Question What's your safety stop depth?

[Hoag]

Considering that this is the Basic Scuba Forum, I am curious why deco dives are even being discussed.

For the moderators, perhaps it is time for this thread to be split into two threads, one based on what the OP asked, and the other discussing a similar topic but with deco considerations.

 

[dmaziuk]

... at 30 msw ...
If we are breathing air, the nitrogen has a partial pressure of 4 * 0.79 = 3.76. Since this is more than the nitrogen pressure in our body, nitrogen enters our tissues and increases the nitrogen pressure there.

That is the advantage of breathing nitrox. If we breathe 32% nitrox on such a dive, the nitrogen partial pressure is 4 * 67 = 2.68, so we take on less nitrogen.

Nothing's ever simple, however: the rate of gas exchange is driven by the pressure difference. That is, on the air dive we're on-gassing at the rate of 3.76 to 1 -- assuming we started "clean" and out tissue pressure is 1 atm. On the nitrox dive the rate of on-gassing is 2.68:1 i.e. quite a bit slower -- but you will on-gas to the same tissue pressure if you stay down there longer, as your computer would let you, if you configured your %O2 correctly.

I.e. it's important to remember that you have to "pin" one of the variables, e.g. exposure time, for this to work.

 

[happy-diver]

Because smart experienced people come in here too and if they're not they want to get that way

Considering that this is the Basic Scuba Forum, I am curious why deco dives are even being discussed.

For the moderators, perhaps it is time for this thread to be split into two threads, one based on what the OP asked, and the other discussing a similar topic but with deco considerations.

 

[clownfishsydney]

In Australia it is practice to do 5 minutes at 5 metres. It has been this way for at least 25 years. Before that, it was 3 minutes at 3 metres. I think DAN Australia (John Lippmann) pushed for the change. Not hard to do an extra two minutes.

 

[clownfishsydney]

You do a safety stop on top of deco stops?

When I do a deep dive (40+ metres) and well into deco, yes. Makes it far safer.

 

[Dan]

You have a variety of tissues in your body, and they absorb and release gas at different rates. The fastest tissues had the most pressure when you began the ascent

some slower tissues are still on-gassing, even during the safety stop. That on-gassing will stop when they are at equilibrium

What are examples of the fastest & slowest tissues?

What are the reasons for someone getting skin bends?

 

[scubaozy]

When I do a deep dive (40+ metres) and well into deco, yes. Makes it far safer.

When I go well into deco, I normally have the last stop at 3m. Do you go back to 5m for additional safety stop or do you extend your deco stop further? Not long ago someone I dived with went into deco, despite same profile, his dive computer was way more conservative, so, while I was doing my optional safety stop at 5 he went to 3 to clear his computer and joined back to me. I thought that was unusual, I would have either stayed at 5 until my computer showsno deco obligation + safety stop or straight to 3m and clear and surface.

 

[rongoodman]

What are examples of the fastest & slowest tissues?

What are the reasons for someone getting skin bends?

Faster tissues would be blood and nerves, slower ones would be bone and ligaments. The half times for N2 in the 16 compartments in the Buhmann ZHL model range from 4 to 635 minutes. It would require 6 half times for a compartment to reach equilibrium at a given pressure, so while the fastest compartments would completely clear during a surface interval it would take several days for the slowest to do so.

 

[Dan]

635 minutes are very long time. Recreational divers wouldn't need to worry about those tissues then.

 

[rongoodman]

635 minutes are very long time. Recreational divers wouldn't need to worry about those tissues then.

Saturation divers would, though.