Rule of Thirds & Shallow Rec diving

[nimoh]

I'm an extremely novice diver, 11 dives in fact, and I've gained a lot from this thread. I clearly see the benefit of Rock Bottom/minimum gas planning over the "back on the boat with 50b" theory....

The back on the boat with 50b thing is like asking you to drive around town for a few hours then show up at my house with exactly an 1/16th of a tank of gas in your car...all the while making sure your buddy's vehicle (possibly a gas guzzling s.u.v) makes it as well by donating your fuel to him should he need it. The way I see it you're gonna do one of two things, show up with either too much gas or not enough. Thus cutting your sight seeing short or ending it either dangerously low or running out. I'd rather know exactly what fuel level I needed to head for home safely (R.B.) than to know how much fuel to show up with. (50b 500psi)

Rock bottom or minimum gas gives you the tools to get you back with enough gas, for you and your buddy, without having to GUESS. I've watched the entirety of this thread and even with the stubbornness of some and frustrations of others, one thing has been clear the whole time, DOING THE MATH IS BETTER THAN GUESSING. I thank all of you who participated in this and helped teach me something along the way.

to me, this post makes this thread worthwhile.

 

[Doppler]

I'm an extremely novice diver, 11 dives in fact, and I've gained a lot from this thread. I clearly see the benefit of Rock Bottom/minimum gas planning over the "back on the boat with 50b" theory....

The back on the boat with 50b thing is like asking you to drive around town for a few hours then show up at my house with exactly an 1/16th of a tank of gas in your car...all the while making sure your buddy's vehicle (possibly a gas guzzling s.u.v) makes it as well by donating your fuel to him should he need it. The way I see it you're gonna do one of two things, show up with either too much gas or not enough. Thus cutting your sight seeing short or ending it either dangerously low or running out. I'd rather know exactly what fuel level I needed to head for home safely (R.B.) than to know how much fuel to show up with. (50b 500psi)

Rock bottom or minimum gas gives you the tools to get you back with enough gas, for you and your buddy, without having to GUESS. I've watched the entirety of this thread and even with the stubbornness of some and frustrations of others, one thing has been clear the whole time, DOING THE MATH IS BETTER THAN GUESSING. I thank all of you who participated in this and helped teach me something along the way.

Really neat to see someone who should get it, gets it! ONYA mate.

 

[boulderjohn]

When I am teaching OW students and we reach this point in the class, I tell them that a lot of boats have a policy saying that you are to get back on the boat with 500 PSI. I then give them a depth and ask them how many PSI they should have at that depth before they ascend if they are to get on the boat with that much gas. This is usually followed by different versions of "I have no idea." Then there is some guessing, and when they guess, it is usually far too little. So we talk about it and get what is still a rough idea on what THEY would need to get on the boat with that many PSI. Then I ask, "OK, what if your buddy signals OOA at the moment you are about to ascend?"

Then I give them my handout on simple gas management.

 

[lilnik10]

This thread has also made me think more about my gas planning. My husband and I learning 1/3 out, 1/3 back, leaving 1000 PSI to ascend and do our safety stop (we usually rent Al 80). I have never really thought how much actual air that leaves is with and if it's enough to get both of us to the surface in an emergency. Luckily, most of our dives have been above 60-70 ft so I we have a been safe, but I am definitely going to doing some better planning for our deeper dives. Thanks for this thread!


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[tattooedjeeper]

Really neat to see someone who should get it, gets it! ONYA mate.

This thread has also made me think more about my gas planning. My husband and I learning 1/3 out, 1/3 back, leaving 1000 PSI to ascend and do our safety stop (we usually rent Al 80). I have never really thought how much actual air that leaves is with and if it's enough to get both of us to the surface in an emergency. Luckily, most of our dives have been above 60-70 ft so I we have a been safe, but I am definitely going to doing some better planning for our deeper dives. Thanks for this thread!

I knew I wasn't the only one gaining something from this ...

 

[Foxfish]

All of these numbers were gotten post-incident, as I wasn't watching their air or profile....but it's what I remember being told. I think they were at 90ft when they started the ascent. I think they were at 1100psi when they started their ascent. Neither of them had any reason to be breathing heavy as it was a good dive. Their depth/time profile was fairly square, descending to the sand and then working their way up from 100ft-90ft along the wreck. It was the second dive of a two-tank dip, all running PDCs with no added conservancy and relatively aggressive algorithms. We were all running a really hot nitrox mix.

The point is: They were diving exactly as you prescribed. 80b is 1176psi, which you said you'd use to ascend from 100ft (30m). They ascended slightly later from slightly shallower. It would've been fine, but right at ascent one of the yoke o-rings blew out and emptied the tank with surprising speed. They ascended together like a textbook (we had been practicing for our DM course, so they had just done like 30 air-sharing ascents in a pool) and ran out of gas right before they reached their safety stop. Both were pretty comfortable in the water. Literally everything you were saying you'd do almost got them killed.event really sticks out in my mind. It just so happens that it was fantastically similar to what your described profile would be.

You previously said:

.I know 5 or 6 divers that were taught to use the "50b rule" that misjudged the air needed to get to the surface and have run out and performed a CESA from 50ft.

Did these divers do the CESA from 50 ft (about 15 m)?

