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Professor Nemo
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Good news, I will be able to use Steel 100s rather than AL80s on the dives.
Safe tank pressure at which to begin accent.
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that's good, the CF requirement doesn't change though. For my calculation, true rock bottom ascent pressure will now be a 1400psi turn pressure from 100ft or around 1000psi from 50-70ft
Remind me not to buddy with you. I like bottom time more than you do.
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your decision, but with my SAC rate I can almost guarantee that I'll be down just as long as you. Just remember, I am holding YOUR emergency air supply if you have a catastrophic loss of gas. If I don't have enough gas, guess what, you aren't getting a regulator if you come at me out of gas because I'm not going to make a CESA from 100ft. I can, but I won't. If you tell me you want me to violate those gas rules, I have no problem doing it, but you better be prepared with your own gas supply because you will not be getting any of mine. I plan conservative because I have experienced situations like this where that type of gas planning was just enough to complete the dive without any sort of emergency procedures *quick ascent rates, skipped safety stops, or CESA*. If you want to gamble with your buddies gas supply, just make sure he is aware because if he needs it, you won't have it
Divetech99
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@boulderjohn
Just checking my math. Will this rule of thumb,
D * 10 + 300
Get two divers back to the surface with one of them OOA? Will it permit a 3min stop at 15 ft? Assume AL80 at 1cf/min SAC for both.
Thanks
halocline
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I don't think it's necessary to use technical diving formulas to safety plan and execute open water, recreational no-decompression dives. OW, NDL diving means that all divers have immediate access to the surface. As long as divers maintain that condition in their dives, all that is really necessary for OW diving is enough gas to get yourself and a buddy to the surface safely. I also think it's foolish to arbitrarily assign a PSI or cft requirement from depth X without understanding the conditions of the dive, like currents, exertion level, etc.. A square profile is far different than a multi-level profile in which the later part of the dive is shallow.
It's definitely a good idea to include a safety stop in your emergency plan, but it's not required. A typical diver resting at 15ft for 3 minutes is only going to use 4-5 cft of gas anyway, so double that for two divers, that's 10cft (based on a SAC of 1cfm). 500 PSI in an AL80 is about 13cft. This means that if two divers breathing the same reg get to a safety stop with 500 PSI. there should be plenty of gas to get them a reasonable stop and safely to the surface.
So it comes down to how much gas you might need to get yourself and a buddy from what ever depth you are at to a safety stop with 500 PSI.
OW, recreational diving is much more forgiving than technical diving. That's why most OW divers do little if any gas planning and yet it's statistically very safe. It's usually divers that fail to monitor their SPG and/or dive really foolishly that get into trouble with gas on OW dives.
It's definitely a good idea to include a safety stop in your emergency plan, but it's not required. A typical diver resting at 15ft for 3 minutes is only going to use 4-5 cft of gas anyway, so double that for two divers, that's 10cft (based on a SAC of 1cfm). 500 PSI in an AL80 is about 13cft. This means that if two divers breathing the same reg get to a safety stop with 500 PSI. there should be plenty of gas to get them a reasonable stop and safely to the surface.
So it comes down to how much gas you might need to get yourself and a buddy from what ever depth you are at to a safety stop with 500 PSI.
OW, recreational diving is much more forgiving than technical diving. That's why most OW divers do little if any gas planning and yet it's statistically very safe. It's usually divers that fail to monitor their SPG and/or dive really foolishly that get into trouble with gas on OW dives.
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rock bottom is an inherently recreational formula.... also note that when he posted the depths I said that I would have two "rock bottom pressures" one for the deep part of the dive, and one for hanging out at the deck. Condoning reckless gas planning is not ok....
Yes to all.
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To the OP ... there have been several suggestions offered, some better than others and with rules of thumb that will generally work if nothing goes wrong on the dive.
