Failed CESA in OW

[wetb4igetinthewater]

Strangely enough my 'Other Half' is a Padi OW instructor & First Aid Instructor (in case I drown!) but I didnt want her to teach me...or the company that she works for. There's no one I trust more with my safety, so perhaps I'll hire gear & finish the dives with her after all...

It is wise to not have a partner/spouse be one's instructor. when I was a ski patroller and we'd observe a couple bickering as one was teaching the other, we called those DIPs.

Divorce In Progress.

 

[SlugLife]

It is wise to not have a partner/spouse be one's instructor. when I was a ski patroller and we'd observe a couple bickering as one was teaching the other, we called those DIPs.

Divorce In Progress.

While it's probably not ideal, in OP's case, it's probably better than dealing with the incompetent or overbooked instructors described. (I'd also have to imagine most weren't formal experienced ski-instructors.)

 

[dlofting]

Nope, I have never had to do one and do not see myself ever needing to do one. You may ask why would I never see myself doing one...then again you may not ask that but I will tell you anyway.

I typically dive sidemount so I am not reliant on a single cylinder of gas or a buddy that needs to stay close enough to rescue me. If I am not teaching and required to be in a single cylinder, I dive sm or ccr. When on CCR I carry a bailout with me, even on shallow dives. Redundancy is important to me as is the ability to save myself in an OOG emergency.

Makes sense.

Do you think that PADI should teach redundancy, maybe how to rig and carry a pony, or a brief introduction to doubles or sidemount and do away with CESA ?

 

[boulderjohn]

The vague answer doesn't surprise me. It's probably the best answer.

As I learned when discussing overhead environments with PADI, there is an element within its leadership that much prefers hard and fast rules to anything advocating the use of judgment.

As I also learned through the years, their literature is created by a team effort, and not all members of the team agree on or even know about everything that goes into it. I am quite sure that the person with whom I was discussing overhead environments had nothing to do with the current OW manual, and he seemed surprised when I pointed out the strong, absolute language in that manual.

That is reminiscent of the old hard and fast rule against reverse profiles--doing a deeper dive after a shallower dive. A workshop on this topic about 20 years ago asked where this rule came from and why it existed. No one knew. The earliest reference to it was a suggestion in a 1972 PADI OW manual, and the PADI representatives in the workshop had no idea who made that suggestion or why. Other agencies picked it up, and over the following decades, that suggestion morphed into a rigid rule.

In all likelihood, there was a very good reason for the suggestion. In those days everyone was using the US Navy tables, with those long, long surface intervals. If you wanted to do two dives, you could do them with a much shorter surface interval if you did the deeper one first. That is probably why that was suggested, but no one can say for sure, and many people today still treat it as an hard and fast rule with important (but unknown) safety implications.

 

[Angelo Farina]

As I learned when discussing overhead environments with PADI, there is an element within its leadership that much prefers hard and fast rules to anything advocating the use of judgment.

As I also learned through the years, their literature is created by a team effort, and not all members of the team agree on or even know about everything that goes into it. I am quite sure that the person with whom I was discussing overhead environments had nothing to do with the current OW manual, and he seemed surprised when I pointed out the strong, absolute language in that manual.

That is reminiscent of the old hard and fast rule against reverse profiles--doing a deeper dive after a shallower dive. A workshop on this topic about 20 years ago asked where this rule came from and why it existed. No one knew. The earliest reference to it was a suggestion in a 1972 PADI OW manual, and the PADI representatives in the workshop had no idea who made that suggestion or why. Other agencies picked it up, and over the following decades, that suggestion morphed into a rigid rule.

In all likelihood, there was a very good reason for the suggestion. In those days everyone was using the US Navy tables, with those long, long surface intervals. If you wanted to do two dives, you could do them with a much shorter surface interval if you did the deeper one first. That is probably why that was suggested, but no one can say for sure, and many people today still treat it as an hard and fast rule with important (but unknown) safety implications.

