 hrs out of the water. more is always better but do whatever you are comfortable doing.

 

[TMHeimer]

2 atmospheres (10 mtrs / 33 feet) is the limit you are looking for. "theoretically" you can stay indefinitely at that depth with no decompression obligation. as pointed out above, you still need to control ascent rate and breath continuously.
i would also add that if flying anytime after breathing compressed gas (diving in the pool for example) you may want to consider making sure you have at least 12 hrs out of the water. more is always better but do whatever you are comfortable doing.

I'm sure I did read somewhere it is either 18 or 20 feet. What is your source?
On the PADI RDP the NDL for 35' is of course 2 hours, 5 minutes. Does only 5 feet shallower mean you can stay down indefinitely?

 

[dmaziuk]

I'm sure I did read somewhere it is either 18 or 20 feet. What is your source?

John Scott Haldane and a whole lot of caisson workers. The model was later refined to account for gas composition, so on air that 2 atm / 10 msw was adjusted to 79% nitrogen and thus 7.9 msw or 24-ish feet.

 

[Jim Lapenta]

My tech instructor trainer told me that a 1/2 hour playing in the pool with gear and things like buoyancy and trim is like 2 hours in open water. If you can do skills in less than 6-8 feet doing them deeper is a piece of cake.

 

[Scuba Lawyer]

Similar question, but kind of the opposite end of the OP's question. While I did a s**t-ton of decompression diving in the 70s and 80s, I haven't kept up with the theory in decades. One thing we always discussed and wondered about was the issue of 100% saturation of nitrogen in the tissues on dives less than 2 atm. So here's my question.

SCENARIO: Let's say you do a series of dives throughout the day and on your last dive you encounter a decompression obligation. However, just as you ascend back above 33 feet (24ish, whatever) your computer algorithm says you are now cleared of a deco obligation.

QUESTION: Can you now still spend unlimited time in less than 33 feet of sea water, or are your saturated tissue compartments still loading nitrogen that would place you out of no-deco range? [I think I said that right, but again, its been years ]

I'm pretty sure I know the answer but am throwing it out there for a topic of discussion.

 

[dmaziuk]

Similar question, but kind of the opposite end of the OP's question.

It's a trick question: it's not about how much gas is in your tissues in terms of volume, it's about pressure difference between gas in your tissues and the surface.

 

[broncobowsher]

Yes, if you can keep in control in 6-8 feet of water, the deeper stuff is way easy.
But remember it isn't just about being neutral at 6-8 feet, it is getting neutral. Once you are comfortably neutral, pick a weight off the bottom and reset yourself. Than drop that weight and reset again. Swim up to the 3' end without breaking the surface or touching bottom. Go back to the deep end. Don't forget to play with your kicks. Set a GoPro in the pool and film yourself, check trim and stuff. There is a lot you can do in a pool. Don't have a GoPro? One of the knock off clones will do just fine.

 

[rick00001967]

I'm sure I did read somewhere it is either 18 or 20 feet. What is your source?
On the PADI RDP the NDL for 35' is of course 2 hours, 5 minutes. Does only 5 feet shallower mean you can stay down indefinitely?

John Scott Haldane and a whole lot of caisson workers. The model was later refined to account for gas composition, so on air that 2 atm / 10 msw was adjusted to 79% nitrogen and thus 7.9 msw or 24-ish feet.

haldane found the body could safely tolerate some supersaturation. meaning on ascent, the gas we are breathing can be lower than the pressure our tissues are loaded with. how much lower ? essentially half. so he stated we could handle a 2:1 ratio without a problem.
workman further refined that theory. because we are concerned only with nitrogen loading when calculating NDL's etc, he stated that since the partial pressure of nitrogen on the surface (at sea level) was 0.79 atm, we could tolerate a partial pressure of nitrogen of twice that, or 1.58 atm.
if breathing air, we have a partial pressure of nitrogen of 1.58 atm at 33 fsw or 10 mtrs.

 

[dmaziuk]

workman further refined that theory. because we are concerned only with nitrogen loading when calculating NDL's etc, he stated that since the partial pressure of nitrogen on the surface (at sea level) was 0.79 atm, we could tolerate a partial pressure of nitrogen of twice that, or 1.58 atm.
if breathing air, we have a partial pressure of nitrogen of 1.58 atm at 33 fsw or 10 mtrs.

Good one.

It's still Workman's fault, though: Haldane had 2:1 across the board and things were easy. Workman came up with different M-values for different tissue compartments so now you have to pick the least "tolerant" compartment in your model to work out the no-limit depth.

 

[TMHeimer]

I'm still a little confused, having heard numbers of 24' and 33'. Also that NAVY tables say 371 no deco minutes for 30 feet (according to that 33' doesn't fit the bill as "unlimited"). Seems some disagreement. Maybe where I read 18 or 22 feet was someone being very conservative?