How much gas should one surface with

[Rockhound]

was pretty close to what I believe, as well. I plan my dives to be at the safety stop with a certain amount of air, not on the boat or at the surface--but at the stop. My plan is for 700 psi on my AL80 on non-deco dives, but I'm often at the hang bar with more than a 1000 due to my buddy's air consumption.

However, I'll admit to "hanging" there until I'm at 250 or less. If the current is light, I'll just float around near the boat and look for the big critters that come by to investigate us or the boat.

I try to avoid breathing the tank completely down to keep my 1st stage internals dry, but I do like to breathe my tank pretty low. In the areas where I dive (on the rigs, the Flower Gardens, or Stetson Bank), some of the best parts of the dive happen in those last minutes at 15'. When I'm really breathing well, I can stretch those last few psi a long, long time. I've caught some grief for this on "air checks" at the end of the dive, but most of the time the crew is aware that I'm just taking an "extra" long safety stop.

As my reward, I've seen a whale shark, lots of manta rays, a big tiger shark, several school sof hammerheads and silkies........I've seen all those on my hang stops at one time or another.

 

[JimC]

Originally posted by MikeS
Jim,

Would you agree, that since a controlled ascent from 15 fsw is not only possible but also highly practical, once you safely reach that point your obligation for maintaining a reserve for your buddy has been met? If not when is it?

Great description. Can you provided some insight and perhaps and example of how you decide what the “find the line” and “start the ascent” psi values should be?

Mike

After you have completed your schedualed and planed stop, if your that close to surface.. I would say you have met your buddys gas obligations. I would also make sure your buddy is aware that after the safty stop, you are going to be swimming around at 10fsw.


As for figuring the numbers, take your SAC, your buddys SAC, add in a bit of safty margin. Then figure your total accent gas requierments, including deco/safty obligations. Figure that back to psi on your tank. Record that number. For my 'find the line' gas I figure the lenght of the swim depending on the size of the wreck, add in a 2-3 minute safty margin for finding the line or seeing someting real cool on the swim back, figure the gas, and convert to psi.

Eg:

My SAC is 0.5, my wifes SAC is 0.4, total is 0.9 so I round out to about 1.5 for safty and stress factors and typicaly cold water. We both dive the same tank, an AL80. (Which dousn't hold 80cf @ 3000psi.. none do)

I like deep stops, so my typical safty stops are 1 min @ 40, 30, 20 and 10 with a 30f/min accent. Those are the constants I typicaly run most dives.


Say we are diving a wreck that is 150 feet long, with a mild current that is 90fsw to the sand.

Accent gas:
90fsw to surface is 3 minutes + safty stops.
Time * SAC * Depth (in abs atmospheres) = cf gas used is the basic formula.

Accent from 90fsw ~12cf
Safty stops ~ 10cf

So I would set my accent gas somewhere between 22 and 25cf, depending on comfort level and my gut feel. Which when turned into a PSI rating is: 937

So I dilegently fudge up some more and write 950 on my slate.

Then I figure out from instict, chating with other divers, etc that its a 2 minute swim from the firthest place to the line, add in 2 minutes of lost/oh cool look at that time, all at the sand for

4min@90fsw ~ 24cf
or 900psi, add that to my 950 from eariler and I have a 1850psi 'find the line'


So if I am on the wreck and I hit 1850, I start to head for the line. No rush, still enjoying the dive, still having a blast. If I hit the line and have more than my 950, I can putz around. If I am not at the line by 950.. its up we go for a nice drifting accent.

I'll probably have a good 600psi left after my safty obligations with which I can swim to the boat with, etc.

 

[SubMariner]

Got your attention? Good!

If more people paid attention to safety rather than obsession with the MINIMUM amount of air to surfaced with, I think the're be fewer dive problems.

In the case of no-decompression diving there is the determination of the SAFE time limit. This means not just the no decompression limit (NDL) but taking into account physical activity, temperature, hydration level, etc. IOW, not diving to the NDL limits.

THEN you can consider air use.

Frankly, I am a strong believer in using the caver diver's "Rule of Thirds":

One third out.
One third back.
One third for contingency.

This obsession with having to dive until you have 500 vs 700 vs 1000 psi (or whatever) is left at the end of the dive is puerile. Aren't we all grown up enough to leave that in the playpen where it belongs?

Getting off my :box: now,

~SubMariner~

 

[Rockhound]

has a valid point. While I was focused on "answering the question" regarding "how much air", I left out the reality that all proper dives are PLANNED, and that plan includes much more than just "how much air" the diver wants to have at the "safety stop" or at the surface.

In most cases, this means SAFE adherence to NDL's, "higher rate" buddy's SAC and even any future planned dives subject to this nitrogen load. Air management is a key part of that planning, but it is not the end-all, tell-all of a successful dive or dive trip, nor should it be.

At Stetson Bank, my typical 6:00 am morning dive is number 6 of 8, with my last dive, a night dive at the Gardens, ending only about 7 or 8 hours earlier. Even diving on an AL80, I am up against the NDL before I run low on air. But, I still plan a turnaround for the mooring at 2000psi, with my ascent starting at least 5min before I hit deco on my computer. In that case, "air" is the least of my considerations, unless for some reason my SAC is higher than normal (due to the infamous mask blaster current at Stetson, for example). I know my consumption well enough on most dives to predict what will drive my decision making process, but I am always willing to adjust. My safety stop will be extra long, with stops at 20' and 15', just for grins. After all, getting old I can't avoid without dying, but getting bent is another thing.........

