Rule of Thirds & Shallow Rec diving

[Storker]

Why do you think divers are required to surface with 50 b?

I really wouldn't know. I'm not an instructor, I'm a basic rec diver with just a few dozen dives in my log.

But I do teach. And I know about... let's call them "not top-achieving students". So I've got a hypothesis: Maybe it's a very, very, very simple rule ensuring that even those who are too dense to understand the concept of rock bottom gas planning still has some kind of safety margin. It's pretty far from perfect, and it's probably inadequate at depth, but it sure beats the heck out of routinely sucking your tank empty because you haven't even been introduced to the idea of reserve gas.


BTW, what has your 50 bar question to do with the post that you quoted?

 

[Foxfish]

Your take-home messages from all the crap contained in this thread is surely that gas management is not so complex that it can be ignored; and secondly and perhaps most importantly THERE ARE LOTS OF VARIABLES. One can choose to take that into account or ignore it. Seems most of the folks responding to your post choose the former.

I don't have a problem with people wanting to do rock bottom calculations. Personally I'm not convinced it is necessary. It implies that the approach taken by the main diving agencies is inadequate or wrong. I haven't seen any practical evidence to support that claim.

---------- Post added December 5th, 2013 at 09:42 AM ----------

But I do teach. And I know about... let's call them "not top-achieving students". So I've got a hypothesis: Maybe it's a very, very, very simple rule ensuring that even those who are too dense to understand the concept of rock bottom gas planning still has some kind of safety margin. It's pretty far from perfect, and it's probably inadequate at depth, but it sure beats the heck out of routinely sucking your tank empty because you haven't even been introduced to the idea of reserve gas.

I taught secondary mathematics for a while. Some students found it intimidating. I see no reason to make this an obstacle for these people to enjoy recreational diving.

 

[supergaijin]

I don't have a problem with people wanting to do rock bottom calculations. Personally I'm not convinced it is necessary. It implies that the approach taken by the main diving agencies is inadequate or wrong. I haven't seen any practical evidence to support that claim.

Accidents and Incidents

and... take a look at this:
[video=youtube;W30cufYc_ZI]http://www.youtube.com/watch?v=W30cufYc_ZI[/video]

 

[victorzamora]

I know it because it does not comply with what divers are taught in their OW and AOW which is to surface with 50 b or 725 psi. Do you think that is wrong?

Yes. It is absolutely and unequivocally wrong.

Well, while the number and intent isn't wrong, it's absolutely insufficient and instills absolutely the wrong idea in the heads of divers. Saying "surface with 50b" (500psi is what is taught over here) is woefully insufficient and completely misleading. I asked my instructor if there was a better way of knowing, and he said "No, just experience." He was taught 500psi, he taught me 500psi. It's what my wife was taught, and what all my friends have been taught. A few of us have started realizing that there's OBVIOUSLY a better way of doing it, and have sought those ways out. Rock bottom/min gas is it. Do I do this every dive? No. Do I have these tables cut and on my slate at all times? No (but I will). Have I run my tank dangerously low? Absolutely. However, on every dive I use knowledge of these principles to make educated guesses. I was in a river max depth 40ft on EAN24 (residual nitrox, topped with air). We dove for a while, then got shallower as tanks got low. By 500psi, we were in 6ft of water. My buddy sucked his tank DRY. We were having fun, so he asked me for my long hose. I donated. When we sucked MY tank dry, we stood up. That was two tanks at 0psi. Smart? Not really. Dangerous? Not overly so. The conditions were incredibly benign.

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.

 

[MXGratefulDiver]

I don't have a problem with people wanting to do rock bottom calculations. Personally I'm not convinced it is necessary. It implies that the approach taken by the main diving agencies is inadequate or wrong. I haven't seen any practical evidence to support that claim.

I have seen plenty of evidence to support that claim. All anyone has to do to see that evidence is spend some time in the Incidents and Accidents forum ... but I've also seen it up close and personal. Every year we have one or more fatalities in Puget Sound. During the 13 years I've been diving here, by far the most common reason for those fatalities has been someone running out of air. Of those, some that I knew of were people who thought they were following their training, and found out the hard way that their training was inadequate for the dive they were attempting to do.

