Why 15 feet?

[jimmyboy]

Why is it that your safety stop is at 15 feet, where did that number come from? Why not 10 or 20 feet why is 15 the magic number??

 

[Kim]

Probably because it's shallow enough to allow for plenty of off-gassing - dumping your nitrogen - but still deep enough so that it's easy to maintain your neutral bouyancy. The closer you get to the surface - the more your wetsuit will expand and add to your bouyancy. This - coupled with the fact that your tank will be lighter being empty - can make it harder to actually hold the stop.

 

[theskull]

Simply because 15 is easier to maintain than 10. Ten feet plus a little current pulling the ascent line plus an inhale can easily become 5, and then you pop to the surface.

20 is pretty good, too. Most dive computers start the safety stop countdown at 20 or 19 feet; they will stop counting any deeper than that, and will give you a warning any shallower than 10.

theskull

 

[String]

My decompression tables assume a stop depth of 6m (19.6ft) and the same for the safety stop but to kee my computer happy i tend to do it at 5.5m (18ft). Any shallower than 5m and the stop is very hard to maintain given normal surface conditions and currents.

As for why this particular range of depth is chose in the first place, its to do with pressure and pressure gradients designed to allow off gassing at a safe but effective rate without being too fast or too slow.

 

[MikeFerrara]

We use 10 or 20 for our last (safety) stop...or even 15.

 

[jlyle]

Why is it that your safety stop is at 15 feet, where did that number come from? Why not 10 or 20 feet why is 15 the magic number??

At 15 feet the water pressure is exactly twice what it is at the surface, allowing for reasonable off-gassing while keeping the dissolved nitrogen in solution to avoid bubbles; less than 15 feet and bubble formation is more likely; more than 15 feet and off-gassing is too slow. At fifteen feet, you have a nice compromise between minimizing bubble formation and maximizing off-gassing.

There is some evidence that in addition to the 15 foot safety stop, doing deeper stops for a minute or two on the way up is an even better way to avoid DCS.

 

[Kim]

At 15 feet the water pressure is exactly twice what it is at the surface, allowing for reasonable off-gassing while keeping the dissolved nitrogen in solution to avoid bubbles; less than 15 feet and bubble formation is more likely; more than 15 feet and off-gassing is too slow. At fifteen feet, you have a nice compromise between minimizing bubble formation and maximizing off-gassing.

There is some evidence that in addition to the 15 foot safety stop, doing deeper stops for a minute or two on the way up is an even better way to avoid DCS.

Now you've got me confused! I thought that the water pressure was twice the surface pressure at 10 meters/33ft? Have I misunderstood a basic principle? :11:

 

[Sideband]

At 15 feet the water pressure is exactly twice what it is at the surface,

No it isn't. It is ~1.5 times the surface pressure. The rest may be right but 33fsw or 34ffw = 1atm

Joe

 

[MikeFerrara]

Now you've got me confused! I thought that the water pressure was twice the surface pressure at 10 meters/33ft? Have I misunderstood a basic principle? :11:

No. You're correct Kim. 1 ATM at sea level is a pressure uqual to about 33 ft or sea water or about 34 ft of fresh water.

So...in general 33 ft is 2 ATAS or twice the pressure at the surface which is 1 ATA.

 

[Kim]

No. You're correct Kim. 1 ATM at sea level is a pressure uqual to about 33 ft or sea water or about 34 ft of fresh water.

So...in general 33 ft is 2 ATAS or twice the pressure at the surface which is 1 ATA.

Thank goodness for that! For a moment there I thought that someone had changed the laws of physics and I'd have to do deco planning for any dive under 65ft!!