13 replies to this topic

[WreckWench]

IS IT TRUE... that the first 15 ft is the most dangerous part of the entire water column and more people get bent going up in the last 10 ft to the boat/surface?

[Bubble2Bubble]

That would be True WW

Alot of old and new divers seem to forget there training in this very important area of diving.

Bubski

[Hipshot]

It's true. That's where the greatest relative pressure change takes place. For example:

* A diver at 99 ft (salt water) ascends 11 feet. The absolute pressure on the diver goes from 4 atm (absolute) to 3-2/3 atm. This is a relative pressure change of approximately 8%.

* A diver goes from 11 feet to the surface. The absolute pressure on the diver goes from 1-13 atm to 1 atm. This is a relative pressure change of 25%.

The relative pressure change is what determines the degree of volume change in a gas, per Boyle's Law. In other words, a diver holding his/her breath while ascending from 99 to 88 feet will experience a gas expansion of approximately 8%, whereas the same diver holding his/her breath from 11 feet will experience a gas expansion of approximately 25%. The risk of embolism is that much greater in the 11 feet to the surface interval.

Since gas solubility in a liquid is also proportional to ambient pressure, that last 10-15 feet also plays a critical role in DCS. Notice that the shallowest decompression stop is at 10 feet.

This also explains why a diver generally has to vent the BC more frequently approaching the surface and has to equalize pressure in the ears less frequently when descending past the first 30 feet or so.

Rick

[Landlocked Dive Nut]

Rick, that explanation is succinct, to the point, and in diver-layman terms. Good job!

[ScubaTex]

It's true. That's where the greatest relative pressure change takes place. For example:

* A diver at 99 ft (salt water) ascends 11 feet. The absolute pressure on the diver goes from 4 atm (absolute) to 3-2/3 atm. This is a relative pressure change of approximately 8%.

* A diver goes from 11 feet to the surface. The absolute pressure on the diver goes from 1-13 atm to 1 atm. This is a relative pressure change of 25%.

The relative pressure change is what determines the degree of volume change in a gas, per Boyle's Law. In other words, a diver holding his/her breath while ascending from 99 to 88 feet will experience a gas expansion of approximately 8%, whereas the same diver holding his/her breath from 11 feet will experience a gas expansion of approximately 25%. The risk of embolism is that much greater in the 11 feet to the surface interval.

Since gas solubility in a liquid is also proportional to ambient pressure, that last 10-15 feet also plays a critical role in DCS. Notice that the shallowest decompression stop is at 10 feet.

This also explains why a diver generally has to vent the BC more frequently approaching the surface and has to equalize pressure in the ears less frequently when descending past the first 30 feet or so.

Rick

Thanks for the excellent explaination of the importance of making sure ascents should be even slower the last 10-15 feet. But you lost me on the last statement, personally, I always have to equalize MORE frequently, when descending the first 30 feet.

Bill

[scubaski]

IS IT TRUE... that the first 15 ft is the most dangerous part of the entire water column and more people get bent going up in the last 10 ft to the boat/surface?

Consider the dangerorness ie: injury rate with in both 15 feet of the dive ladder both verticle axcess and horizontal axcess . I'm sure there's more slip/fall accidents/broken bones, divers giant strides onto non cleared divers, diver fallbacks onto in water divers while climbing ladder. Has to be many more accidents/injurys this way vs overexpansion ascents.

[pmarie]

It's true. That's where the greatest relative pressure change takes place. For example:

This also explains why a diver generally has to vent the BC more frequently approaching the surface and has to equalize pressure in the ears less frequently when descending past the first 30 feet or so.

Rick

Thanks for the excellent explaination of the importance of making sure ascents should be even slower the last 10-15 feet. But you lost me on the last statement, personally, I always have to equalize MORE frequently, when descending the first 30 feet.

Bill

I read the statement from Rick as being that as you go past the 30' area that you equalize less frequently; and more within the first 30'. Great explanation all the way around Rick, Thanks.

I have seen some divers shoot up after that 15-20' safety stop while I am still taking it slow....

Edited by pmarie, 28 August 2009 - 11:50 AM.

[Landlocked Dive Nut]

I have seen some divers shoot up after that 15-20' safety stop while I am still taking it slow....

You don't even have to kick that last bit to the surface after the safety stop.....just relax, and you slowly (and safely) rise in the water column.

Assuming you have proper weighting and good buoyancy control.....

[WreckWench]

I have seen some divers shoot up after that 15-20' safety stop while I am still taking it slow....

You don't even have to kick that last bit to the surface after the safety stop.....just relax, and you slowly (and safely) rise in the water column.

Assuming you have proper weighting and good buoyancy control.....

You would not believe how many people are very conscientious about their assent TO the safety stop. However once they finish their safety stop their computers SCREAM to the surface following them and their bubbles.

As Tammy said...there is NO reason to kick up...you should be working HARD on not rising too fast.
Many computers have assent meters on them which can help you ensure that you are NOT going up to fast in that last 10-15 feet.

[Bubble2Bubble]

Whats is really interesting, is that divers spend so much of their money and time on preparing to go on a diving trip... not including preparing their home/work life etc... to go diving.. just to get in a hurry to get out of the water/sea/ocean. When your in 15 ft or less of water your SAC rate sky rockets... so dont worry about only having 300 psi in your tank when coming from a 100 ft dive (for example)


Bubski

[Houston Squid]

However once they finish their safety stop their computers SCREAM to the surface following them and their bubbles.

Wow ... that would be me. I never knew about this. In the past there have been many times when I was done with a 5 - 20 minute Safety Stop (I tend to hang around down there wait for the other divers if my buddy runs low on air) and then just bolt to the surface.

My thinking was always, "Hey I just did a 20 minute safety stop, I am super safe now."

Thanks for this great topic.

C

[Jerrymxz]

Consider the dangerorness ie: injury rate with in both 15 feet of the dive ladder both verticle axcess and horizontal axcess . I'm sure there's more slip/fall accidents/broken bones, divers giant strides onto non cleared divers , diver fallbacks onto in water divers while climbing ladder. Has to be many more accidents/injurys this way vs overexpansion ascents.

Hey Stan you hit it close to home on these and I will certainly second this motion.

On 4th of July this year a guy did a Giant stride onto my head. Needless to say end of dive day and 12 stainless steel staples in my head later the last 15 feet get my vote as the greatest potential for injury. I certainly have my share of beaten up shins from ladders and on one occasion tossed off the ladder in high seas. That one turned into the closest call I've ever had to drowning. Real fear of God stuff.

lessons learned: 1 Keep regulator in my mouth until standing on the boat. 2 Dive isn't over until I'm sitting at my station with my rig bungied in.

[VADiver]

My buddies and I take 3-6 minutes from 20' (usually closer to 6 minutes @ 20'/3 minutes from 10'). that solves the ascent issue and gives you a nice gradual ascent to the surface.

[WreckWench]

However once they finish their safety stop their computers SCREAM to the surface following them and their bubbles.

Wow ... that would be me. I never knew about this. In the past there have been many times when I was done with a 5 - 20 minute Safety Stop (I tend to hang around down there wait for the other divers if my buddy runs low on air) and then just bolt to the surface.

My thinking was always, "Hey I just did a 20 minute safety stop, I am super safe now."

Thanks for this great topic.

C

Squidly...if just one diver learns from this I am THRILLED! If its a friend like you.... PRICELESS!!!


I am sooooooooooooooooooooooooooooooo glad that this has been helpful! I'm hoping others will also realize the danger of this all too common practice.