You seem to be of the impression that OW/AOW training is the end-all/be-all of recreational diving. As a dive instructor I can assure you that is not the case. Training doesn't teach you how to dive ... it teaches you how to learn diving. You learn by doing ... by experiencing different environments ... by questioning everything, particularly asking why they taught you what they did.

OW/AOW training leaves a lot of gaps in a diver's knowledge ... especially for the diver who's planning on diving in more than a warm-water, guided dive environment. Perhaps the most significant value to a forum like ScubaBoard is learning what those gaps are, and using the knowledge of more experienced divers to help you fill them. Or you can assume that you learned everything you'll ever need to know in two rather limited classes and argue with those people ... which makes for interesting conversation, but doesn't do much to increase your knowledge about diving ...

... Bob (Grateful Diver)

 

[runez]

Why do you think divers are required to surface with 50 b?

I have questioned the reason behind the "magic" 50 bars reserve gas advocated by many dive agencies, why not say, 40 bars or 60 bars? I made some calculations before to "interpret" this requirement, appended below.



I learnt in my OW that the 5m/3min safety stop is not mandatory, so, 50 bars is actually sufficient for most scenarios under the stipulated guidelines (i.e. 18m/min ascent rate, optional safety stop).

"Ascend at 50bars" is also hell a lot easier to remember than say: 110bars@30m, 70bars@20m, 40bars@10m etc. It is also easier at dive briefings for the divemaster to communicate to his/her divers.


So while I believe and practice minimum gas calculations, I also don't think it is suitable for every diver. I believe a part of diver safety is also making it simple to teach and simple to remember (i.e. KISS - Keep It Simple Stupid!). This is probably the approach the major dive agencies have taken.

What're the chances that we'll see more bent divers from human error if all OW courses taught detailed gas planning with varying minimum gases at different depths, depending on SCR and other variables? We can't tell for certain, but my bet is that we will....

On a positive note, the "50 bar" rule can't be all that bad, since we're not seeing a huge number of bent or dead divers, although the actual number of bent divers or close calls can be hard to estimate as it is possible many cases go unreported. I guess "huge" in this case is also a relative word..

An open question to fellow divers: if you disagree with the "50 bar" rule, how would you propose to change it?

I taught secondary mathematics for a while. Some students found it intimidating. I see no reason to make this an obstacle for these people to enjoy recreational diving.

I agree, KISS is sometimes the best approach!

 

[GCullen94]

I note that you've calculated that you need 110 b (1600 psi) to ascend safely from 30 m (100 ft) with an al80 (about 11 L). That seems very conservative to me. Maybe you have a very high air consumption rate?

Foxfish it is not about the pressure to get you home but rather the cubic footage. If I was doing this dive on double 100s, my min gas pressure would be 700 PSI, this was planed using the following criteria:

-Ascent rate of 30FPM
-A stressed SCR rate of 1 CFM for each diver
-Sharing gas on ascent
-One minute at depth to get on buddy's gas, solve immediate issues.

So now to walk you though the 3 of the profiles I posted for the deep 100' dive on an AL80

Profile one: stop at 50 for one minute and 15 for 3

So we need (for each diver)
1 minute of gas at 100
2 minutes of gas to go to 50
1 minute at 50
1 minute to get to 15
3 minutes at 15
.5 minute to go to the surface

Because we are sharing gas we need double the amount of breathable gas time we have. Because we are using a SCR of 1 for each diver just multiply gas time needed x ATA = CF needed to reach surface. Following?

2 minutes of gas at 4 ATA = 8 CF
4 minutes of gas at 3.27 ATA = 13 CF
2 minutes of gas at 2.51 ATA = 5 CF
2 minutes of gas at 1 ATA = 2 CF
6 minutes of gas at 1.5 ATA = 9 CF
1 minute of gas at 1.2 ATA = 1 CF

total= 38 CF, rounded up to 40 to make things easy.

That took about 60 seconds to do. If you wanted to make it even faster, take gas time needed X 1/2 max ATA... 17 minutes of gas *2.5 ATA = 42.5 CF... easy enough?



Now let's look at a GUE/UTD/DIR ascent. They use an ascent rate of 30FPM until half max depth, then 30 second stops every 10 feet from 1/2 max to the surface and spend 30 seconds moving in between stops.

