Gas Planning

[Codyjp]

since depth can change the bottle PSI, how much weight do you give to that when thinking about turning around?

none. Temperature also affects your gas gauge in your car. Do you take that into account when your driving?

The pressure change just doesn't have an effect worth considering.

Good discussion all!

 

[Hepcat62]

Gas planning of this kind is not covered in the PADI recreational curriculum. This was the answer that Peter got to the question, when he asked it of the instructor for his IDC.

I've been working on a "seminar" presentation on this type of gas planning for the shop I DM with - no certification provided or expected, open to all certified divers. I've done two prototype runs on it to refine the presentation and worksheet. So far, we've had a pretty good response from the people who've sat in on it (guinea-pig DM candidates and instructors). Most of these people have never been exposed to this level of planning, and found it interesting and illuminating. For the DM candidates it's a good exercise in safe thinking, and a good review/prep for the physics section of their exam.

I'm looking forward to the day when we (hopefully) decide to roll it out as an actual scheduled event and start offering it to our OW and AOW students...

 

[DevonDiver]

none. Temperature also affects your gas gauge in your car. Do you take that into account when your driving?

The pressure change just doesn't have an effect worth considering.

In an OOA emergency, a diver can 'stroke' the outside of their tank. The resulting friction causes temperature increase - thus increasing the internal pressure of the tank. That increased pressure may provide extra gas to reach the surface with.

It's comparable as an emergency technique to using Spare Air.

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[Divedoggie]

I'm starting to understand why the accidents and incidents site is so busy.

How does today's lack of gas planning help you understand a busy A and I section?
A quick look at the A and I section showed the following:

Yukon Incident.(cause not known.)
Police training exercise resulting in a death.
A health issue resulting in a death.
An exploding tank resulting in a death.
A non-disclosed issue in the Great Lakes resulting in a death.
Diver caught in rocks off Laguna resulting in a death.
Lost buddies resulting in a close call.
Elderly diver in Cayman, cause not disclosed, resulting in death.
A missing Cave Diver. Not actually a cave diver. No training. Possible hoax.
Downwelling resulting in close call in Bali.
Deniseegg's incident, health related, resulting in a close call.
Separation of a diver from a threesome in Oakland resulting in death. Dive computer showed he only used his air for 10 minutes in 60fsw. Cause not known.

It seems that few, if any of these accidents had anything to do with the gas planning/training that the victims received. The statistics don't support your assumption.

Don't get me wrong, gas planning is a key component of diving, and it surely is taught in the PADI AOW college course that I teach. (Its an 8 week long course).

On the other hand,
Boyle's Law is taught in basic OW and emphasized in the book, in the video, and by the Instructor. Paying attention to the SPG, staying close to buddy, communicating remaining air pressure, and not pushing limits are also emphasized. The result is a student who manages their gas by not letting it get too low.
Since it is recommended that new divers use 60 feet as a max depth, and it is recommended that they begin ascent with at least 800 psi, the planning is not far off average warm water Rock Bottom calculations for an AL80.

As much as I'd like to see more thorough OW training, the system seems to work.

 

[DevonDiver]

Further to the A and I threads here.... a glance at the annual DAN and BSAC reports will ease your fears about OOA being a big 'killer'. Here are some interesting reads....

DAN Publication - Common Factors in Diving Fatalities

BSAC Publication - A review of the nature of diving in the United Kingdom and of UK diving fatalities in the period 1st Jan 1998 to 31st Dec 2009

Not inclouding diving in overhead environments, OOA fatalities are not a major statistic in modern recreational scuba diving.

According to BSAC, in UK dive fatality statistics, OOA only accounts for 8.6% of fatalities.

[FONT=&quot]UK Diving Fatalities, by percentage
[/FONT]
[FONT=&quot]Non-diving medical problem 27.1%[/FONT]
[FONT=&quot]Rebreather use 10.7%[/FONT]
[FONT=&quot]Equipment problem 9.1%[/FONT]
[FONT=&quot]Out of Gas 8.6% (see below)[/FONT]
[FONT=&quot]Inadequate pre-dive checks/brief 8.6%[/FONT]
[FONT=&quot]Inexperience 7.1%[/FONT]
[FONT=&quot]Buoyancy – diver light 7.1%[/FONT]
[FONT=&quot]Buoyancy – diver heavy 5.7%[/FONT]
[FONT=&quot]Narcosis 3.6%[/FONT]
[FONT=&quot]Tangled (rope, debris) 3.6%[/FONT]
[FONT=&quot]Trapped in wreck 3.6%[/FONT]
[FONT=&quot]Other trauma 2.1%[/FONT]
[FONT=&quot]Other rapid ascent 0.7%[/FONT]
[FONT=&quot]DCI 0.7%[/FONT]
[FONT=&quot]Unconsciousness 0.7%[/FONT]
[FONT=&quot]Separation (no data available) 0.7%[/FONT][FONT=&quot][/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]Out of Gas (12 cases) Accident Analysis[/FONT]
[FONT=&quot]1) Out of gas incident often followed by a failed attempt to use a secondary gas source[/FONT]
[FONT=&quot]2) Loss of buoyancy often occurred (sometimes due to the inability to inflate a BCD or drysuit)[/FONT]
[FONT=&quot]3) 2 cases involved divers re-entering the water after a dive, with low gas supplies, to recover lost[/FONT]
[FONT=&quot]equipment.[/FONT]
[FONT=&quot]4) 36 of the 140 cases involved divers running out of breathing gas, although in many cases this was a secondary or tertiary factor.[/FONT]

