A big tank could bend you????

[MechDiver]

notabob once bubbled...


Correct me if I'm wrong, but unless my reasoning is off, making progressive stops only makes sense once a diver is shallow enough that he/she starts off-gassing. Typically about 30ft, is it? As long as one is deeper than that, the nitrogen intake does not stop, though it does slow down at the shallower depths.

-Roman.

30' has nothing to do with it. On our last dive, the first deco stop was at 130 fsw, offgassing started at 136. The offgassing depth depends on what you are breathing and how deep you've been.

 

[Dryglove]

This is an example of a dive profile that is on the v-planner website .If you notice at the bottom of the page, offgassing starts at 201.9ft

http://www.v-planner.com/enlarge.html

 

[Charlie99]

notabob once bubbled...
Correct me if I'm wrong, but unless my reasoning is off, making progressive stops only makes sense once a diver is shallow enough that he/she starts off-gassing. Typically about 30ft, is it? As long as one is deeper than that, the nitrogen intake does not stop, though it does slow down at the shallower depths.
-Roman.

Lets look at a 80' dive on air to the PADI NDL limit, 30 minute.

The 5 minute compartment is almost fully saturated, and will be offgassing at any depth shallower than 77'.

OTHOH, the 60 minute halftime compartment will not offgass until you are shallower than 22'.

Slow ascent rates, and short stops at 40' and 30' give the faster compartments time to offgass a bit, while not unduly adding to the loading of the slower compartments.

Even better, when the dive site is suitable, is a multilevel dive, such as 80', 20 minutes, followed by 45', 15 minutes, then a couple minutes at 25-30', followed by a safety stop.

 

[notabob]

Live & learn they say. Guess I was oversimplifying things. Thanks for the info!

 

[jbd]

I think?!?!?

I agree that depth and time have the most to do with on gassing or tissue loading but I don't think thats all there is to it in the case described by the original poster.

Lets say Walter and I go on a dive together. Same gas mixture. Perfect square profile with both of us side by side throuhgout the dive. Lets say that I complete two respiratory cycles for each one that Walter completes throughout the dive. It would seem to me that I have exposed my body(particularly the lung tissues) to twice as many nitrogen molecules which need to go somewhere. I'm thinking that the laws of physics or chemistry that pertain to osmosis will dictate that I absorb more nitrogen--in theory maybe twice as much as Walter.

In reality I think that if I figure the dive based on the profile it would show me in say pressure group "G"; but in reality I would be(due the extra ongassing) in pressure group "I".

Your thoughts are welcomed

 

[jonnythan]

jbd once bubbled...
I think?!?!?

I agree that depth and time have the most to do with on gassing or tissue loading but I don't think thats all there is to it in the case described by the original poster.

Lets say Walter and I go on a dive together. Same gas mixture. Perfect square profile with both of us side by side throuhgout the dive. Lets say that I complete two respiratory cycles for each one that Walter completes throughout the dive. It would seem to me that I have exposed my body(particularly the lung tissues) to twice as many nitrogen molecules which need to go somewhere. I'm thinking that the laws of physics or chemistry that pertain to osmosis will dictate that I absorb more nitrogen--in theory maybe twice as much as Walter.

In reality I think that if I figure the dive based on the profile it would show me in say pressure group "G"; but in reality I would be(due the extra ongassing) in pressure group "I".

Your thoughts are welcomed

Those nitrogen molecules definitely go somewhere.... they go out in exhaled breath. The ventilation of the lungs is *not* the limiting factor in nitrogen uptake, as I recently read from a Doc on the board.

 

[Charlie99]

jbd once bubbled...
Lets say that I complete two respiratory cycles for each one that Walter completes throughout the dive. It would seem to me that I have exposed my body(particularly the lung tissues) to twice as many nitrogen molecules which need to go somewhere. I'm thinking that the laws of physics or chemistry that pertain to osmosis will dictate that I absorb more nitrogen--in theory maybe twice as much as Walter.

Your thoughts are welcomed

My 1st thought was, boy are you confused.

As various people have posted, the primary factor in nitrogen absorption is the partial pressure of N2 in the lungs. That is primarily determined by what mix you are breathing, and the depth. Once the blood is saturated with N2, it won't hold anymore, no matter how fast you pant.

Activity level changes the circulation to tissues, and therefore the halftimes, and activity is often reflected by changes in breathing rates. I.E., heavy breathing is sometimes an INDICATOR of higher than normal rate of loading. It is not a CAUSE.

 

[Zept]

jbd said...
It would seem to me that I have exposed my body(particularly the lung tissues) to twice as many nitrogen molecules which need to go somewhere. I'm thinking that the laws of physics or chemistry that pertain to osmosis will dictate that I absorb more nitrogen--in theory maybe twice as much as Walter.

Nope. The rate of absorption depends on the partial pressure of nitrogen in your lungs (the concentration, if you like). If you are both breathing the same mix at the same depth, you are exposing your lungs to the same partial pressure of nitrogen and will absorb it at roughly the same rate.

Zept

 

[Zept]

Boy, we all jumped on that one at the same time!

 

[jbd]

I understand that the lungs are not the limiting factor in N2 loading. What I am suggesting is that if you put 100 N2 molecules on one side of a cell membrane and there are zero molecules of N2 on the other side then to achieve equilibrium the mebrane will "let through" 50 N2 molecules so you end up with 50 on each side. Lets say that is the case with Walter on his dive. I on the other hand put 200 molecules on one side of the cell membrane. To achieve equilibrium the membrane will "let through" 100 molecules. Now I have 100 on each side; twice as much as Walter in this case. IMHO this would put me at a higher risk of DCS.

If I recall correctly(it has been a long time since I studied this stuff) the transfer from one side to the other is fastest when there is a larger difference between the two sides of the membrane. If I'm wrong about this, someone please make the necessary corrections.

Charlie99---dare I ask what your second or third thoughts were? Yes I agree that ppN2 is the primary factor in ongassing. The key word being primary. I also think that there is more to it than just ppN2. This is partially why, in my mind, that decompression theory is not clear cut. It has been noted by many people on scubaboard that the line between low risk of DCS and very high risk is not a line at all; rather it is a band of variable width determined by many things. I happen to think that overall air consumption in a given time is one of those things.