Rule of Thirds & Shallow Rec diving

[Foxfish]

Foxfish it is not about the pressure to get you home but rather the cubic footage....

Calculating min gas is very simple and really doesn't take that much time at all. I'd rather take the minute it takes to do and dive with a solid gas plan.

You'd know however that for a given tank volume, more pressure means more volume.

8 minutes to ascend from 30 m seems reasonable. Remember though that the average depth is not simply the total depth divided by two, it is the time weighted average. I calculate a pressure of 80 b or 1150 psi with a 12 L tank. If I do a normal ascent from 30 m starting with 80 b I reach the surface with 50 b. I'd suggest that is more than a coincidence. Someone posted a link to the DIR website previously showing the calculation with similar results. I noticed that Natalie Gibbs also runs through a calculation and came up with 70 b or 1000 psi though she notes this is on the low side. You and a few others are saying you need 110 b or 1600 psi.

---------- Post added December 5th, 2013 at 06:53 PM ----------

and... take a look at this:

The problem here was stupidity or gross incompetence not the rules. Show me cases where a person died as a result of running out of air when the began their ascent with adequate pressure to hit the surface with 50 b. Not that I'm any real judge on the matter given my limited experience.

---------- Post added December 5th, 2013 at 07:08 PM ----------

Here's a perfectly good example of why it's so wrong: A seemingly reasonable diver with a decent quantity of experience is absolutely convinced that basic math should NOT be used to plan for safety because the major agencies say "surface with 50b." You simply can't be convinced, nor can you even admit to seeing that there might be some benefit due to the dogma ingrained so deeply by such a simple phrase. As has been proven by anecdote, video, and math, it's insufficient and you can't see it. THAT is why it's wrong. I'm not saying you should HAVE to abide by the rules I like, but you should at least be able to admit there is merit to it.

In post #134 I did say the approach had merit. I think it is helpful to gain at least some insight into how you do a gas plan for an emergency ascent. I see the benefit of a simple rule like surface with 50 b in the OW and AOW context and it seems to tally with the air required to ascend from 30 m in an emergency.

 

[GCullen94]

Foxfish I don't know how to make this any clearer... It depends on the ascent profile.

I don't know why you are having such trouble understanding that. To make a 3 minute stop at 15 feet you need 35 CF of gas the way I plan it or 1400 PSI. To do a GUE ascent or deep stop ascent you need 40 CF or 1600 PSI in an AL80. If you go straight to the surface you need 23CF. How'd you find out what point to start your ascent? trial and error? Personally I'd rather know I have the gas reserves no matter what. I think it's been shown there are cases where that amount WILL NOT be sufficient even in the recreational depths also what happens if you use a smaller or larger tank? Still sticking to 50B even though it could be 15CF or 30CF? It is apparent that you want to stick to your old ways of exactly what your agency taught you. I take it you still wear a snorkel all the time no matter what and use a fin pivot to establish buoyancy because thats what you were told to do as well?

and also for the record there is no DIR website. DIR is an idea that was established many years ago. There are agencies that teach DIR style... there is no one central "DIR website" Agencies that I can think of off the top of my head

GUE
UTD
NAUI tech programs
ISE
5thD

Also "min gas" is not limited to DIR, I know DIR followers and non DIR divers who use the system.

 

[Foxfish]

As Bob said, every year, we have deaths due to running out of gas. Look at the DAN statistics. No one should ever run out of gas, and very few of those incidents involve any kind of equipment malfunction that results in a catastrophic gas loss. It's people who don't know that the dive they are planning is too much for their tanks, and people who get distracted and don't check their gas. Neither should happen.

Emergencies are typically chaotic events. You could carry a spare 12 L tank of air at all times and it would still not be adequate in some emergencies. In diving it's always a compromise between the risks and what is practicable. Establishing rules for an ascent requires experience and judgement not just the ability to push buttons on a calculator. At this stage I'm prepared to back the judgement of the major instruction agencies.

Lamont's page on gas management was written and posted as a result of him having been first on scene to try to rescue a woman who died as a result of poor gas planning and poor gas monitoring -- but she would have survived the poor monitoring, had she had an adequate gas supply in the first place.

So did the person who died begin the ascent with adequate pressure to hit the surface with 50 b. If not what does that prove? If so I'd be interested in a link to the article please.

 

[TSandM]

No, she didn't -- but don't you get it? SHE DIDN'T KNOW HOW TO FIGURE THAT OUT!!!! She didn't have any tools. And in the particular case that gave rise to Lamont's article, surfacing in the middle of the dive could get you killed by getting run over by a large water taxi.

I'll be the first to admit that it's unlikely you'll run out of gas if you start your ascent with enough gas to surface with 50b. (You could still have a freeflow, or run into a low on air or out of air diver who'll eat your reserve gas, if you haven't planned for two.) What we're arguing about is how you go about PLANNING so that you can be sure that you will be able to start your ascent in a place where you WANT to do it, with enough gas to do it safely.

I'm really quite done with this thread. I appreciate the opportunity to explain a very valid and safe way of doing gas planning to the audience of the thread, which, as always, is much larger than the group of people posting in it. But I realize that you are making me angry and life is too short to waste energy on getting mad at people on the internet. You've heard what we've all had to say, and for whatever reasons of your own, you absolutely do not want to acknowledge that it is useful information. Safe diving to you. (And kudos -- you're one of three people on SB who have made me give up!)

