Rule of Thirds & Shallow Rec diving

[Foxfish]

I just realized foxfish is the same person that asked and started this thread http://www.scubaboard.com/forums/advanced-scuba-discussions/470422-how-low-spare-air.html We are dealing with someone that likes to push the envelope in terms of running low on gas. Foxfish there is NO POINT in running low. Get bigger cylinders or doubles if you want more gas. Don't put your and your buddy's life at risk because you want to run liberal gas plans.

Are you seriously suggesting that anyone who begins an ascent at around 80 b or 1160 psi on a recreational dive from 30 m with a 12 L tank is showing a reckless disregard for safety. How many other people here consider that to be correct.

Yes, have a good read of the thread. You'll read similar arguments to the one on this thread. You may be relieved to know there isn't much comment from me. If you do find any of my comments that indicate I like to push the limits in a way that is unsafe let me know.

---------- Post added December 9th, 2013 at 09:37 PM ----------

I said they HAD been calm. No idea what happened to their breathing rates after all hell broke loose

That's not how I'd read it.

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.

 

[GCullen94]

First off this thread started out with an 11L tank, putting you at 100B (96.8B exact but I've never seen a gague like that so round up). Second of all even with a 12L tank you need at a MINIMUM 83B... SO YES I AM. The minemum gas number is exaclty that. It has you on the surface with a drained tank. If you go below that minimum number then you just DO NOT have the gas to reach the surface. If we were using a 15L you would only need 60B for Min Gas. Foxfish I have no clue why you are being this stubborn, many divers use this method because we like to go home with out a major incident if something does go wrong. EVERYONE I dive with uses a min gas format. Everyone, not just GUE, but also the PADI trained divers I dive with. They have either already learned min gas from somewhere else or they get a quick lesson on the boat and realize how much more useful and safe it is than surface with 50B.

 

[DaleC]

I am not a technical diver. Nothing more than AOW right after OW and basic Nitrox. Nothing about RB taxes or confuses me. and I am an admitted math klutz.

I was confused about Foxfish's position regarding his gas planning (or lack thereof).

First he said planning was unnecessary for AOW diving. All one needed was the "be back on the surface with 50b" rule. Now it seems he has a spread sheet of preset values for different depths and tank sizes. That's a lot of calculating for not needing to calculate.

Sort of sounds like basic gas planning to me.

So, it would appear he initially told others not to do something he does himself (calculate his gas needs) or he has been making up the numbers as the thread progresses, to justify his position. If the latter, it's weird that someone would do that work to win an internet argument but not to know how much gas they needed to dive safely.

Either way, I'm glad he's finally agreed rock bottom gas planning has a place in AOW diving.

 

[Foxfish]

I was confused about Foxfish's position regarding his gas planning (or lack thereof).

First he said planning was unnecessary for AOW diving. All one needed was the "be back on the surface with 50b" rule. Now it seems he has a spread sheet of preset values for different depths and tank sizes. That's a lot of calculating for not needing to calculate.

Have a look back at post # 134. Nothing has changed.

Do you seriously believe that beginning an ascent at around 80 b or 1160 psi on a recreational dive from 30 m with a 12 L tank is showing a reckless disregard for safety.

If you look back at the start of the thread, a number of people seemed to think it was okay or even conservative.

 

[GCullen94]

foxfish... lets do a little math one more time. After all you are an engineer, you should be able to follow some BASIC, calculations. I will do them in metric and imperial.
For metric we are assuming

30L stressed breathing rate for both divers
1 minute at depth to solve issue
10m/minute ascent
safety stop at 5m for 3 minutes

Imperial we are assuming
1CFM for both divers
1 minute at depth to solve issue
30FPM ascent rate
safety stop at 15feet for 3 minutes


Math time!

30L x 2 divers x 1 minute x 4 ata = 240L
60L/M x 3 minutes x 2.75 ata= 495L
60L/M x 3 minutes x 1.5 ata = 270L
60L/M x .5 minute x 1.25 ata = 38L

= 1043 L of gas needed


1CF/M x 2 divers x 1 minute x 4 ata= 8CF
6 minutes gas time x 2.75ata = 16.5 CF
6 minutes gas time x 1.5 ata= 9 CF
1 minute gas time x 1.25 ata = 1.25 CF

= 35CF of gas needed

Now we will calculate gas in different tanks: For metric divided L required by tank capacity in L to get B required. For imperial we use tank factors, take CF required divided by tank factor and multiply by 100 to get PSI needed.

