Hold that line boys! How precise should you hold depth?

[boulderjohn]

Which of those figures is the most relevant to getting bent I dont know.

My understanding is that no one knows.

The stuff I have read tells me that different people argue for size, total volume, and number of bubbles as being the most critical factor, with no clear and decisive case being made for any one of them.

 

[Doc Harry]

PO2 plays a definite role, but also the gradient you are trying to achieve and total exposure (effectively RT). Doing deeper stops than required has more negative outcome than doing the stops shallower. Doing your 20ft stop at 25ft is worse than doing it at 15ft.

Especially with longer hang times, achieving the correct depth (but at a minimum shallower if you cannot) is crucial.

Doesn't that depend on what deco theory to which you subscribe? Gradient versus bubble?

 

[Meng_Tze]

Doesn't that depend on what deco theory to which you subscribe? Gradient versus bubble?

Gradient was probably was not the right word. I used it synonymously with change in ambient pressure, not per se as a true gradient as in the GF overlay.

As for the shallow vs. deeper stop, I personally would rather take the risk of increased bubbles/gradient ahead of schedule within the suggested 5ft than the increased risk of toxing. With the (safety) margins, either selected or defaulted by the algorithm, to me there is less risk with increasing the ambient pressure differential. My typical O2 time these days is around 45-60+ minutes, hanging at a PO2 of 1.45 is less taxing than at 1.75.

 

[Big John]

Not a tech diver, but have future aspirations. With ocean conditions being mentioned what effect would the waves themselves have on depth. Given a wave height of 4ft wouldn't that give you a pressure variation of +/-2ft from crest to trough?

 

[Doc Harry]

Gradient was probably was not the right word. I used it synonymously with change in ambient pressure, not per se as a true gradient as in the GF overlay.

I mean, wouldn't the gradient deco diver want to ascent quicker to shallow depths to maximize the gradient? And wouldn't the bubble deco diver want to make deeper stops and slower ascent?

 

[boulderjohn]

I mean, wouldn't the gradient deco diver want to ascent quicker to shallow depths to maximize the gradient? And wouldn't the bubble deco diver want to make deeper stops and slower ascent?

As complex as that sounds, it is not as simple as it sounds. (OK, shoot me. I'm dong the best I can with limited brain power)

We tend to think of bubbles as if they are all one size at all times. They are not. They are different sizes, and because the surface tension of each size is different, the internal pressure at equilibrium of each is different. At any given depth, the pressure of the tissues will be greater than the internal pressure of some bubbles and lesser than the internal pressure of others. That means that at any given depth, some bubbles will grow and some bubbles will shrink.

If we stay at a depth where all (or almost all) bubbles are shrinking, then we are still on gassing rapidly and might as well consider it to be an extended bottom time. It does not help us in the long run.

The trick (and the basis of the different theories) is to do stops at depths that maximize the number of bubbles shrinking while minimizing the amount of on-gassing of dissolved gas. This goes back to what I said earlier--people aren't sure if it is the number of bubbles of a particular size, the overall bubble volume, or the volume of bobbles in general that is most dangerous. Thus we have different theories on how to ascend to achieve that balance.

At least, that's how I understand it. I welcome corrections.

 

[Meng_Tze]

I mean, wouldn't the gradient deco diver want to ascent quicker to shallow depths to maximize the gradient? And wouldn't the bubble deco diver want to make deeper stops and slower ascent?

In abstraction; yes that is the intent and difference of both approaches.