My simple answer would be don't go to 100 feet using an AL80. I'd even be hesitant to suggest going there using an HP100, given what you've said about your relative size and air consumption rate. The reason is that going to 100 feet for a new'ish diver (you indicate 0-24 dives experience) is like driving across a deserted stretch of highway, where you know there won't be any gas stations available for 100 miles. You can look at your gas gauge, but in order to know that you'll be able to make it you need to know not just how many gallons your tank will hold, but how many miles per gallon you get. Then you have to factor in a reserve for worst-case conditions. Otherwise the position of the needle on the gauge only gives you a relative indication of how much gas you have relative to a "full tank", and no way of knowing how fast you'll actually use it up. Scuba diving is like that too ... you need to know not only how many cubic feet your tank holds, but how quickly you will use it up. But in scuba diving, your "miles per gallon" is variable not just with depth, but also with conditions ... both physical and mental. New divers tend to go through their air quickly, and even more so when faced with new or stressful circumstances. New divers are often taken by surprise by how quickly their SPG goes down as they go deeper, even though they were taught that it would happen ... there's a big difference between academic knowledge and practical application.
When I was teaching I would have my students calculate how much gas they would need for a 100-foot dive, based on a supplied dive plan. They would measure their air consumption rates, both under reasonable (typical) diving conditions and under stressful conditions, and then use those measurements to perform the calculations. They often (usually) came to realize why that AL80 tank they were renting wasn't adequate for the dive. And it usually came down not to the expected consumption rate if the dive went according to plan ... but rather to how much gas they would need to make a controlled ascent while sharing air with their buddy if the worst case (OOA) occurred while at maximum depth.
Some things to think about ...
1) When someone goes OOA, they are unlikely to remain calm. A diver's consumption rate goes up with stress ... sometimes it goes WAY up. I'm not talking about a panicked diver here ... just someone who has to react to an unexpected circumstance while at 100 feet, while probably dealing with some level of narcosis.
2) You're going to be working harder than normal making the ascent while sharing air. This means both divers will be ascending with an accelerated air consumption rate.
3) If you haven't practiced making an ascent while sharing air since you did it in your OW class, don't assume it will go without a hitch. A more likely circumstance is that you will not have an easy time controlling your buoyancy. It's likely you'll end up cycling more air through your BCD than you normally would.
4) If one of you goes OOA on this dive, how likely is it that your dive buddy is going to have air to spare? Particularly if you haven't both planned your reserves in advance with the circumstance in mind that you might both need to be breathing off of only one of your tanks.
A good rule of thumb for calculating air consumption is that under stressful conditions your "typical" consumption rate could double ... I've measured some people's rate going much higher than that, but double is pretty typical. For new'ish divers, that could put your stressful consumption rate well over 1 cubic foot per minute (x depth pressure in ATAs) for EACH DIVER. At 100 feet, that would mean that two stressed divers could potentially use up 8-10 cubic feet of gas in the first minute ... and that could be just how long it takes to sort out the situation and begin your ascent. You then have to factor in sufficient gas to get you to the surface ... hopefully with enough in reserve to at least power-inflate the donor's BCD (since they'll be helping their dive buddy remain afloat while they orally inflate theirs).
Safety stops are optional. Far better to make a measured ascent and skip the stop (unless you have plenty of gas left by the time you get there, and are confident enough in your buoyancy control to not have to struggle maintaining safety stop depth). The priority at this point isn't making the stop ... it's making it to the surface before you run out of gas ... keep your priorities straight and mindful of how much air you have left to work with before you decide to stop.
There is no magic formula ... rules of thumb are convenient, but we're all different. The only way to know for sure is to take measurements, and calculate your "miles per gallon" under both normal and stressful conditions. Your controlling factor on when to begin ascending won't be how much gas you will need to surface, but how much you'll need if a worst-case emergency (OOA) occurs at the deepest point in your dive.
