ccx2
Contributor
How does one estimate the volume of air needed to turn around or end the dive and have enouhf air to safely ascend and have 500 or 600psi remaining when reaching the surface like most DM want and most recommend. Thanks
Return to surface air volume
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You are going to get some lengthy replies to this one... in short: most conventional recreational divers are told make sure they start their ascent with either 500 PSI or 50 bar in their tanks as that will invariably ensure they have enough gas to make it to the surface with time for an adequate safety stop from recreational depths.
In more advanced forms of diving, specifically planned decompression diving (but other types as well) divers normally calculate the exact gas usage for the dive plan based upon their own consumption rates (which they ascertain beforehand). For those plans normal practice is to leave a full third of your carried gas as a "reserve" for emergencies.
That is the simple version. I'll leave it to others to post long tedious details about SACs, RMVs, rock bottom, turn pressures and other nomenclature beloved of gas planners.
In more advanced forms of diving, specifically planned decompression diving (but other types as well) divers normally calculate the exact gas usage for the dive plan based upon their own consumption rates (which they ascertain beforehand). For those plans normal practice is to leave a full third of your carried gas as a "reserve" for emergencies.
That is the simple version. I'll leave it to others to post long tedious details about SACs, RMVs, rock bottom, turn pressures and other nomenclature beloved of gas planners.
Shallow dive of say 30-40' about 500psi should do it with a three minute safety stop. If I'm diving a ship at around 100-120' I go up at around 1000psi. This gives me time for a two minute stop at 50', 30' and a three minute stop at 15'. I generally have a surplus of air at this rate.
In "normal" dives where you would do the deeper part of your dive first and you swim from a to b and backt to a
starting pressure - 5 or 600 psi = usable gas
usable gas / 2 = usable gas 1st half of the dive
Turnpressure = usable gas - usable gas 1st half of the dive
example:
3000- 600 = 2600
2600/2=1200
You should turn the dive at 2600 - 1200 = 1400 psi to return to your entry point
No estimating needed, just some simple math
of course, there are other situiations ( drift dive ) that require other "formulas". but to prevent my post from getting long and tediuous, I'll leave it at this..
Enjoy !
A quick and easy rule of thumb I use From Lamont's guide on how to calculate end of dive air needed to safely get you and a buddy to the surface safely (called Rock Bottom ) article here ... ... http://www.scriptkiddie.org/blog/2010/09/19/rock-bottom-and-gas-management-for-recreational-divers/
Depth + 0 + 300 psi ... IE if at 60feet add a 0 = 600 + 300 = 900psi to get both of you to surface ... I can do that in my head and change it if we are at a different depth
turn pressure would be half of what you have when reaching dive depth, minus that reserve needed to get both divers to the surface ... check your pressure when you get down, subtract the 900psi or whatever from that and divide the remaining pressure in half
Depth + 0 + 300 psi ... IE if at 60feet add a 0 = 600 + 300 = 900psi to get both of you to surface ... I can do that in my head and change it if we are at a different depth
turn pressure would be half of what you have when reaching dive depth, minus that reserve needed to get both divers to the surface ... check your pressure when you get down, subtract the 900psi or whatever from that and divide the remaining pressure in half
Ok thanks to you both, the two tank dive trips vie done it seem the DM always wants us to start back/ accend at 1000psi , these are shallow dives 40fsw, maybe it's because of time constraints for two tank dive.
Seems 600 or 700 would be plenty conservative from that depth, and I've seen where some people say you MUST have 500 when you pop the surface to be safe/conservative.....maybe I'm missing something.
Seems 600 or 700 would be plenty conservative from that depth, and I've seen where some people say you MUST have 500 when you pop the surface to be safe/conservative.....maybe I'm missing something.
Please remember two things when discussing this topic.
1) 50 bar or 500 psi... this is standard minimum advice for basic open-water divers (18 metres/60 feet maximum depth).
2) few -- if any -- SPGs have ever been calibrated and therefore are most likely to be grossly inaccurate.
1) 50 bar or 500 psi... this is standard minimum advice for basic open-water divers (18 metres/60 feet maximum depth).
2) few -- if any -- SPGs have ever been calibrated and therefore are most likely to be grossly inaccurate.
A comment like return with 500 psi is kinda meaningless without the tools to calculate it
.. I think it's more of an admonishment to novice/new divers to watch your gages and don't run low and to give you some solid figure in the hope that you'll notice if it's close
.. I think it's more of an admonishment to novice/new divers to watch your gages and don't run low and to give you some solid figure in the hope that you'll notice if it's close
If I didn't have to go to work right now I'd type a lengthy reply stating you should ascend with more psi. I'll just leave you with this to think about till I get back.
With 700psi in your tank and say a SAC rate of 0.70cf/min, will that last you from 40ft if you have 2 divers breathing off that same tank? Assume both are stressed and breathing heavier than normal.
Are you going to be empty or cutting it close? Are you going to make a clean ascend to the top, or have a few hiccups in the middle? Are you positive your answers will cover all OOA situations you might face?
With 700psi in your tank and say a SAC rate of 0.70cf/min, will that last you from 40ft if you have 2 divers breathing off that same tank? Assume both are stressed and breathing heavier than normal.
Are you going to be empty or cutting it close? Are you going to make a clean ascend to the top, or have a few hiccups in the middle? Are you positive your answers will cover all OOA situations you might face?
THIS article will give you a whole bunch of information on this VERY good topic.
In short, you not only need enough gas to get you to the surface . . . you need enough to get your buddy there, too, if something interferes with his own ability to breathe from his equipment (freeflow, clogged dip tube -- there ARE things other than carelessness that can lead to sharing gas). There are tools to calculate your gas usage, and they aren't difficult. They allow you to plan, so that you know that, if the dive is briefed at 70 feet, how long you will be able to dive before you have to start up. Bob's article has the details, and Lamont's quick page on the topic is very good, too.
In short, you not only need enough gas to get you to the surface . . . you need enough to get your buddy there, too, if something interferes with his own ability to breathe from his equipment (freeflow, clogged dip tube -- there ARE things other than carelessness that can lead to sharing gas). There are tools to calculate your gas usage, and they aren't difficult. They allow you to plan, so that you know that, if the dive is briefed at 70 feet, how long you will be able to dive before you have to start up. Bob's article has the details, and Lamont's quick page on the topic is very good, too.
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