 

[Jax]

Since it is recommended that new divers use 60 feet as a max depth, and it is recommended that they begin ascent with at least 800 psi, the planning is not far off average warm water Rock Bottom calculations for an AL80.

As much as I'd like to see more thorough OW training, the system seems to work.

I keep seeing this "Rock Bottom" term, but it doesn't come up in the SDI / TDI manuals or classes. Does it mean a flat bottom profile?

 

[Peter Guy]

"Rock Bottom" = "Minimum Air Reserve" = The amount of gas (air) that it would take to get you, and your buddy, safely to the surface doing a normal ascent profile with all the stops.

The minimum MAR is 500 psi (generally thought to be due to the unreliability of pressure gauges at the lower end of the scale and NOT because of some idea about "being back on the boat with 500 psi").

A couple of things people forget about MAR/RB:

a. It is a dynamic number -- there is no one MAR/RB for a dive -- there is a MAR/RB for a depth -- thus at 100 feet, 40 cubic feet is the MAR/RB -- but once you leave that depth and go shallower, the MAR/RB decreases accordingly. The gas that is "released" from the MAR/RB reserve becomes yours to breathe.

b. Half of the MAR/RB is yours to breathe so once you "hit Rock Bottom" and start up, you are breathing YOUR half of the gas -- the other half is your buddy's gas. Using this concept, you should ALWAYS be on the surface with at least 250 PSI! (But do you want to trust your gauge that much?)

 

[D_B]

Peter has it right, it's the amount of gas needed to get you and your buddy up safely from depth

Take a look at this article on Rock Bottom calculations.

take a look above at Lamont's excellent article about it

I use his rule of thumb rock bottom calculation all the time for my recreational OW diving as I can adjust them on the fly as needed ...

IE ... HP100 tank, 90ft depth, start ascent with 1200psi ... but if I go up to 70ft then it's 1000psi
(BTW it's depth + a 0 + 300 (HP100) = psi needed)

As he says, this is a rule of thumb and is less accurate at any of the extremes
"Be aware that this simplified extrapolation breaks down as you go deeper."

 

[Jax]

"Rock Bottom" = "Minimum Air Reserve" = The amount of gas (air) that it would take to get you, and your buddy, safely to the surface doing a normal ascent profile with all the stops.

The minimum MAR is 500 psi (generally thought to be due to the unreliability of pressure gauges at the lower end of the scale and NOT because of some idea about "being back on the boat with 500 psi").

A couple of things people forget about MAR/RB:

a. It is a dynamic number -- there is no one MAR/RB for a dive -- there is a MAR/RB for a depth -- thus at 100 feet, 40 cubic feet is the MAR/RB -- but once you leave that depth and go shallower, the MAR/RB decreases accordingly. The gas that is "released" from the MAR/RB reserve becomes yours to breathe.

b. Half of the MAR/RB is yours to breathe so once you "hit Rock Bottom" and start up, you are breathing YOUR half of the gas -- the other half is your buddy's gas. Using this concept, you should ALWAYS be on the surface with at least 250 PSI! (But do you want to trust your gauge that much?)

Ahhh -- thanks! Now I recognize it.

 

[Jax]

Peter has it right, it's the amount of gas needed to get you and your buddy up safely from depthtake a look above at Lamont's excellent article about it

I use his rule of thumb rock bottom calculation all the time for my recreational OW diving as I can adjust them on the fly as needed ...

IE ... HP100 tank, 90ft depth, start ascent with 1200psi ... but if I go up to 70ft then it's 1000psi
(BTW it's depth + 0 + 300 (HP100) = psi needed)

As he says, this is a rule of thumb and is less accurate at any of the extremes
"Be aware that this simplified extrapolation breaks down as you go deeper."

:huh: Sorry, bud; you lost me at adding zero?