I did start diving with the US Navy tables, and at those times every dive did require deco stops.
You nailed the problem: if the first dive is long and shallow, you saturate the slow tissues, which take a lot of time to release gas.
If instead the first dive is very deep and short, you only saturate the fast tissues, which degas quickly.
Please consider that this is true with EVERY tables you use or using a DC. It is physics and physiology. So the recommendation to plan for a first deep, short dive followed by a longer, shallow dive is for minimising the time required between the two.
When PADI developed their own dive tables, they did it by removing the slower tissues from calculations. Albeit this revealed in practice to be "safe enough", I still prefer to employ the US NAVY tables.
What PADI did was to factor in their calculations the fact that the dive profile is never square: usually the diver surfaces slowly, with a trapezoidal profile. And this allows to reduce the nitrogen load at the end of the dive and hence the surface interval before diving again.
This did mean that the recommended dive profile was going first at maximum depth, and then ascending progressively. Because this did adhere to the computational model employed for their new dive tables.
A modern DC computer calculates correctly the nitrogen load in each tissue also if the dive profile was reversed (shallow at the beginning and deeper at the end): so, with a modern DC the profile shape recommendation has not any value now. But the recommendation of planning a first deep and short dive, followed by a longer, shallow dive, still holds unchanged, as in the seventies.

 

[tursiops]

What PADI did was to factor in their calculations the fact that the dive profile is never square: usually the diver surfaces slowly, with a trapezoidal profile.

No, not according to their published report on the work, The RDP is for square profiles.

Dropbox

www.dropbox.com

 

[Angelo Farina]

No, not according to their published report on the work, The RDP is for square profiles.

Dropbox

www.dropbox.com

Thanks, a very interesting document, which I did not own. I will read it carefully later this evening.
At the time, when the new PADI tables and the "wheel" were introduced, some PADI instructors (who were previously my CMAS students) explained me that the new tables did take into account "multilevel dives", which means "not square" to me.
And in the document you posted, at pag. 22, in the part which describes how the new tables were computed, I read the following:

So my understanding has always been that the new PADI tables take into account these not-direct ascent profiles.
Said that, I am NOT a PADI instructor. I NEVER used their dive tables or the "wheel", and always employed for myself and my students - customers the good old US Navy tables.
This means that I admit that I do not know well the PADI literature, so I could have been misinformed, both by my former students, and by reading documents such the one you posted.

 

[tursiops]

Thanks, a very interesting document, which I did not own. I will read it carefully later this evening.
At the time, when the new PADI tables and the "wheel" were introdoced, some PADI instrutors (who were previously my CMAS students) explained me that the new tables did take into account "mutilevel dives", which means "not square" to me.
And in the document you posted, at pag 22, in the part which describes how the new tables were computed, I read the following:

So my understanding has always been that the new PADI tables take into account these not-direct ascent profiles.
Said that, I am NOT a PADI instructor. I NEVER used their dive tables or the "wheel", and always employed for myself and my students - customers the good old US Navy tables.
This means that I admit that I do not know well the PADI literature, so I could have been misinformed, both by my former students, and by reading documents such the one you posted.

The RDP table was for square profiles. The Wheel allowed for simple multi-level dives, but was far more complex to use. Many people incorrectly use the flat tables for multi-level dives. They are wrong.

 

[boulderjohn]

What PADI did was to factor in their calculations the fact that the dive profile is never square: usually the diver surfaces slowly, with a trapezoidal profile. And this allows to reduce the nitrogen load at the end of the dive and hence the surface interval before diving again.

As Tursiops said, the PADI RDP is for square profiles and the wheel is for multi-level dives.

The US Navy tables of those times used the 120 minute compartment to determine surface intervals, and they made the decision on that without doing any real research on it. In fact, they created the 120 minute compartment when they did it. PADI did extensive research using doppler bubble imaging on divers to determine what compartment should be used for surface intervals for the kind of dives done by recreational divers, and they determined that for the overwhelming majority of those dives, the 40 minute compartment would do nicely. They ultimately decided to use the 60 minute compartment, adding some extra time for long repetitive dives (the WX and YZ rules). PADI thereby created much shorter intervals on their tables by implementing three major changes:

1. They changed from the 120 minute compartment to the 60 minute compartment.
2. The nearly doubled the number of pressure groups to limit rounding errors.
3. They shortened first dive limits. (The US Navy limit for 100 feet was 25 minutes; the RDP was 20.)

 

[John C. Ratliff]

The RDP table was for square profiles. The Wheel allowed for simple multi-level dives, but was far more complex to use. Many people incorrectly use the flat tables for multi-level dives. They are wrong.

Actually, rather than being “wrong,” they were more conservative.

SeaRat