AL80 (TF 2.5)/11L= 1400 PSI, 100B (ROUND UP to more conservative 10B)
HP100/12L= 1200PSI, 90B(1166, rounded up to more conservative 100 PSI)
HP120/15L= 1000PSI, 70B
HP130/18L= 900PSI, 60B


again these are MINIMUM numbers. Everything depends on tank choice. For the record, you started out with saying "surface with 50B" Then with was "50B + 10B for every 10M" next thing you make a spread sheet.

 

[Foxfish]

foxfish... lets do a little math one more time. After all you are an engineer, you should be able to follow some BASIC, calculations. I will do them in metric and imperial....

gain these are MINIMUM numbers. Everything depends on tank choice. For the record, you started out with saying "surface with 50B" Then with was "50B + 10B for every 10M" next thing you make a spread sheet.

You're getting there. After you've read my posts and checked the cacluations another ten times you should have it about right.

 

[GCullen94]

No fox fish... holy crap if anyone is interested in some eh light deco theory reading I suggest this http://www.rebreatherworld.com/gene...6994-deep-stops-debate-split-ascent-rate.html I swear foxfish and ross are the same person. Foxfish Those calculations have never changed, I added larger tank sizes which changed the ascent pressure. There is a fundamental difference between the two methods that you are absolutely FAILING to grasp. Min Gas/Rock Bottom (the way I have been doing things) uses a minimum cubic footage/liters of gas and then converts that into pressure. You just take a pressure for all tanks and say that will be good enough when CLEARLY it is TOO LIBRAL for some tanks/depths and TOO CONSERVATIVE for some tanks/depths. You are impossible to talk to or reason with.

And also way to cut the math out of my post where I SHOW YOU THAT YOUR METHOD IS INSUFFICIENT and instead try and quote me out of context. Thanks buddy

 

[Storker]

There is a fundamental difference between the two methods that you are absolutely FAILING to grasp. Min Gas/Rock Bottom (the way I have been doing things) uses a minimum cubic footage/liters of gas and then converts that into pressure. You just take a pressure for all tanks and say that will be good enough when CLEARLY it is TOO LIBRAL for some tanks/depths and TOO CONSERVATIVE for some tanks/depths.

...as demonstrated clearly here:



For both cases, I've used a normal gas consumption rate of 15L/bar·minute, since that number has been floating around in this thread for some time now. In one case, I haven't allowed one iota of margin for an inaccurate SPG or having just a tad of air after surfacing for inflating my BCD and/or my dry suit (the latter, of course is not bloody likely for Foxfish, as he doesn't regard dry suit diving to be "basic level"). In the second case, I added an extra margin of 20 bar after surfacing to allow for DS inflation or other minor deviations from the standard parameters for the rock bottom calculation.

Assuming we reach the surface just at the time we're sucking my tank empty and stress levels increase even further, we see that:

• With a 15L tank, the 50b+10b/10m "rule" is more conservative than rock bottom at all recreational depths
• With a 12L tank, the 50b+10b/10m "rule" is more conservative than rock bottom at depths down to 30m
• With a 10L tank, the 50b+10b/10m "rule" is more conservative than rock bottom at depths down to 20m and inadequate below 20m.

Allowing for an extra margin of 20 bar residual gas at the surface, for inflating BCD and DS, or to have just a little extra in case we need a few more seconds at depth than the one minute we've calculated for, or in case my SPG is slightly inaccurate, we see that:

• With a 15L tank, the 50b+10b/10m "rule" is more conservative than rock bottom at depths down to 25m and inadequate below 25m
• With a 12L tank, the 50b+10b/10m "rule" is more conservative than rock bottom at depths down to 12m and inadequate below 12m
• With a 10L tank, the 50b+10b/10m "rule" is inadequate at all depths below 5m

There's no problem, of course, to include more conservatism than the standard rock bottom method allows for, but considering Foxfish's apparent reluctance in previous posts to surface with more air left in the tank than strictly necessary it seems kind of weird that he wants to use a gas calculation model that's more conservative than min gas/rock bottom.

Also, I included the 10L size since a 10Lx300bar is the standard rental tank up here. But I guess that's not relevant for Foxfish. As we all know now, we dive dry up here and he doesn't regard dry suit diving to be "basic level".

 

[Storker]

By the way, a small piece of trivia:

I just downloaded my latest dives from my AI PDC to my PC. As previously mentioned, my normal gas consumption rate these days is some 14-15 SLM. My last couple of dives were in crappier viz than I've experienced before, some 2m viz above ~15m depth, and apparently that stressed me more than I realized. My consumption jumped from below 15 to almost 20 SLM in those dives. From very moderate stress caused by low viz and a bigger challenge to keep contact with my buddy.

I'm starting to think that 30SLM in a real emergency isn't much of an exaggeration. Rather the opposite...

 

[Foxfish]

Refer to post # 316.