500 psi for two divers?

[decompression]

Rock bottom....rock bottom....rock bottom.....rock bottom..


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[Errol Kalayci]

As a general rule of thumb, if you want enough gas to share with another diver for you both to ascend making your safety stops which we essentially refer to as minimum deco then you need to leave the bottom with what we call minimum gas: In volume this translates into
At 100': need to leave bottom with at least 40cft
At 80': need to leave bottom with at least 30 cft
At 60': need to leave bottom with at least 20 cft or 800psi in Al80
An aluminum 80 is about 2.5cft per 100psi so 500psi is approximately 12.5 cft. figure breathing rate per diver of 1cft per minute x ata equals your burn rate of gas per minute

 

[DeputyDan]

OOA story:

Took my niece (age 14) on her first dive trip to Bonaire.
She had completed her OW in her home town before we went.

She ran out of air on her first dive and thought it was funny - but when my 17 year old daughter ripped her a new one over her actions (and I do mean ripped her one) - there was never another issue with anything related to diving.

I didn't have to say a word.

 

[tbone1004]

if your instructor said 15 mins then he's a moron and wasn't teaching you much of anything. As we have all been stating, 500psi doesn't mean anything because it has no correlation so the volume of air that you have left and the volume of air that you are breathing.

The only way to avoid the situation is to punch the other guy in the face and leave him there, but as we have also said, you should never put yourself in that situation, and should never base anything off of PSI, but always off of volume of air left.

you also have to almost double your normal breathing rates for emergency situations because very few divers actually have the mental ability to stay calm in stressful situations, mainly because they aren't put in those situations regularly.

Errol had a pretty good layup there, and with an AL80 being ~2.5cf/100psi your instructor is right, you have about 15 minutes at 30 feet IF your SAC is 0.5cfm. If you are at 66 feet, then that becomes 10 minutes if you stay there which is why before every dive you have to do the math.

Looks like this
My sac=0.5
buddies sac=0.8
Max depth =66 feet *used because of 2ata for easy math
assuming ascent of 30ft/minute with a 3 minute precautionary stop at 15 ft

Assuming everything goes hunky dory, you need 11.4 cf of air to make that ascent safely. That equates to about 450psi in an AL80 plus at minimum 150psi of reserve, so you know that you have to turn the dive around 700psi to make it back safe. That is best case scenario, and the very likely scenario is that during that emergency ascent your SAC will multiply by at least 1.5 which is now close to 700 PSI just to make the ascent, so you should turn that dive around 1000psi. Most recommend 2.0 adjustment, but 1.5 is bare minimum

Now, here is why that psi number has to be done as a factor of tank. Using a 200bar 18L *lp108 for us Americans* steel tank, the cf used doesn't change, but the PSI becomes 400 for the safe ascent instead of 700psi for the aluminum. Same cubic footage but you have to learn to think in cubic feet of air instead of PSI because the PSI is only useful when you only use one tank.

 

[doctormike]

guys, im not expecting this to happen and i kind of know how to avoid it, i say kind of because i only have 4 dives in my log book so dont expect much from me as a diver . thats why im here its hard to just jump to the water without having all the situations solved in my mind .

my dive instructor told me once 500 psi would last like 15 min ( average) so , in case it happens to me, i dont want to panic

Hey,

I hope that you don't take this the wrong way. A lot of very experienced divers are going out of their way to try to help you.

It's always OK to ask questions if they are asked in earnest (I don't think that you are trolling). But what we are trying to tell you is that memorizing a protocol for what to do if you find yourself and your buddy with 500 psi between you is NOT a good idea at all. It's not a good idea, because you should be totally focused on your gas reserves and your gas consumption. You should be working on honing your situational awareness. If you end up in the situation you described, it is because of a failure of that process, not because you just had the bad luck to encounter that condition through no fault of your own.

The fact that you are writing thing like "my dive instructor told me once 500 psi would last like 15 min ( average)" means to us that you do not have a basic understanding of the mechanics of breathing compressed gas underwater, even at the OW level. Again, don't take this the wrong way, we are not beating up on you, we are trying to direct your learning and practice towards productive ends.

Dive safe!

M

 

[boulderjohn]

Tatiana, I think you asked a great question, one that a lot of brand new divers would not think to ask. I am sorry that you got some fairly harsh answers, especially since you asked you question in the new divers area, where all questions are supposed to be welcomed and answered politely.

The simple answer to your first question, as has been shown several times, is no, it won't be enough gas for two divers to do a safety stop and ascend.

As for your second question, I am not sure I understood it properly. I think you were taking a second diver out of the question and wondering if you were at 500 PSI, should you do a safety stop or just ascend. If it were me, I would do the safety stop, all the while thinking that I won't let myself get that low on air again.

There was a lot of talk about concepts like rock bottom, as if brand new divers are supposed to have been taught that in their beginning OW class. They aren't, and you should not feel bad for not knowing that concept.

 

[TatianaSilva]

Hey,

I hope that you don't take this the wrong way. A lot of very experienced divers are going out of their way to try to help you.