---------- Post added December 9th, 2013 at 04:34 PM ----------

Near the start of this thread Foxfish had no minimum gas plan, just the 50 bar rule. A few pages later minimum gas was 70 bar on a 30m dive to arrive on the surface at 50 bar, and now it is 81 bar.

My guess is that 50 more pages in this thread, and Foxfish will have a minimum gas plan of 110 bar for a 30m dive.

Have a look at post #134. 80 b for a 12 L tank, 70 b for a 15 L tank.

 

[victorzamora]

Did these divers do the CESA from 50 ft (about 15 m)?

Two of them did, and ended up slightly bent. Mostly they were lucky because they were diving shallow, conservative profiles in EXTREMELY benign conditions. Two had to ditch weights, but made it. I retrieved their weights. Two had to be saved by other divers that just happened to see them.

 

[Foxfish]

I'm an extremely novice diver, 11 dives in fact, and I've gained a lot from this thread. I clearly see the benefit of Rock Bottom/minimum gas planning over the "back on the boat with 50b" theory....

The back on the boat with 50b thing is like asking you to drive around town for a few hours then show up at my house with exactly an 1/16th of a tank of gas in your car...all the while making sure your buddy's vehicle (possibly a gas guzzling s.u.v) makes it as well by donating your fuel to him should he need it. The way I see it you're gonna do one of two things, show up with either too much gas or not enough. Thus cutting your sight seeing short or ending it either dangerously low or running out. I'd rather know exactly what fuel level I needed to head for home safely (R.B.) than to know how much fuel to show up with. (50b 500psi)

Rock bottom or minimum gas gives you the tools to get you back with enough gas, for you and your buddy, without having to GUESS. I've watched the entirety of this thread and even with the stubbornness of some and frustrations of others, one thing has been clear the whole time, DOING THE MATH IS BETTER THAN GUESSING. I thank all of you who participated in this and helped teach me something along the way.

Glad you've found the thread helpful. As others have said, Scubaboard is a very small subset of divers. Many of the people posting on the thread arguing the merits of rock bottom calculations have done technical training or more advanced training than your average OW/AOW diver. In my experience, and I'd suggest in general, most recreational divers can't be bothered with this kind of quibbling and the hassle of doing gas management calculations. It is not necessary. They simply need to start with a couple of numbers or simple rules such as the one I give in #134 to get them back to the surface with 50 b.

Oh and 50 b is about 725 psi, not 500 psi as you noted.

---------- Post added December 9th, 2013 at 04:55 PM ----------

Two of them did, and ended up slightly bent. Mostly they were lucky because they were diving shallow, conservative profiles in EXTREMELY benign conditions. Two had to ditch weights, but made it. I retrieved their weights. Two had to be saved by other divers that just happened to see them.

So what about the divers in the story you just related. Did they also need to do an emergency CESA from 50 ft?

All of these numbers were gotten post-incident, as I wasn't watching their air or profile....but it's what I remember being told. I think they were at 90ft when they started the ascent. I think they were at 1100psi when they started their ascent. Neither of them had any reason to be breathing heavy as it was a good dive. Their depth/time profile was fairly square, descending to the sand and then working their way up from 100ft-90ft along the wreck. It was the second dive of a two-tank dip, all running PDCs with no added conservancy and relatively aggressive algorithms. We were all running a really hot nitrox mix.

The point is: They were diving exactly as you prescribed. 80b is 1176psi, which you said you'd use to ascend from 100ft (30m). They ascended slightly later from slightly shallower. It would've been fine, but right at ascent one of the yoke o-rings blew out and emptied the tank with surprising speed. They ascended together like a textbook (we had been practicing for our DM course, so they had just done like 30 air-sharing ascents in a pool) and ran out of gas right before they reached their safety stop. Both were pretty comfortable in the water. Literally everything you were saying you'd do almost got them killed.event really sticks out in my mind. It just so happens that it was fantastically similar to what your described profile would be.

 

[Foxfish]

I came up with 88B in a 12L cylinder. It's 95B in an 11L The problem is foxfish, the numbers we are throwing out has you sucking the tank empty at those numbers. You, using your method, would run out of gas somewhere in your safety stop and have to make a bolt to the surface, causing the biggest pressure change to happen (last 5M) VERY quickly which increases your chance of taking a hit. I'd rather follow min gas laws or use a short cut that actually works (I have outlined it before) rather than your "surface with 50B" rule. Yes you are being rather impossible. You fail to recognize the word minimum.

One plus of this thread, I am much more comfortable working in metric now.

So here are my numbers. For a 12 L cylinder you need about 81 b. For an 11 L cylinder it goes up to about 89 b.

 

[victorzamora]

Why would you go through all of the hassle of creating that huge Excel table and not do it properly? We've given you PLENTY of examples of why that is woefully insufficient. We've asked you plenty of specific questions that you've dodged.

You've also dodged the most important question: what happens if your buddy goes OOA right before you start ascent? The answer: Bent or Dead. The better question is: Are you aware of this?? Are you aware that it's not much more work to just do it right? Are you aware that by spreading this, you could end up bent or dead? Are you aware that I have 7 acquaintances practicing exactly what you're preaching that could've ended up dead, and 2 that got bent, were it not for luck.

I'm asking you all of that VERY specifically all of the questions above. The other question I have for you is this: Would you use MinGas/RockBottom if I sent you the spreadsheet and custom-tailored it to any and all of your needs? I'm willing to make this all in metric for you.