Rule of Thirds & Shallow Rec diving

[Storker]

I'd probably just calculate a couple rock bottom depth pressures for a 12 L tank (only ever filled to 232 b), remember those, and then estimate other pressures and then vary them mentally for depth and tank size.

So the numbers to remember may be:

12 L tank ascent from 15 m - 60 b
12 L tank ascent from 30 m - 80 b

And mental estimates derived from those numbers would be:

15 L tank ascent from 10 m - 55 b
15 L tank ascent from 30 m - 70 b
12 L tank ascent from 23 m - 70 b

If those are your rock bottom numbers, you're either:

• Basing your rock bottom calculation on a very low air consumption (~12 SLM/0.42 cu.ft)
• Omitting the safety stop at 5m

or

• Not including the one minute at bottom before ascent

Just sayin'.

 

[Doppler]

In my experience and area, PADI and SSI are the major OW and AOW diving instuction agencies. The others you mention are either unheard of or at best uncommon in this area of instruction. I've noted the difference in approach specifically to gas planning for an ascent between these major instruction diving agencies and the approach by DIR and apparently the others you mentioned.

You wrote: "I'd have to conclude that the major agencies consider it an unnecessary complication to their procedures. Maybe they just don't like it because it is recommended by DIR. The feuding between DIR and the major agencies is well known."

I disagree. All agencies have a gas management module... even the sport ones. We are complicating matters somewhat by applying the common-sense approach promoted by the tech agencies to sport-style exposures, but you are making an incorrect assumption... a couple, I believe. Since you seemed unsure of the facts in a broader sense than Perth, I simply wanted to help you out. The major SPORT training agencies do not give a rat's ass about who recommends what to whom, and I am unaware of any feuding going on. I am shocked to learn that there is one and it's well-known. I also did not realize that DIR is now a certifying agency in Australia. None of my contacts there informed me of it.

 

[Foxfish]

If those are your rock bottom numbers, you're either:

• Basing your rock bottom calculation on a very low air consumption (~12 SLM/0.42 cu.ft)
• Omitting the safety stop at 5m

or

• Not including the one minute at bottom before ascent

Just sayin'.

I did include those times but frankly my intuition is that in an emergency I'd probably fore go the safety stop. Intuition is not always a good guide.

---------- Post added December 4th, 2013 at 05:43 AM ----------

You wrote:

I disagree. All agencies have a gas management module... even the sport ones.

Agree. I've simply noted that the major recreational agencies have a very different approach to the ones you mentioned. People are questioning the validity of their approach. I'm not so sure.

Wiki has lengthy and detailed descriptions regarding the feuding between DIR and other agencies.

 

[Storker]

I did include those times

*Checking my math* Oops.

My spreadsheet included a safety margin of 20bar after reaching the surface. If you trust your SPG completely and you're comfortable with sucking your tank totally empty in an air-sharing emergency, 80bar should be enough in a 12L tank at 30m.

 

[MXGratefulDiver]

Agree. I've simply noted that the major recreational agencies have a very different approach to the ones you mentioned. People are questioning the validity of their approach. I'm not so sure.

No one's questioned the validity of their approach ... all any of us have done is offer an alternative, more thorough approach that would work under a broader set of circumstances.

And you seem more sure of yourself than your apparent knowledge of the subject should warrant.

Wiki has lengthy and detailed descriptions regarding the feuding between DIR and other agencies.

Those of us who are more familiar with DIR than you seem to be tend to view that Wiki article as propaganda. The "feuding" you refer to didn't occur between agencies. It was between individuals ... generally pretty egotistical ones. And with the exception of Florida, where it has more to do with access than technique, it ended long ago. In my area, where we have a robust population of both DIR and non-DIR divers, people tend to get along ... regardless of which agency they trained with.

As Doppler keeps trying to tell you, DIR is not an agency ... it's a process. For that matter, there's nothing "unique" about it. The people who created it didn't invent any of it. They simply took things that were being taught "ad hoc" by instructors from other agencies and turned it into a system that could be marketed as a package. Gas management is just one aspect of that package, and has its roots in some air planning rules that were first promoted by Sheck Exley for cave diving. He first published those rules in 1968 ... nearly 30 years before anyone ever heard of DIR. In those days, the "major" sport agencies were YMCA, NAUI, and LA County. PADI was barely two years old, and SSI didn't exist yet.

If you're going to argue with people who have many times your knowledge and experience, it would help if you first learned a little bit about the topic.

... Bob (Grateful Diver)

 

[Foxfish]

If you trust your SPG completely and you're comfortable with sucking your tank totally empty in an air-sharing emergency, 80bar should be enough in a 12L tank at 30m.

The problem with calculations, as you are aware, is that people make significant mistakes. The more complicated the calculation the more chance of a mistake. It is relatively easy for a diver to build up empirical knowledge of the ascent pressure required from varying depths to surface with 50 b. I'd say for many recreational OW/AOW divers that is a big plus for the 'surface with 50 b' approach.

 

[nimoh]

The problem with calculations, as you are aware, is that people make significant mistakes. The more complicated the calculation the more chance of a mistake. It is relatively easy for a diver to build up empirical knowledge of the ascent pressure required from varying depths to surface with 50 b. I'd say for many recreational OW/AOW divers that is a big plus for the 'surface with 50 b' approach.

in order to build up empirical evidence without math, you would need to ascend under stress while sharing gas. It seems to me that making some estimates and doing some calculations is easier, and there is no requirement to do a bunch of dives without a gas plan in order to formulate a gas plan.

 

[victorzamora]

I was about to post that the only way to build up empirical knowledge was to have multiple ACTUAL emergency ascents and survive all of them and have the presence of mind to constantly check tank pressures.

Sent from my Samsung Galaxy S4 using Tapatalk

 

[Foxfish]

in order to build up empirical evidence without math, you would need to ascend under stress while sharing gas. It seems to me that making some estimates and doing some calculations is easier, and there is no requirement to do a bunch of dives without a gas plan in order to formulate a gas plan.

I think you've missed the main difference between the methodologies in question.

 

[Storker]

The problem with calculations, as you are aware, is that people make significant mistakes. The more complicated the calculation the more chance of a mistake.

Quite right. And exactly the reason for me not to do those calculations under water. Comfortable in front of my PC at home I decide on my safety margins, do the calculations and put the results on my slate. At the diving site I stick to those numbers, whether I'm planning my dive topside or changing my plan while submerged.