Anyway, I'm not sure about the "playpen" comment, but I do understand the point, and I'm in agreement with the sentiment, if not the language.....

 

[JimC]

SubMariner, you are right, gas managment is only part of dive planing. It is however a very important part, probably the most important. It is the FIRST thing I consider. Pulse, Breath, Blood, remeber your first aid?

You can survive a deco obligation. Even if you blow it, you still may survive.
You can survive getting lost. Even if its embarasing being rescued by helecopter on natonal TV.
You cannot survive without gas. No gas, ~1 minute later, no life.

Interestingly its also some thing your computer can't tell you. But thats a whole other messy road to go down.


Plan your dive, dive your plan. You are right.


As for the rule of 3rds. It a good rule, but one that does not apply to your typical OW dive. There is a minimum amount of gas you need. Now running the minimums every time will eventualy catch up to you.. but why on a 30fsw dive, where I need somewhere around 400psi for my buddy and I to surface would I want to start accending at 2000psi?

I'm all for planing for the worst, but part of that is planing for the type of dive. Most of the dives most people do do not need the 1/3rds rule. Most of the dives that do need 1/3rds, need more anyway. Thats why they teach 1/6th in Cave 1.

 

[Scuba]

Thank you all for some great tips on gas management.

Does any one have a recommendation on a book, course, or other source that will cover dive planning and gas management in detail?

 

[metridium]

Keep in mind that in some conditions, it's a good idea to surface with a couple hundred PSI or more than you normally would.

E.g. conditions where you've surfaced away from the boat and it's better to descend again and swim at 20' or so.

 

[scottbeach]

Last week we dove with two different operators in Cozumel.

Company 1: Standard "low on air" was 750 psi which initiated ascent with 3 min safety stop. My buddy and I would usually finish up with 650 or 600 psi

Company 2: Never asked anyone's pressure through the whole dive until he/she went and told him that they were at 500 psi in over 90 fsw with signicant time over 110 fsw. I watched divers ascend without their buddies and saw 2 divers come out of 100 fsw without safety stops because of running out of air.

Current plays a big role too... We noticed on the day when we had 4 knot currents that we burned an extra 150-200 psi during our ascent and safety stop.

While I want to maximize bottom time as much as the next guy, I'd rather spend 5 less minutes on bottom than perform a CESA from 100 feet.

 

[GDI]

Scuba once bubbled...
not in terms of pressure but of volume cf. or bars. I read in another post that in countries outside the U.S. the standard is 700 psi or 50 bars as opposed to 500 psi. here.

Which amount, please refer to volume, as pressure varies dependant on cylinder size and pressure, is optimum, in terms of safety without unduly cutting dive time.

Peter

:doctor:
Ok well I must agree with some of the others whom I read and some I do not. I teach my OW students to calculate their SAC (surface air consumption) Because at some point they will begin to dive expanding their limitations with training or without. Hopefully they stay within the limits of their training and I've taught them well to do so. (NEVER DO OVERHEAD ENVIRONMENTS OR ANY DIVE FOR WHICH YOU ARE NOT TRAINED) If you know your SAC then you will be able to maximize your diving based on the air or bottom time you are allowed depending on what depth you are at. One should not plan their dive based on PSI but rather on Cu. Ft/min. The Cu. Ft. of a cylinder never changes but the PSI will. Also always plan your dive for the return trip to the surface keeping in mind the weakest link in the dive team, you may need to provide that person with gas for the return trip and keep in mind that they will be a little bit excited if they had a major gas loss, be conservative! Now also keep in mind that SAC rates are personal to the diver and that you should never allow one problem to result in compounding into another, in short so what if you come up with less than 25-50 bar after a dive where you did all your safety/deco stops. Does the cylinder still have air in it? And are you Alive? Did you Violate any part of your dive plan? Did your dive plan have a contingency plan? If you ran out of gas you can alwas breath O2 for a couple of minutes. Basically calculate your SAC rate and plan your dive based on the total amount of gas you will need and then match that gas to the weakest link of your dive team.

 

[GDI]

Scuba once bubbled...
Thank you all for some great tips on gas management.

Does any one have a recommendation on a book, course, or other source that will cover dive planning and gas management in detail?

:doctor:
Hey scuba there are many courses out there. First take a look at the most common size of cylinder you use, example 80cuft (actually closer to 77cuft) but lets go with 80cuft at 3000psig, 207 bar
Find the PSI/cuft.
PSI/cuft= 3000/80=37.5psi/cuft

Now take your starting air - ending air= air used
ie: 3000-1500=1500 psig

Now factor your bottom time and depth:
66 ft for 30min
66ft = 3 ata
psi/minute used =1500/30=50 psi/min @ 66ft
Ok now bring it back to the surface 66ft =3 ata, therefore
50 psi/min divided by 3 = 16.7 psi/min = SAC in psi/min

convert to cu. ft. : psi/min divided by psi/cuft
example 16.7psi/min divided by 37.5 psi/cuft = 0.44 cuft/min
0.44 cuft/min=SAC
Now this is only one way to calculate your SAC. I'm sure that others may have different ways some might be easier. I hope this helps, because I'm tired now and not sure if I explained this right.

GDI