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