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.
I understand the math, I don't agree with the application.
You have one diver who is now in an emergency situation (OOA) according to all I have learned in six months of diving and reading profusely here and about 20 books, and probably 10-20 blogs... that one single fact becomes the limiting criteria in the scenario. How much air diver B has in their tank is now IMMATERIAL, and given the depth, you surface immediately on shared air and do not worry about a safety stop... even if the dive profile included 12 minutes at 100ft, you might go to 20 feet and ride it out until your tanks on fumes, but it seems that surfacing and being done for the day is the better part of valor, and safer choice overall.
---------- Post added August 22nd, 2014 at 09:08 AM ----------
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
Tatiana, I too an new certified, though I've been trying to log as many dives as I can... and as some here know, I have already broke the rule about monitoring my gas and ended up OOA (with an instructor mind you).
As a fellow new diver, let me pass the wisdom of that lesson on to you. Most divers use AL-80s, the most common tank around... at 500psi or 50bar, that's where the redline on your SPG starts, and that's where your ascent to the surface starts no matter what. (wait wait wait you heavy handed meatheads, I'm getting there...).. In truth, you should be ON THE SURFACE at 500psi.
If you hit the redline anywhere else, you should be signalling your buddy the "LOW AIR" sign, then the "ASCEND" sign in that order. If they are giving you the "OUT OF AIR" sign, they better be moving the same direction as your alternate air source.
Other than making sure you are monitoring your air (I had to set my watch to go off every 6 minutes, now it's set for 8 - a little annoying to the peace seekers, but its for me not them)... you don't have to do much THINKING... your plan is to be on the surface before you go into the RED. PERIOD.
---------- Post added August 22nd, 2014 at 09:18 AM ----------
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.
I fail to see why that is pathetic. Most people here won't have this experience, but I grew up in farm country. Kids got a driver's license at 14. It was good for DAYLIGHT ONLY, not much farming goes on in the dark. And we knew that "BE HOME BEFORE DARK" was the rule. That's pretty easy to define, if you have to turn on the lights of the vehicle, or another vehicle can't see you clearly without lights, then it's DARK, and you should be HOME.
For a new diver, I think teaching an absolute is the best way to go about it. There's nothing pathetic for a new diver to be taught "BE ON THE SURFACE AT 500PSI" (actually, I'd just say, BE ON THE SURFACE BEFORE YOU ARE IN THE RED)... it's a clearly defined line, and unless they are under the direct supervision of a more experienced diver who is going to be responsible for their safety, they should stick to it like the gospel.
I don't argue that there is MORE to learn about gas management. I think NWGrateful Diver's blog should be in the AOW book, AS IS, because it's clear, concise information on the BEST way to manage your gas supply.
But for some reason, our society has decided not to set boundaries for ourselves. Children shouldn't be told no, we can drink 'a little' and still drive, 500psi isn't really 500psi,... now that is pathetic.