For more information, go read this article ... NWGratefulDiver.com
... Bob (Grateful Diver)
My simple answer would be don't go to 100 feet using an AL80. I'd even be hesitant to suggest going there using an HP100, given what you've said about your relative size and air consumption rate. The reason is that going to 100 feet for a new'ish diver (you indicate 0-24 dives experience) is like driving across a deserted stretch of highway, where you know there won't be any gas stations available for 100 miles. You can look at your gas gauge, but in order to know that you'll be able to make it you need to know not just how many gallons your tank will hold, but how many miles per gallon you get. Then you have to factor in a reserve for worst-case conditions. Otherwise the position of the needle on the gauge only gives you a relative indication of how much gas you have relative to a "full tank", and no way of knowing how fast you'll actually use it up. Scuba diving is like that too ... you need to know not only how many cubic feet your tank holds, but how quickly you will use it up. But in scuba diving, your "miles per gallon" is variable not just with depth, but also with conditions ... both physical and mental. New divers tend to go through their air quickly, and even more so when faced with new or stressful circumstances. New divers are often taken by surprise by how quickly their SPG goes down as they go deeper, even though they were taught that it would happen ... there's a big difference between academic knowledge and practical application.
When I was teaching I would have my students calculate how much gas they would need for a 100-foot dive, based on a supplied dive plan. They would measure their air consumption rates, both under reasonable (typical) diving conditions and under stressful conditions, and then use those measurements to perform the calculations. They often (usually) came to realize why that AL80 tank they were renting wasn't adequate for the dive. And it usually came down not to the expected consumption rate if the dive went according to plan ... but rather to how much gas they would need to make a controlled ascent while sharing air with their buddy if the worst case (OOA) occurred while at maximum depth.
Some things to think about ...
1) When someone goes OOA, they are unlikely to remain calm. A diver's consumption rate goes up with stress ... sometimes it goes WAY up. I'm not talking about a panicked diver here ... just someone who has to react to an unexpected circumstance while at 100 feet, while probably dealing with some level of narcosis.
2) You're going to be working harder than normal making the ascent while sharing air. This means both divers will be ascending with an accelerated air consumption rate.
3) If you haven't practiced making an ascent while sharing air since you did it in your OW class, don't assume it will go without a hitch. A more likely circumstance is that you will not have an easy time controlling your buoyancy. It's likely you'll end up cycling more air through your BCD than you normally would.
4) If one of you goes OOA on this dive, how likely is it that your dive buddy is going to have air to spare? Particularly if you haven't both planned your reserves in advance with the circumstance in mind that you might both need to be breathing off of only one of your tanks.
A good rule of thumb for calculating air consumption is that under stressful conditions your "typical" consumption rate could double ... I've measured some people's rate going much higher than that, but double is pretty typical. For new'ish divers, that could put your stressful consumption rate well over 1 cubic foot per minute (x depth pressure in ATAs) for EACH DIVER. At 100 feet, that would mean that two stressed divers could potentially use up 8-10 cubic feet of gas in the first minute ... and that could be just how long it takes to sort out the situation and begin your ascent. You then have to factor in sufficient gas to get you to the surface ... hopefully with enough in reserve to at least power-inflate the donor's BCD (since they'll be helping their dive buddy remain afloat while they orally inflate theirs).
Safety stops are optional. Far better to make a measured ascent and skip the stop (unless you have plenty of gas left by the time you get there, and are confident enough in your buoyancy control to not have to struggle maintaining safety stop depth). The priority at this point isn't making the stop ... it's making it to the surface before you run out of gas ... keep your priorities straight and mindful of how much air you have left to work with before you decide to stop.
There is no magic formula ... rules of thumb are convenient, but we're all different. The only way to know for sure is to take measurements, and calculate your "miles per gallon" under both normal and stressful conditions. Your controlling factor on when to begin ascending won't be how much gas you will need to surface, but how much you'll need if a worst-case emergency (OOA) occurs at the deepest point in your dive.
For more information, go read this article ... NWGratefulDiver.com
... Bob (Grateful Diver)
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yes, but the caveat is it gives you little to no time to get situated at the bottom.
1300psi in an al80 is 34cf of gas.
At that sac rate, you need 12 of that for the ascent, and 9 for the safety stop, and 5 as the 200psi buffer in the tank to keep the regs working properly. 26/34 is spoken for. That gives you 8cf to play with at the bottom, at that SAC rate and at that depth, that is 30 seconds. It is long enough for most situations and you will likely be fine, but again, it's how much you want to gamble. It's a good rule of thumb, unlikely to get you into any real trouble, but I still prefer to do the math and have my buddies/students do the math as well
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