Inexperience resulted in OOA at 66 feet (long)

[lamont]

Don't ya love it? In their user profiles, some people have three lines listing every course, class or card they have. Lamont's just says, PADI OW.
Great post, Lamont, thanks!

technically that's the only little plastic card that i've got, unless you coun't the DAN cards which are starting to really stack up...

 

[jeckyll]

Great info lamont. Copied & saved that for some additionl reading and future reference.

Thanks.

Bjorn

(hope people don't get too upset about 'resurection threads'

 

[miketsp]

Coming back to the original topic of this thread.
When we dove Cuba it became obvious after the first couple of dives that the "DM" does not act as a DM but as a guide to show you the route.
Before each dive we had an excellent briefing where the "DM" would describe the route and the profile and indicate how many minutes the route would take.
Once we set off he never looked back and he always reached the ascent point in the exact number of minutes he had indicated.
It was then up to each diver to manage his gas. If you wanted to do part of the dive deeper than the "DM" then you could count on having to do part shallower, mid water above the group.
Vis was good and in a couple of cases gas guzzlers would end up surface snorkelling above the group.
The whole turn pressure concept discussed above meant nothing because most of the dives were from point A to point B and there was no upline to return to.
You had to complete the route, whether on the surface or under it.

 

[TSandM]

I did two deep dives for the PADI specialty yesterday. Dive plan for the first one was to go to 120 feet. The briefing from my instructor talked about no deco times and using the tables and the wheel to plan them. There was some discussion about being aware of how much faster you use air at that depth.

Everybody, including the divemasters assisting the class, was shocked when I told them I had calculated rock bottom for 120 feet and it was half the air I was carrying. I went on to have a discussion with one of the divemasters about calculating the expected air consumption for the dive we planned to make sure I had enough air with me to do it. She had never calculated her rmv, had never done rock bottom calculations, and was astonished that I was running those numbers.

I don't know why this sort of thing isn't included, if not in open water, at least in the AOW deep dive, or at minimum, in the deep dive specialty. I wouldn't have this information if it weren't for NW Grateful Diver's gas management seminar I attended.

My husband refers to these ideas as "DIR stuff". Sad to think that intelligent management of one's life support would be viewed as solely the province of technical divers . . .

 

[lamont]

My husband refers to these ideas as "DIR stuff". Sad to think that intelligent management of one's life support would be viewed as solely the province of technical divers . . .

It definitely isn't limited to DIR. There's nothing about gas management that you can't sit down and figure out from what was presented in PADI BOW since it is all just application of Boyle's Law and some rule of thumbs. And while it may come from cave diving and the 'rule of thirds' developed by Exley and others, that's just because in cave diving the idea of "your buddy's gas supply explodes at the worst possible moment -- can you get both of you out without drowning?" is a little easier to mentally crystallize for cave divers.

 

[Web Monkey]

It's sad, but not surprising.

I got cr*p from one of the DMs on a dive in FL for coming back to the boat with 1000PSI. He wanted to know why I had all that air left. I said "Because nothing bad happened".

Terry

Everybody, including the divemasters assisting the class, was shocked when I told them I had calculated rock bottom for 120 feet and it was half the air I was carrying. I went on to have a discussion with one of the divemasters about calculating the expected air consumption for the dive we planned to make sure I had enough air with me to do it. She had never calculated her rmv, had never done rock bottom calculations, and was astonished that I was running those numbers.

I don't know why this sort of thing isn't included, if not in open water, at least in the AOW deep dive, or at minimum, in the deep dive specialty. I wouldn't have this information if it weren't for NW Grateful Diver's gas management seminar I attended.

 

[Ayisha]

Rock Bottom - Table Values

I would suggest using tables like the following for an AL80:

30 fsw - 700 psi
60 fsw - 1000 psi
100 fsw - 1300 psi
130 fsw - 1800 psi

This table makes it really simple. All the rest is amazing, but holy complicated...

So based on your example of Rock Bottom - Turn Pressure, here are some calculations for Rock Bottom - Turn Pressures assuming an AL 80 tank and a dive that must end where it begins like at an ascent line:

30 fsw = 3000 psi - 700 psi = 2300 psi/2=1100 psi
3000 psi - 1100 psi = 1900 psi

60 fsw = 3000 psi - 1000 psi = 2000 psi/2 = 1000 psi
3000 psi - 1000 psi = 2000 psi

100 fsw = 3000 psi - 1300 psi = 1700 psi/2 = 800 psi
3000 psi - 800 psi = 2200 psi

130 fsw = 3000 psi - 1800 psi = 1200 psi/2 = 600 psi
3000 psi - 600 psi = 2400 psi

So a Rock Bottom - Turn Pressures Table would look like this:

30 fsw - 1900 psi
60 fsw - 2000 psi
100 fsw - 2200 psi
130 fsw - 2400 psi

If my calculations are correct, then at 30 feet, even assuming that it is a dive where you must ascend where you began, to turn around at 1900 psi seems really over-cautious. Even at 60 feet, to turnaround at 2000 psi seems over-cautious. I can see the need for the deeper depths. Are my calculations off or is that much contingent gas necessary for the shallower depths? :06:

I'm afraid to calculate what those numbers translate to for my AL 63's!! Even with excellent consumption... :11:

 

[spectrum]

I don't know why this sort of thing isn't included, if not in open water, at least in the AOW deep dive, or at minimum, in the deep dive specialty.

Because it would take a lot of remedial work beyond the scope of dive instruction for many students to get it right. Time and $$

If they don't get it do they fail?

Congratulations on taking this seriously and being a "thinking womans" diver, I look forward to finding similar instruction locally but have amazingly learned most (probably not all) of it here thanks to the great online mentors that make this board such a resource.

Thanks again everyone.

Pete

 

[fldiver327]

Thank you to all that contributed to this thread. I sat here tonight reading through this thread and all the linked threads added...man did I learn alot!!! I am almost embarrassed to say that I had never heard of Rock Bottom Caculations before tonight! I sure hope it wasn't because I was sleeping during my certification classes and BECAUSE my instructors did not cover it...although as I say that I guess that's the real shame of it, isn't it! Thank you again!

Now a safer diver!

 

[lamont]

If my calculations are correct, then at 30 feet, even assuming that it is a dive where you must ascend where you began, to turn around at 1900 psi seems really over-cautious. Even at 60 feet, to turnaround at 2000 psi seems over-cautious. I can see the need for the deeper depths. Are my calculations off or is that much contingent gas necessary for the shallower depths? :06:

I'm afraid to calculate what those numbers translate to for my AL 63's!! Even with excellent consumption... :11:

Well, a turn at 1500 psi leaves you OOA at the upline, so you must be more conservative than that.

At 30 fsw you're probably burning through less than 60 psi / min, so for a 1900 psi turn that's 20 mins out and 20 mins back for a 40 min dive. And that's with a 0.75 SAC rate. Even my SAC rate is usually a little better than that, and yours is almost certainly better than mine, which is probably why you can get away with the Al63...

And you should be thinking worst case. So, when you get back to the upline you find a diver who is entangled in it and hoovering and goes OOG on you (their buddy is either non-existant or has deer-in-headlights issues...).