 

[GCullen94]

Emergencies are typically chaotic events. You could carry a spare 12 L tank of air at all times and it would still not be adequate in some emergencies. In diving it's always a compromise between the risks and what is practicable. Establishing rules for an ascent requires experience and judgement not just the ability to push buttons on a calculator. At this stage I'm prepared to back the judgement of the major instruction agencies.



So did the person who died begin the ascent with adequate pressure to hit the surface with 50 b. If not what does that prove? If so I'd be interested in a link to the article please.

The MOST ironic comment of the thread is in there... HOLY The simple rules were taught to tourist divers and vacation divers that hug a DM's side because thats unfortunately who a lot of divers are. Almost all of us in this thread adapted the idea of min gas because it is flexible to depth, tank size and is calculated on the cubic footage of gas we need, not a pressure number. We use min gas because it is SAFER than trying to hit the surface with 50B.

Why is it that the "major instruction agencies" teach the idea of min gas to higher level classes then?? It's because most OW divers are tourist divers or divers that quit after certification. Some agencies have chosen to teach min gas straight from OW.

I hope you can see this as a way to reduce risk.

 

[Foxfish]

Uh... Have you ever considered that your ideas are wrong (or at least limited)? Personally, Dive #6. First Dive after OW training.
I've never dived tropical water.

I've spent some time trolling through the BSAC incident reports and what always amazed me was the number of injuries and fatalities that occur due to problems with dry suits.

I'd accept that what I and the major diving agencies that have trained millions of divers say regarding gas planning for an ascent on recreational courses may be wrong, I just don't find the arguments against it so far very convincing.

OW/AOW gas management calculations? Nil.

Here's what the PADI Open Water Manual 1999-2006 has to say about it.

Under "Dive Planning", beginning on Pg.144 It describes four stages of planning:

• Advance planning: Nil
• Preparation (a day or two ahead of time): Nil
• Last minute preparation (just before you leave for the dive): Nil
• Pre-dive Planning (at the dive site): Item#6. Agree on time, depth and air supply limits

Later on Pg.151, under "Boat Diving", the following is said:

On the bottom, get your bearings and swim into the current. Plan your dive and navigate so you finish near the boat with enough air so you'll be back on board with 20-40bar/300-600psi left in your tank.

That's it for gas planning according to the manual. I stand to be corrected by other examples.

Providing people stay within recreational limits and surface with 50 b I don't see a problem with this approach. Those who need more extensive training because they're exceeding recreational limits or because of the increased risks with the type of diving they do could take specialty courses or advanced training. Some appear to have done. Personally, I'd rather dive somewhere it is safer. Not everyone can.

I do have a problem with people who've done more advanced training getting onto a forum intended for divers with basic recreational training making it sound more complicated than it needs to be and denigrating their training without proper justification.

 

[GCullen94]

Alright stay with your "50B comments" I pray thats enough to get you and your buddy out of trouble if you have something come up at the end of a dive (doubt it). Hope you have a large enough tank were 50B actually means something. If not, well guess we'll hear about it in another forum.

You math in front of you... WHERE THE DO YOU NOT FIND THIS CONVINCING??? Hell if me and my buddy were trying to ascend from 100 feet STRAIGHT to the surface 50B would JUST if get you there if SHTF... JUST. And at the end of the day how smart is that? If you run out of gas at 100' you are likely coming up on NDL limits, how smart is skipping that safety stop. What if it was a repetitive dive? I'd rather play it safe according to what I have learned in higher up classes. There is a reason why PADI re-wrote the whole OW manual this year, not only to work on trim but gas management. Instead you insist on being stubborn and hey good luck to you, hope we don't end up hearing about you in a thread.

 

[victorzamora]

Min gas is for WITHIN RECREATIONAL LIMITS! People diving beyond recreational limits have much more involved gas planning techniques and plan things much more rigorously than a super-simple formula. They have to plan for multiple stops with multiple gasses, including losing the option for certain gasses and they have to accomodate longer deco in case of getting caught deep. All of these are variables that add greatly to the complexity of gas management. This is why Min Gas or Rock Bottom is so useful for rec divers, it's DESIGNED for rec divers specifically. It has to be extrapolated out drastically to fulfill tech diving needs.

 

[dumpsterDiver]

If you cant recognize the inherent stupidity (or excessive over simplification) in making up a rule that says "be on the surface with XX psi", then nobody can explain it to you. Clearly, it makes more sense to tell a diver to leave the bottom with XX psi (or cu-ft, really) in order to reach the surface safely.

You can argue over how exactly one should arrive at the "rock bottom" reserve volume, but it makes a hell of a lot more sense (and it is vastly more practical) to know WHEN one needs to leave for a destination, rather than to simply say: "you need to arrive at XX hundred hours".


Sorta like: the plane leaves at 8.. great! But what time do we need to leave for the airport?

 

[Foxfish]

No, she didn't -- but don't you get it? SHE DIDN'T KNOW HOW TO FIGURE THAT OUT!!!! She didn't have any tools. And in the particular case that gave rise to Lamont's article, surfacing in the middle of the dive could get you killed by getting run over by a large water taxi.

Did you have the link to the article. I mentioned how I 'figure it out' previously. As I got recall, I got told a pressure, I think it was 70 b, during my OW/AOW pressure that I'd need to begin the ascent with to hit 50 b on the surface. I've refined the number with experience and almost do it unconsciously. Seriously, how hard is that? I never have trouble hitting the surface with 50 b. Up till now it's never been something I've pondered deeply and neither has anyone else I've dived with. Many recreational divers WILL have trouble figuring it out if the only way they can get the information is to do a rock bottom calculation.