It's always OK to ask questions if they are asked in earnest (I don't think that you are trolling). But what we are trying to tell you is that memorizing a protocol for what to do if you find yourself and your buddy with 500 psi between you is NOT a good idea at all. It's not a good idea, because you should be totally focused on your gas reserves and your gas consumption. You should be working on honing your situational awareness. If you end up in the situation you described, it is because of a failure of that process, not because you just had the bad luck to encounter that condition through no fault of your own.

The fact that you are writing thing like "my dive instructor told me once 500 psi would last like 15 min ( average)" means to us that you do not have a basic understanding of the mechanics of breathing compressed gas underwater, even at the OW level. Again, don't take this the wrong way, we are not beating up on you, we are trying to direct your learning and practice towards productive ends.

Dive safe!

M

of course i do not take it wrong ! im really happy you all tried to help me , just sometimes people expect too much from new divers and until now i havent had any problems but the " what if ..." are always in my head . i just find this specific subject quite hard to get with the volume, cf, calculating bottom times, etc ...

im quite slow understanding calculations ! so guess will take time and more dives

 

[tursiops]

guys, im not expecting this to happen and i kind of know how to avoid it, i say kind of because i only have 4 dives in my log book so dont expect much from me as a diver . thats why im here its hard to just jump to the water without having all the situations solved in my mind .

my dive instructor told me once 500 psi would last like 15 min ( average) so , in case it happens to me, i dont want to panic

No problem! But your dive instructor was not quite correct....it might last 15 mins *at the surface* but not at depth, and it would probably not last 15 minutes if you were stressed. And it makes a difference if you are talking about the LAST 500 psi in your tank, because that is really only 300-350 psi that is useful to drive your regulator.

You cannot estimate this kind of thing without knowing the size of your tank, and your breathing rate, as others have said. In (very) round numbers, your tanks holds 80 cuft at 3000 psi, so 300 psi is 1/10 of a tank, or 8 cuft. That is how much gas you safely have to breath if your SPG shows 500 psi. You might get a little more, but not much. How fast does that 8 cuft go away? At the surface, unstressed, 0.5 cuft/min is a nice number. So, 16 mins....what your instructor said. But at 10m, the pressure is twice as much, so the gas only lasts half as long....8 minutes! If you are at 20m, and head for the surface, your average depth is 10m, which is why to use 10m for the estimate. How long does it take to get to the surface from 20m? It ought to take 2 minutes, or longer. Right? If you go faster than that, you are putting yourself in jeopardy. So you've got four minutes of gas, and you use 2 of that to get to the surface. And if there are two of you, you use all four minutes of gas just to get to the surface...no safety stop! And this assumes you are not stressed. If you are stressed, your breathing rate can easily double or triple.

I hope you see why 500 psi is NOT what you start coming up with, especially if you have a buddy in need.

 

[TatianaSilva]

well i know the basics to dive ... but i dont think many OW divers know much about what i asked ... even the padi elearning doesnt say much about what i asked , keeping in mind that i still havent done dives with a dive computer or dive table i think i did it just fine !

it was just the OW course so you basically rely a lot on the instructor the first times

---------- Post added August 21st, 2014 at 07:37 PM ----------

No problem! But your dive instructor was not quite correct....it might last 15 mins *at the surface* but not at depth, and it would probably not last 15 minutes if you were stressed. And it makes a difference if you are talking about the LAST 500 psi in your tank, because that is really only 300-350 psi that is useful to drive your regulator.

You cannot estimate this kind of thing without knowing the size of your tank, and your breathing rate, as others have said. In (very) round numbers, your tanks holds 80 cuft at 3000 psi, so 300 psi is 1/10 of a tank, or 8 cuft. That is how much gas you safely have to breath if your SPG shows 500 psi. You might get a little more, but not much. How fast does that 8 cuft go away? At the surface, unstressed, 0.5 cuft/min is a nice number. So, 16 mins....what your instructor said. But at 10m, the pressure is twice as much, so the gas only lasts half as long....8 minutes! If you are at 20m, and head for the surface, your average depth is 10m, which is why to use 10m for the estimate. How long does it take to get to the surface from 20m? It ought to take 2 minutes, or longer. Right? If you go faster than that, you are putting yourself in jeopardy. So you've got four minutes of gas, and you use 2 of that to get to the surface. And if there are two of you, you use all four minutes of gas just to get to the surface...no safety stop! And this assumes you are not stressed. If you are stressed, your breathing rate can easily double or triple.

I hope you see why 500 psi is NOT what you start coming up with, especially if you have a buddy in need.

yeah , i get it now

 

[fire_diver]

There was a lot of talk about concepts like rock bottom, as if brand new divers are supposed to have been taught that in their beginning OW class. They aren't, and you should not feel bad for not knowing that concept.

Yep, it's pretty pathetic that new divers aren't taught anything besides "be back to the surface with 500psi". Nobody has been rude to the OP in this thread, poor instruction isn't bashing the OP, only attempting to open her eyes to what she SHOULD know.