Why 15 ft for a safety stop?

[nereas]

Not to split hairs, but does modern deco theory suggest that bubbles will grow even when a diver maintains depth or does it suggest that bubbles will form even when a diver maintains depth? And yes, I understand that the balloon comparison is an over-simplification, but once again this is being made overly-complicated and the base theory that repeated expansion and contraction causes more stress than keeping the size constant.

I will feel better if I use a 15-foot stop. In the Middle Ages people suffering from headaches felt better having holes drilled in their heads because it let the evil spirits out. But I will not just feel better but, more importantly, will know better if, once again, I understand what to do in case I can't make a stop at 15 ft.

The retort "because I said so" or "because that is what the experts agree upon" does not instill any faith in me, especially when it seems that the experts agreed upon a generalization that has a one-size-fits-all appeal.

Ultimately, you will need to find out for yourself what works for you. But before you do that, make sure you get a lot of experience first doing things conservatively.

I normally begin everything in life by reading everything I can get my hands onto, and speaking with everyone who knows anything. Then I decide for myself. You will need to do the same, since you are also a questioning type of person.

If you win at this game, you will live and will never end up in a recompression chambre. If you lose, then you either get the chambre ride, or you die. The game is for keeps. So be careful.

 

[rookers]

You do it because it is at or near an asymptote in a binary exponential function

 

[Steve SDI/TDI HQ]

What I believe he's trying to suggest is that stress (sigma=E epsilon) caused by the expansion (epsilon) in the balloon (Elastic Modulus = E) is analogous to "stress" (DCS) in tissues caused by the expansion of dissolved gases.

Superficially, that's an okay analogy (expansion of gases causes stress in the balloon and expansion of dissolved gases causes DSC in the body).

But it in no way helps one to understand DCS, nor decompression mechanics. It's more appropriate in discussing AGE.

Never heard of Young's Modulus being applied to a balloon but if it works for you, so be it... think you must be clairvoyant to have arrived at that conclusion! :blinking:

An appropriate analogy using a balloon might be to illustrate Graham's Law with regard to the variable permeability of surface tension in a gas bubble in the blood... the larger it gets, the easier it is for gas to get into it...

 

[Blackwood]

Never heard of Young's Modulus being applied to a balloon but if it works for you, so be it... think you must be clairvoyant to have arrived at that conclusion! :blinking:

His question, as I interpret it, was about the stress in the walls of a balloon. In other words, I believe he was saying that, as a balloon inflates, it becomes more and more stressed.

An appropriate analogy using a balloon might be to illustrate Graham's Law with regard to the variable permeability of surface tension in a gas bubble in the blood... the larger it gets, the easier it is for gas to get into it...

Correct. Graham's Law, surface tension, etc. help us to understand bubble growth.

 

[shadragon]

Thanks for the clarification Doppler...!

 

[Doppler]

Thanks for the clarification Doppler...!

No worries... stay safe and dive often

 

[JT]

Haldane theory and Buhlman tables have been formulated from the observation that the 2 ATA point is a critical threshhold related to the development of DCS (decompression sickness/illness/injury). At less than 2 ATAs pressure, DCS does not occur. At pressures greater than 2 ATAs DCS begins to develop given sufficient time (called NDL time).

This is not true. DCS can even happen at depths as shallow as 25 feet. I know of one case: jo-jo diving, playing around at 25 feet, one diver got bent and was later succesfully treated in a reco chamber.

The 2:1 factor from Haldane and friends (for not getting bent) has been lowered to about 1.4 : 1. So 4 meters or 13 feet _should_ still be safe...

And yes, 90 minutes or so at 33 feet/10 meters DOES get you loaded with nitrogen. I sure would make a safety stop and a slow ascent on a dive like that.

 

[MikeFerrara]

The 2:1 factor from Haldane and friends (for not getting bent) has been lowered to about 1.4 : 1. So 4 meters or 13 feet _should_ still be safe...

It was lowered to 1.58 by Workman because FN2 is only about 79%...2 X 0.79 = 1.58

Additionally the 'M' value is depth dependant.

 

[GrandpaScuba]

I was just wondering - it seems a little arbitrary. Does that mean that a safety stop at 20 feet is less safe? Or one that begins at 20 ft and ends at 10 ft 3 minutes later won't allow proper off-gasing? Or, what if there is a very convenient place to stop at 25 or 10 feet, but not at 15? Should I try to hover at 15 anyway?

Actually, it's not arbitrary. First of all, the recommendation is a safety stop between 15 and 20 feet. If you look at the Navy tables, you will see that dives above 20 feet have unlimited no-decompression time.

A safety stop needs to be made at a depth where the ambient pressure is less than the pressure of the gas in your system, so off-gassing will take place. At 20 feet, the ambient pressure is likely to fit this criteria. But if the pressure in your system happens to be less or equal to the ambient at 20 feet, it doesn't matter since there is no NDL for 20 feet and above.

If you have an Oceanic or Aeris computer, you might notice that the automatic 3 minute timer starts as soon as you get to 20 feet. You should stay below 15 feet for 2 reasons. 1) you need to maintain a token amount of pressure, close to the 20 foot level, to prevent too rapid of off-gassing. 2) You need to stay far enough below the wave action so water movement doesn't pull you to the surface.

 

[Ice Mike]

never mind