2 minutes of gas at 4 ATA = 8 CF
4 minutes of gas at 3.27 ATA = 13 CF
2 minutes of gas at 2.51 ATA = 5 CF
2 minute of gas at 2.21 ATA 4.4 CF
2 minutes of gas at 1.9 ATA = 3.8 CF
2 minutes of gas at 1.6 ATA = 3.2 CF
2 minutes of gas at 1.3 ATA = 2.6 CF

total gas needed= 40 CF

Again took about 60 seconds to do. To make it even faster do the same as above, half max depth in ATA X gas time needed to surface = 40 CF
16 minutes * 2.5 ATA= 40 CF



Lets do a 3 minute stop at 15 feet and skip the deep stop this time..
.
2 minutes of gas at 4 ATA = 8 CF
6 minutes of gas at 2.76 ATA = 16.5 CF
6 minutes of gas at 1.5 ATA 9 CF
1 minute of gas at 1.2 ATA 1 CF

34.5 Round up to 35 CF needed for ascent.

Took about 45 seconds that time to calculate. We can also use the shortcut way again because 15 minutes X 2.5 ATA = 37 CF.

So now that we know how many cubic feet we need let's take a look at what pressure that is in different tanks. Different tanks have different tank factors. A tank factor is how many cubic feet there are in 100 PSI. Here is a basic chart


AL 80- 2.5
LP 85- 3
LP 95- 3.5
LP 104- 4
LP 120- 4.5
HP 100- 3
HP 120- 3.5
HP 130- 4

To calculate min gas pressure take CF/TF= min gas pressure x 100

So for an AL 80, our TF=2.5, We need 40CF for our chosen ascent profile. Therefore 40/2.5= 16 (x100) = 1600 PSI needed to ascend safely if SHTF.

If we were using HP130's for this same dive we'd use the following formula: 40/4= 10 (x100) = 1000 PSI needed to safely ascend.

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.

 

[TSandM]

Foxfish, I was taught what you were taught. "Back on the boat with 500 psi, because gauges aren't very accurate below that, and you might be lower on gas than you think." It seemed reasonable, and I didn't give it a great deal of thought, because I was spending most of my thinking trying to figure out how not to cork every other minute.

The difference between you and me is that, when I had about 25 dives or so, I attended a gas management seminar given by none other than NW Grateful Diver. In the seminar, he basically presented the gas management information from the GUE Fundamentals class. I sat, dumbstruck, and wondered, "What ELSE didn't they tell me?" I saw that the information I had been given was truly minimal to inadequate, and realized there were some very good tools to do better, more comprehensive gas planning. I was delighted. You scoff. I don't know why; it seems as though most people would be happy to have more information and better tools.

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.

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.

 

[tracydr]

I don't have a problem with people wanting to do rock bottom calculations. Personally I'm not convinced it is necessary. It implies that the approach taken by the main diving agencies is inadequate or wrong. I haven't seen any practical evidence to support that claim.

---------- Post added December 5th, 2013 at 09:42 AM ----------



I taught secondary mathematics for a while. Some students found it intimidating. I see no reason to make this an obstacle for these people to enjoy recreational diving.

The most common cause of death in scuba fatalities is running out of air. So, I'd say that the common way of diving does in fact,have issues. If it worked, nobody would be running out of air and dying.

 

[DaleC]

Dry suit and cold water? Not exactly my idea of a basic level scuba dive.

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.

...taught secondary mathematics for a while. Some students found it intimidating. I see no reason to make this an obstacle for these people to enjoy recreational diving.

Today's main training agency also doesn't believe you need to know how to swim.

What you are seeing here, is that some of us don't believe this is enough to create independent divers, even at the basic level. We have all been where you are, done what you are doing, and then found it wanting when moving forward.

The only way I could envision non-planning as you are suggesting, is via vintage equipment diving. As I have described in the past though, that form of diving comes with it's own skill set / knowledge base that I have not seen described here.

Although it feels like banging ones head against the wall, this thread has been interesting. I just reviewed my OW and AOW material and was amazed at how much time we spent learning to plan repetitive dives using the tables. A lot of quizzes/worksheets.

Then I looked at my basic EAN course material. Again, lots of MOD and adjusted depth/time calculations. I distinctly remember Slamfire cheating by having the formulas programmed into his cell phone.

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.