Doing the math

[Agility]

It is astounding that anybody survives diving in Cozumel. 100+ feet on an AL80. And they do it over and over again, pushing NDL's if they are on tire gas, with total dive times in excess of an hour. Some of the calculations I hear about for required gas for rec diving are almost amusing when you think about the reality of tourist diving in Mexico.

And they rely on a DM having enough gas at hand in case of an emergency.
Of course you can and will get away with hopingall will go well. Dive equipment does fail, it is just a matter of time. I prefer filling half full tanks to the adrenaline rush of performing a CESA :blinking:

 

[industrious95]

It took us each about 300 PSI to ascend at a leisurely rate (around .5 feet/second) to the hang bar at 15 fsw.
The 3-minute safety stop used another 100 PSI apiece. The boat was bucking, so we were not especially relaxed there.
Swimming to the tag line and getting back on the boat (hard exercise in current and 3 foot waves) used another 100 PSI apiece.
So that means each of us needed 500 PSI to ascend and get back on the boat safely.

You got lots of good suggestions on gas management, but I do have a question and a comment for you:

You mention hang bar. If the boats bucking and the hang bar isn't hanging at a constant depth, don't hold onto it. Having perfect buoyancy skills would be nice and you can just float next to the ascent line/hang bar, but that will take practice, especially in rough water. While you're working on learning that, you can hold onto the ascent line where it meets the hang bar, but hold the rope very loosely so it slides through your hands. That will help you maintain constant depth. And 15 feet is a guideline. If you're at 18 feet that's okay too. 13 won't kill you either.

You mentioned having to swim back in a current. Were you on a dive boat? Any dive boat I've been on anchored with its bow into the current with the ascent line in the front of the boat. So when you finish your dive and you're tired, the current pushes you back towards the ladder. Was this on your friend's new boat who's just learning how to set it up?

 

[DukeAMO]

You mention hang bar. If the boats bucking and the hang bar isn't hanging at a constant depth, don't hold onto it. Having perfect buoyancy skills would be nice and you can just float next to the ascent line/hang bar, but that will take practice, especially in rough water. While you're working on learning that, you can hold onto the ascent line where it meets the hang bar, but hold the rope very loosely so it slides through your hands. That will help you maintain constant depth. And 15 feet is a guideline. If you're at 18 feet that's okay too. 13 won't kill you either.

True. I actually knew that the ascent line would be a better place to hang on, but was having trouble applying it. I was the first of a group of six who ascended at the same time, and someone else got the spots on the ascent line that were above 20 feet and below the bar. :-( The captain said that we'd better not get away from the line, and if we did get blown off, we should deploy our SMBs and be prepared to wait at least an hour for them to come get us. So, I was a little overly careful to stay close to the hang bar. What worked reasonably well was keeping a very light grip on the bar, more for reference, and letting it go when it moved a lot, but I was pretty much finning the entire time. It wasn't scary or anything, but it was definitely work. Practice will definitely help.

You mentioned having to swim back in a current. Were you on a dive boat? Any dive boat I've been on anchored with its bow into the current with the ascent line in the front of the boat. So when you finish your dive and you're tired, the current pushes you back towards the ladder. Was this on your friend's new boat who's just learning how to set it up?

It was a dedicated dive boat a couple hours off of the North Carolina coast, in the gulf stream, so it's a strong current at the surface. Waves were averaging about 3 feet; I think they'll run trips in up to 4-foot waves. The ascent line ran down from the bow, with the current flowing from bow to stern. The hang bar was directly below the boat, running from the ascent line toward the stern. The procedure was to finish your safety stop at the hang bar, then swim well below the propeller and ladder to the tag line, at least 10 feet behind the ladder. Surface, wait far enough back on the tag line that nobody can fall on you, and swim/pull your way back to the ladder along the tag line when it's your turn to get on the ladder. Wait 5 feet back until the crew says to go for it, then get up the ladder (a fins-on ladder) as quickly as possible. We ended up a good 20 feet or so behind the ladder on the tag line, so that was the swim back, and getting up the ladder with fins on was tough. They do exits with fins on and regulator in so you can swim back to the boat (or at least catch the tag line) if you fall off of the ladder. 2 or 3 people fell off each time we finished a dive. Exciting stuff, heh.

 

[Lorenzoid]

Calculating about 1500-1600 psi for minimum gas or rock bottom or whatever one wants to call it from 90-100 feet or so when my wife and I are doing our typical Al 80 dives in FL/Caribbean is a reason why we no longer dive that deep. Learning to do these calculations was an eye opener. My wife actually starting crying, blubbering that "we'll never be able to enjoy diving again." Knowledge hurts. We reached an agreement to limit our depth and to consider learning doubles in the future if we really want to see something at 100+ feet.

 

[Basking Ridge Diver]

Tatiana's thread got me thinking...
Hooray Nitrox.

