Safe CESA depth 35' vs. free divers hitting 130'

[2airishuman]

Once again please bear with me as there is little that I understand.

Least of all do I understand how free divers can dive to 130' and return to the surface, yet a CESA is considered unsafe beyond 35'.

SSI now offers freediving classes with the lowest level of certification being to 66'. https://www.divessi.com/freediving_program

Perhaps it's a matter of training. Would we all be safer divers if we learned freediving?

 

[Doby45]

A free diver takes the breath of air at surface pressure, so as he/she descends the air in his/her lungs is compressed. So if you come back up from whatever depth as a free diver the air can only expand back to it's normal size. If you are a scuba diver and you took a breath at 35' and then surfaced, it would be bad. The air that you took into your lungs at 35' is already compressed and if you surface while holding your breath that air will expand to approximately twice the volume that it was at 35'.

 

[boulderjohn]

I have discussed this personally with PADI. Their wording for it has changed every few years since I have been an instructor.

First of all, recognize that there is a difference between 1) a trained free diver going to 130 while being thoroughly prepared for it, both through training and in the time leading up to the dive and 2) a scuba diver who has no real free diving training and just found out in mid breath that he or she is out of air.

Next, recognize that there is a significant difference between 1) holding a deep breath of uncompressed air taken at the surface and 2) exhaling continually a rapidly expanding volume of compressed air taken at depth.

Finally, recognize that free divers practice their craft regularly, whereas the CESA was likely not practiced since the OW certification dive on which it was performed.

Now, the truth is that a CESA well-performed can be done from just about any recreational depth, even without much practice. I believe I could do one in relative comfort from 100 feet or more. However, the deeper you are, the more likely it will be that you will have a problem, and the main problem is the panicked sense that you won't make it. If you panic, then you might hold your breath during that panicked ascent, and if you do hold your breath...well, that is the number one cause of scuba fatalities (other than heart attacks).

Consequently, PADI advocates that if you are in a CESA situation in which you aren't sure you can make it to the surface with a CESA, you should instead do a buoyant emergency ascent. That way you are sure to make it to the surface. The depth they mention is pretty arbitrary.

 

[TMHeimer]

Another possibility may be that though you keep an open airway you may be tempted (and have to) ascend faster than you should if you are quite deep.

 

[Lorenzoid]

Where does the number 35 (feet) come from? Is that a PADI thing? Why 35?

 

[2airishuman]

More of the expansion of air in the lungs occurs in the last 35 feet than in an ascent from an arbitrary low depth to 35'.

Gas laws.

---------- Post added December 18th, 2015 at 03:50 PM ----------

Where does the number 35 (feet) come from? Is that a PADI thing? Why 35?

It is the lower limit at which a safe CESA can be performed according to the current PADI training materials.

 

[NAM001]

I dont see the corrolation between the 2 sports. the clock is a major difference between them.

Once again please bear with me as there is little that I understand.

Least of all do I understand how free divers can dive to 130' and return to the surface, yet a CESA is considered unsafe beyond 35'.

SSI now offers freediving classes with the lowest level of certification being to 66'. https://www.divessi.com/freediving_program

Perhaps it's a matter of training. Would we all be safer divers if we learned freediving?

 

[Lorenzoid]

More of the expansion of air in the lungs occurs in the last 35 feet than in an ascent from an arbitrary low depth to 35'.

Gas laws.

But why 35 and not 33 or 34 or 36, etc.? There is no magic boundary in the gas laws at 35 feet. One atmosphere is about 33 feet (saltwater), so why didn't they pick 33 feet as the limit?

It is the lower limit at which a safe CESA can be performed according to the current PADI training materials.

Thanks.

 

[NAM001]

could 35 at higher altitudes be equal to 33 at sea level. I dont think that 35 is a padi thing because OW is taught as a class that prepares you to plan and safely execute a dive to 60 ft. CESA should be the last resort self saver action for an OW at their max depth which would be 60 not 35. That is not to say that 35 ft is not the deepest that it can be taught. I think there are other explanations for 35.

 

[TMHeimer]

Coincidently 35' is the shallowest on the RDP. Related?