Rule of Thirds & Shallow Rec diving

[D_B]

Question ...

is the Lamont's rule of thumb I posted earlier too inaccurate to use for recreational depths ?

... being hard to calculate/use/complicated is certainly not a reason where it is concerned

 

[boulderjohn]

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.

Because of the very active presence of both DIR lovers and haters on the Internet, including ScubaBoard, you can get the false sense that the supposed feud is much bigger than it really was then and is now. The overwhelming majority of divers do not pay attention to the Internet forums, and they are unaware of this great feud.

As I mentioned recently, I took my UTD/DIR credentials to south Florida a few years ago with the purpose of crossing over to another agency to complete trimix training. I had to present C-cards many times, and I had to explain my background to a number of prospective instructors. I only found one person that entire time who knew what UTD was. I only found a couple who knew what GUE was. A minority of people had heard of the term DIR, but the only person who really knew anything about it was the only person I found who had taken a class from GUE. These were primarily tech divers I was talking to.

When there was a feud involving the people Bob mentions above, it was primarily tech divers and especially cave divers. I suspect the major sport agencies (like PADI and SSI) have never given a moment of thought to this "feud" as they planned their policies.

 

[DaleC]

The problem with calculations, as you are aware, is that people make significant mistakes.

Paradoxically, the benefit of calculations is that people can also see their mistakes ie. assumptions about gas consumption.

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.

Rock bottom is not complicated and one doesn't have to remake the calculations every dive. For straight forward dives I get the "return to the surface with 500psi/50b" concept. What I never got, and was never taught, was how to plan an AOW dive so I could accomplish that intelligently.

__________________________________________________________

Let's look at a very simple AOW shore dive: 60' for 50 minutes on air. Swim down, swim around, swim up - no overheads, no need to return to a specific exit.

A group is planning this dive and invites a newer AOW diver to join. With a sac of .75 he wants to rent a tank and wonders if an Al 80 is enough or should he go for a HP 100 for such a dive.. HP 130?
He also wonders if he should take that course and use EAN to extend the dive. So many questions...

He asks the shop employee who has a sac of .50 and they say a 100 is fine. They do that same dive and always surface with at least 800psi. They also say Nitrox is the safer way to dive and sign him up for Basic EAN and pre-sell him tank wraps and an analyzer.

He goes on the dive but finds he either has to run low on air to stay with the group or call the dive early. Why is he such a terrible diver. Why does he still run into trouble???

The shop suggests he take a PPB course or consider "turning pro".

But then he meets Fred, who introduces him to rock bottom calculations.

First, Fred notes the air tables state that 50min at 60' requires a mandatory SS so with that he sits the newer diver down with a calculator...

Reserve Volume (base on 1cuft/min): 20cuft (rounded)
Ascent volume + SS volume (5+6) = 11 X 2 (air sharing)

Bottom Volume (based on .75sac): 112cuft
atm's X sac X time (3X.75X50)

BV+RV = 132cuft. That's why the 100cuft tank wasn't big enough for the newer diver.

But that shop employee has a sac of .50 so...
RV = 20cuft.
BV = (3X.5X50) 75cuft.
RV+BV = 95cuft. That's why the 100cuft worked for them.

But, the diver asks, If I was using EAN would I need less gas? Would this change the calculation?

Sort of, says Fred, who is happy he is now thinking; it potentially eliminates 12cuft from the reserve (if his buddy is also on 32%) because a SS is now not mandatory and it somewhat reduces the risk of DCS if bolting. But why use EAN to extend a dive when you are limited by available volume? EAN would be nominally safer but the main issue is running out of air. Get the volume right and you will not need to bolt. Don't put the cart before the horse.

Oh... say's the new fellow, I'm not a putz after all. I just had no clue how to figure out what I needed to do the dive. I'm going to practice these calculations and make a preset chart for different depths and tank sizes.

Bingo!

Usually, without a Fred, this diver calls the dives early and feels embarrassed about being such an airhog and wonders if he is cut out for advanced diving after all. All it does is make him feel inadequate. If it persists he either buys a big tank and lugs it around on all his dives or sells his gear on craigsl_st.

Very occasionally he and his buddy are at the end of the dive, about to ascend, when one of them goes OOA. The other doesn't have enough reserve volume and they have a close call. They don't talk about that because it's embarrassing or it scares them and they quit diving altogether.

Very very occasionally they both drown at depth or embolize/suffer DCS when bolting to the surface.

Every diver should run into a Fred, whether he is a professional or not. That's one thing this board is good at. Just in this one thread a novice diver could talk to instructors, authors and experienced laymen/women who could explain exactly how to do rock bottom calculations, while having an ongoing peer review process to ensure accuracy of information. For free!

 

[TSandM]

I find it amusing that the one person who is inveighing against the utility of a gas planning strategy is someone who has never used one. Somehow, all the rest of us who HAVE seem to think it works fine and is useful.

 

[NAM001]

Bob
I agree with you. I think the issue is what is considered a simple dive. It is not that same thing as an average dive. For some the average dive is in currents low vis boat traffic and a number of factors that most others would think of as more than a simple dive. I supose it is the same delema as discussions relating to "the level of training a ow needs to be an ow". Simple is just that... without complexities. As far as my comments about using a table to determine min gas for a dive . WE do that all the time. Every time we use the dive table we accept it as good info. the same premis can be done with a min gas table for simple dives. OWs are not taught how to determine ndl for a depth, we give them a table, and the plan from that. We all know the problems with tables, but they work for the simple dive so long as the simple dive is a square profile. deviate from the square profile and it no longer a somple dive in regards to the ndl aspects.