I am curious - When you took your Nitrox class and I am assuming you took AOW - but I don't know this to be true. Did they review any of the calculations to determine your Gas requirements?
It surprises me that the classes have not done calculations to determine your Dive Plan needs - this is not a cut - I am only asking.
I am OW certified from YMCA - have been for a long time and last year finally took the SDI Solo Dive class (at my wife's request) - where it was hammered into us how to determine your Gas needs. I have not taken an AOW class - never needed it for the dives that I do - 100ft or less but my expectation was Nitrox or AOW would have covered this material. Is this not correct? My son was Jr.OW last year and I was thinking he is almost ready for Jr.AOW - he has more than 30 dives in various conditions and I am getting comfortable he is getting a handle on diving.

BTW in my opinion - spend the $80 and get the cable - you can use it to do your calculations and learn / look back at your dive profile - I learn a lot reviewing old dives and reviewing my actual vs planned.

 

[Roman_66]

The reserve gas plan? The DM said, "I don't use much air, so you can share air with me if there's any problem." Sound familiar?

I met the same "Gas managment strategy in Cozumel". Due my gas consumption is little bit higher than normal, and I knew that local DM is NOT responsable for your dive, as you are certified diver, I didn't find better strategy as to go up few meters and continue dive on half deep, to continue to keep enought gas for safety ascending for two divers. I didn't like it, because 30 ft between buddies, even on vertical, is NOT dive with buddy, but more solo.
I need to say that I used doubles, so had full redundancy of first and second stages. I have no idea what to do with one tank dive in this case, maybe only to not dive at all....

 

[NorCalDM]

In the dives you described your limiting factor was your NDL not the amount of air you had but you also didn't say how much air you actually had left when you got back on the boat. If your limiting factor is your NDL not your air why go to a larger heavier cylinder or cylinders when you will not get anymore bottom time out of it without getting trained for deco.

 

[Errol Kalayci]

Here is the math:
1) 1' Time to get squared away after donating gas to other diver so you are not rushing and cause another problem and let everyone relax a little
2) Ascent rate is 30'/min with safety stops (we spread them out starting at 50% depth by 30" pause and slower 30" to next stop) so to go from 100' to 50feet round to 2 minutes of travel time
3) 5 stops at 1' each as 30" pause and 30" to get to next 10'
4) SAC rate of 1 to account for slight stress
5) avg depth of ascent 4ata @ 100' + 1ata @ surface / 2 for average is 2.5 avg ata during ascent
6) 2 divers
work it out as (1' on bottom + 7' total ascent) x 2 divers is 16minutes of gas x sac of 1 = 16cft x 2.5avg ata = 40 cuft or 1600psi in AL80 where 100psi = roughly 2.5 cft.
100' dive need 40cft or 1600 psi in AL80
80' dive need 30cft or 1200 psi in AL80
60' dive need 20cft or 800 psi in AL80
if you are using nitrox 32 then you are allowed 30 minutes of bottom time before you hit NDL
On a true 100' dive you have 1400psi or 35cubic feet of useable gas before you hit minimum gas and time to start ascent with a single AL80 so that you would have enough gas for 2 divers to share gas making their prescribed ascent and safety stops all the way to surface.
Average diver has sac of .75 cubic feet per minute x 4ata (100'depth) they are using 3 cubic feet per minute and have 35 cubic feet of useable gas so they can stay less than 12 minutes before it is time to go. If they hit 1600psi and move shallower to say 80 feet then they must leave when it says 1200psi on AL80 spg....this is the way it works.
if you want to solve for other tanks just convert them to a number such that you know every 100psi equals x cubic feet which we call tank factor. for example 3500psi steel 100 tank would have a tank factor of 3 so every 100psi is approximately 3 cubic feet...close enough for us divers

 

[Basking Ridge Diver]

Errol -
Your numbers check out fine - of course.
My question for me - why am I carrying redundant air for 2? If I am on a charter in NJ - everyone is required to carry their own pony or redundant air source.
Diving in general - I have never seen or heard of two divers having catastrophic failure at the same time. So if each diver cares for themselves - I would cut your numbers in half. Everyone has their own approach - I am not saying yours is right or wrong - just my approach is different. But since I am recreational and dont cut razor thin margins - it seems to work for me.
If I have a true emergency - I am not doing deep stops with or without a buddy - I may chose to do a safety stop as normal but that will be decided by me based on the failure point(s) and how close I think I came to the NDLs.

 

[fire_diver]

It is astounding that anybody survives diving in Cozumel. 100+ feet on an AL80. And they do it over and over again, pushing NDL's if they are on tire gas, with total dive times in excess of an hour. Some of the calculations I hear about for required gas for rec diving are almost amusing when you think about the reality of tourist diving in Mexico.

I think the reason we don't hear about it more often is because 90%+ of divers will never be on scubaboard or any other diving forum. That means we won't have the chance to hear just how often things go wrong. Even with the low% we see here, there are still a surprising number of people who tell stories of being ignored when trying to tell the DM they are low on air.

I believe the reason we don't have more diver deaths and DCS stories is because of the conservation built into our dive tables, along with the fact that we often really can make fast ascents without getting bent as long as we are within NDLs. But you better not make that a routine habit or it will bite your butt one day.