Planned deco on a recreational dive?

[boulderjohn]

... my cave training taught me to consider how much gas I needed for the dive I'm planning to do. Turning the dive on thirds isn't a plan ... it's feedback that helps you stick to the plan you made before you ever got in the water. If I hit thirds before I reach my objective, it's telling me that my plan was crap to begin with ...

You added something to my original comparison--the need to reach a specific objective. I never said anything about either of the compared dives having to reach an objective, and that was quite intentional to make them comparable. Reaching an objective is not necessarily a part of a cave dive--in fact, goal-oriented dive plans are not allowed in training. I did not have any specific objectives in most of the cave dives I have done other than to see the sights in the cave.

 

[EricTheDood]

Is this a joke? If not, I suggest you should go talk to your OW instructor and ask for a refund ... he apparently neglected to teach you some fairly fundamental concepts about decompression and gas management.

Breathing is not an afterthought ...

... Bob (Grateful Diver)

This is a wording issue.

I don't consider blanket guidelines, like turning the dive at 2/3 or 1/2 tank, or beginning an ascent at 700-800psi, to be "gas planning". Some do, and that's fine. I just disagree with them.

 

[BRT]

Simpler, OK. But not safer. And what is good if it's not done correctly?
The universal gas plan is to be back on the boat with 500psi. Ok, how do we figure out how to do that? We'll swim around and be back at the up line at around 800psi. Here are two examples where this doesn't work.
- We're doing a 60fsw wall dive. On the NAUI table we have a 55 minute NDL, but decide to be conservative and do 50 minutes. Our plan is to go down the mourning line, swim into the current with the wall on our right for 25 minutes, turn and come back to the mourning. Following guidelines, planning a little and being conservative. The problem is we don't have the gas to do this. At 60fsw with a surface RMV of .7 we are breathing 1.97 cf/min. At 25 minutes we've breathed 49cf, we don't have enough gas to complete the dive. 30 seconds of math on a smart phone calculator solves this. Literally 4 math function - divide by 33, add 1, multiply by your RMV and then multiply by planned dive time.
- Now were diving the U352 at 110fsw, breathing 30%. Using the NAUI tables again with an EAD of 100fsw we stay conservative and decide on a 20 minute dive time. Again the guideline tells us to be back at the up line for our ascent with 800psi. We're diving AL100s so 800psi gives us 24cf of gas. During the dive we are monitoring our air and make our way back to the grapple with 800 psi each. Just as we are getting ready to start our ascent I have a regulator failure and total gas loss. We begin to share air on our way up. The problem is that even without a safety stop we need 26cf to get to the surface and our "guideline" only gave us 24cf.
These are both common dives being executed by OW and AOW divers. I didn't contrive some off the wall scenario to prove a point. Not using basic gas planning leaves gaps. I learned how to gas plan when I was 13, without a calculator or smart phone. This isn't asking too much.

If you can't make it to the surface safely from 110' on 400# in an AL100 you should not be diving to 110'.

 

[Diver-Drex]

If you can't make it to the surface safely from 110' on 400# in an AL100 you should not be diving to 110'.

Or I shouldn't wait until 400psi to start my ascent. Or I should carry a pony bottle. Maybe I can figure out in advance how much reserve I should have. How would you determine if 400psi in an AL100 or 600psi in LP95 or whatever is sufficient??? That's gas planning. That's the point of my post. You do the rock bottom calc, if your comfortable with 12cf for that ascent then good for you. If not have a different plan. But make a plan.

 

[BDSC]

This is a wording issue.

I don't consider blanket guidelines, like turning the dive at 2/3 or 1/2 tank, or beginning an ascent at 700-800psi, to be "gas planning". Some do, and that's fine. I just disagree with them.

Actually, what you said and what Bob said are quite different. You said in certain situations "Gas Planning" can become an afterthought and I agree with you. I don't think I have done gas planning in over 20 years unless you call starting your accent with 500-700psi gas planning. Bob changed that to say "Breathing" in not an afterthought. Of course it's not but that's not what you said.

 

[MXGratefulDiver]

from an objective perspective you are probably right but having a TP is not an either or in that situation is an integral part of the plan and trip alert to correct your crappy plan
Having an objective and a gas plan to suit works for known quantity but if you've not been there before id say TP is very relevant

Turn pressure is always relevant ... but it does not constitute a plan. It's just a data point ...

... Bob (Grateful Diver)

 

[MXGratefulDiver]

You added something to my original comparison--the need to reach a specific objective. I never said anything about either of the compared dives having to reach an objective, and that was quite intentional to make them comparable. Reaching an objective is not necessarily a part of a cave dive--in fact, goal-oriented dive plans are not allowed in training. I did not have any specific objectives in most of the cave dives I have done other than to see the sights in the cave.

... but in the real world, reaching an objective is often a big part of the plan ... even in a place you've never been before. And it's not unusual for divers to get so focused on the objective that they neglect to turn when they should ... that's how OOA/LOA situations usually happen.

... Bob (Grateful Diver)

 

[MXGratefulDiver]

This is a wording issue.

I don't consider blanket guidelines, like turning the dive at 2/3 or 1/2 tank, or beginning an ascent at 700-800psi, to be "gas planning". Some do, and that's fine. I just disagree with them.

Agreed ... and that's my point. A dive plan should include enough forethought to have answered the question "am I carrying sufficient gas to do this dive?" That's the basis for gas planning, and the question should be answered prior to beginning the dive.

"Turn the dive at" a specific pressure isn't planning ... it's reacting. And as long as nothing goes wrong, it's usually sufficient. "End the dive at" a specific pressure is like giving someone directions by telling them to "turn right about a quarter mile before the T-junction" ...

... Bob (Grateful Diver)

 

[RandomGuy1]

Along this line I have been cautioned against having "goal" driven dives. It seems that often times people put the "goal" above their safety. "Well I can go into my reserves just a touch, since my goal is just around the corner". To me, the turn pressure / turn time should be the driving factor of the plan, not the goal.

 

[BRT]

Or I shouldn't wait until 400psi to start my ascent. Or I should carry a pony bottle. Maybe I can figure out in advance how much reserve I should have. How would you determine if 400psi in an AL100 or 600psi in LP95 or whatever is sufficient??? That's gas planning. That's the point of my post. You do the rock bottom calc, if your comfortable with 12cf for that ascent then good for you. If not have a different plan. But make a plan.

The post I answered was the proof of the inability to two people to surface from 110' on 800 psi in an AL100. Absolute calculations that showed you couldn't do it. And absolute nonsense. I don't have to calculate it I know I can do it.