It depends on where you dive ... I can think of a few OOA incidents here, some that resulted in fatalities, that were completely within the parameters of OW and AOW training. One of those was the very dive profile I used as an example earlier in this thread, which never gets deeper than the 100 feet you're supposedly qualified for once you receive an AOW card. Newer divers in particular tend to be gas limited due to poor technique, smallish rental cylinders, and high consumption rates ... so it's relatively easy to find yourself low on or out of gas while staying completely within the depth limits of your training ...

... Bob (Grateful Diver)

 

[DaleC]

KWS, your comparison of tables to min gas presets is a good one for discussion sake.
Using tables you can do square profile dives.
Using a min gas chart you can do straight forward dives.

For multi-level dives a computer is now usually used and while some poo poo the learning of tables, the knowledge of them is what allows a diver to understand what the computer is doing.
It's the same with RB. One can usually fly by presets but it's the understanding of how they are derived that allows this to be done safely.
Otherwise they are just numbers without correlation or cause.

 

[MXGratefulDiver]

There are some differences to consider between using a table to calculate NDL vs using a table to calculate gas consumption.

Dive tables used to calculate dive NDL are based on a "idealized" human body that may or may not bear any resemblance to yours. It makes several assumptions about ongassing and offgassing rates, and factors in enough padding to be "safe" for most people. There is no "line" which on one side denotes safe and on the other denotes unsafe ... it represents a continuum of potential risk and chooses numbers based on what is considered acceptable risk. And even with the safety factors, sometimes people take what they term "undeserved hits" ... which is a misnomer, because if they got bent it means they exceeded their body's capability to offgas acceptably ... and therefore deserved it ... they just don't understand why. Fortunately, DCS is usually treatable.

If we assume the same conditions to create a gas management table we will be doing so using an "idealized" gas usage that may or may not bear any resemblance to yours. It'll make several assumptions about your consumption rate, as well as that of your buddy, and factor in enough padding to be "safe" for most people. This is, in fact, exactly what happens when creating the rules of thumb most commonly used in recreational diving. It knows nothing about your actual consumption rate, and therefore it's possible for you to take an "undeserved hit" because your actual consumption rate may exceed the parameters used to create the table. The difference is that if you do exceed the assumptions of the table, you run out of air. There is no "gray area" ... an empty tank is a definite line that denotes an unsafe condition. If you run out of gas, you'd better have a backup plan readily available ... because unlike DCS, "drowned" is very rarely a treatable condition.

As I keep saying, simplified gas management approaches generally (but not always) work for simplified dive profiles. But the more aggressive the profile, the greater the risk of a failure that can end very badly.

It just stuns me that anyone who dives could reasonably believe that taking that risk is worth more than the few minutes of effort it would take to avoid it ...

... Bob (Grateful Diver)

 

[Foxfish]

I find it amusing that the one person who is inveighing against the utility of a gas planning strategy is someone who has never used one. Somehow, all the rest of us who HAVE seem to think it works fine and is useful.

1. So how do you define a gas planning strategy?
2. What are the essential elements of the strategy?
3. I follow the training I received on my OW and AOW when considering the pressure needed to begin my ascent. I start the ascent so that on a normal dive I surface with 50 b. Why isn't that a gas planning strategy?
4. Is the planning I described in point 3 in line with OW and AOW training an adequate gas planning strategy for the ascent?

 

[TSandM]

Foxfish, one can be proactive or reactive in any given situation. The strategies that have been presented repeatedly in the last 150 posts are efforts to make divers be PROACTIVE about the amount of gas they are taking underwater, and how they will apportion that gas, to make sure that when they reach the point where they need to surface, they will do so with enough gas. Your strategy (for which I don't blame you . . . it's what you were taught) is REACTIVE, which means you reach a point where you need to surface, and you can only hope that you have enough gas to do so, and that you are where you need to be when you do.

I like, teach, and promulgate proactive. You are happy with reactive. You are among the majority; some of them die every year from running out of gas. In all the years I've been diving, I have yet to hear about a diver using our strategy dying from running out of gas.

 

[GCullen94]

1. A gas planning strategy is a gas plan that I am 100% confident in, one that will get me and my teammate/buddy to the surface. I am NOT confident with saying eh surface with 50B/500/750PSI is good enough to ascend from a 30M/100' dive. I showed you in a previous post (#108) about the necessary gas and your normal "surface with 50b" is too liberal for deeper diving and too conservative for shallow diving. I want to know how many CF/L I need if I am sharing gas, I preferably would like to make a safety stop as well as I would most likely be approaching NDL's

2. Elements- KNOWING how much gas I need and not taking a guess based on what an instructor/mental told me, but rather use tools an instructor/mentor gave me to calculate my gas usage. It only takes a couple minutes. I want to get me and my buddy to the surface, safely. I want to take a minute at depth to solve the issue, come up in a slow, controlled ascent, and make my safety stops.

3. Many OW and AOW courses are taught way to fast. Come up with 50b or use the rule of thirds was used as a way to quickly cover gas planning. It isn't a gas strategy because what works at 30' doesn't work at 120'

4. I think by the last 5 pages of post... you will see that no it is not.

GC