How much air to surface with?

[dumpsterDiver]

In an Al80, 50 psi is more than 1 cu ft. The typical volume of a resting breath is about .5 liters. For scuba and breathing deeply as we are taught, lets call it 1 liter. 1 cu ft is 28 liters. WOB will be quite a bit higher if that is the last 50 psi in the tank, but it is there for the taking unless the restricted breathing leads to panic. 50 psi is about 30 breaths.

The pressure in a balloon is only slightly over ambient, like maybe 1 psi over ambient. Scuba regulator opening is a bit smaller but less than an order of magnitude. Scuba is using a longer straw but you could always go to your alt to shorten that - I doubt if it will make any noticeable difference.

The problem is the gas is there but it will be more work to get it. And working harder when you feel you are not getting enough will not help.

Yes,, this information is more aligned with my thinking.. you can figure roughly that you will use 50 psi per minute to hang on a safety stop, if you are really relaxed and trying to conserve air. This is a good rule of thumb.. so if your are low on air,you should know you can often still make a safety stop. 500 psi is around 10 minutes of hang time..

I am really not that knowledgeable about regulators, but some of the information provided here makes no sense to me. drain your tank down to a very low pressure, hook a reg up and breath!

I think you will find that you can continue to sip air slooowly from the tank for a while. I don't believe any of this 130 psi tank pressure means you can not get air from the reg.. Try it yourself. Press the purge instead of sucking and you will get air, without effort for quite a while.

I'm not saying you should run your tank down super low, but some of the information on this thread would lead a diver to assume that terrible things will happen with a low tank.

I used to golf ball dive and often could not see the gauge. I would know when the tank was getting low, because it got hard to inhale, but I could make my way to shore and even along shore for quite a while and when I got out, the tank would be MT! Did it many dozens of times.. And I never got water in the tank either.. just shut the tank down when the pressure gets really low..

Also, the idea of being on the boat with 500 psi is incredibly stupid.. uniformed divers will rush their ascent in order to make it to the boat with 500.. A much better rule would be to leave the bottom with XXX psi.. whatever that figure is, should make sense, but demanding 500 psi reserve is really treating people like imbeciles.

 

[justbillin]

FYI-Stuart

If you go out on charter boats enough you will hear a similar briefing to this almost every time.


This is where the life preservers are and sometimes we have a float off life raft

We have an O2 kit on board

Don’t put anything in the toilet that didn’t go through you first

Puke over the side and not the head if you are seasick

Be back on the boat with 500 PSI – But don’t short change your safety stop for this!!!

 

[HalcyonDaze]

This is why rock bottom is not going to catch on with rec divers. Most would be very surprised to know they have to begin their ascent after using 1/2 of their tank and reaching the surface with 1000 psi or more, still remaining. This means, for nitrox divers with good consumption, not even getting close to their NDL time underwater and giving up a big chunk of their dive, every time. They will sacrifice many, many minutes and eventually many hours of time underwater by anticipating the very worst situation that can happen once in 100,000 times, if even that often, based on exaggerated gas use assumptions that are not necessarily needed in rec NDL diving. I don't think rec divers are going to do this when it means sacrificing 30% of their normal dive time.

Put simply, if I have a buddy with a low/out of air situation underwater, he is getting my primary and we are heading for the surface, pronto. We are not going to worry about ascent rates, and are certainly not going to do a safety stop. A real OOA emergency calls for the surface, as fast as we can short of a full CESA. Formal "rock bottom" planning would be far more gas than needed to accomplish this. In fact, the "surface with 500 psi" guide means leaving the bottom with 700-800 at 80' (in my experience). If it takes me 300 to do a slow ascent and safety stop, even doubling that still leaves 200 left (close, but this is emergency planning). In reality, we would not use that much gas with a faster ascent and no stop. Remember, we are NDL diving so stops are optional and we have the safety margin of the tables or computer algorithm to help offset the fast ascent. I choose to plan this way, considering the very remote likelihood of the event even happening and the remote risk of injury with my plan, rather than sacrifice what, by now in my life, would have been days of underwater time as well as lots of unused gas.

Rock bottom is crucial if you have mandatory deco. At that point, you do have to calculate based on slow/proper ascent rates and the mandatory stops. But otherwise, I just don't see the crusade becoming widespread.

My American Academy of Underwater Sciences instructor/Dive Safety Officer that I first had gave us the working rule of leave the bottom with no less than 500 psi and be on the boat with 300. For the record, this guy was a cave and tech diver with somewhere in the neighborhood of 5-6,000 hours. Granted, a lot of us weren't going to depths over 30 ft and I remember some work dives in Pennekamp where our tanks were practically broaching. Even if I did start sucking on a coke bottle (which I did twice after pushing a dive too hard) the surface was within reach.

I borrow some cave-diving tricks I was taught, but arguing that every diver should follow the gas management rules for it is bonkers. Unlike cave divers, recreational divers in open water can just go up at a nice easy rate. I'm fine with adhering to the 500 psi rule as general practice; depth determines the point at which I'll start my ascent. If I make the surface with between 500 and 300, I'm within my safety margin. Less than 300, that's a screwup in my book and a notice to be more careful.

One recent time where I slipped a little, I was using a nitrox HP120 tank starting at 3800 psi (so I really had 132 cf of gas) on a ledge drift with a max depth of 130 ft. I got a little carried away hunting and killing lionfish and when I left the top of the ledge at 120 ft I had 750-800 psi (on the GoPro video I can be heard saying "uh-oh" when I checked my gauge) in the tank and 1 minute of deco on my computer. Normally my rule would have been to leave the bottom with 1000-1200. I made a normal ascent (slow enough that the 1-minute deco was rescinded before I got to 15 ft) and as a precaution spent 4 minutes at 15 ft. While there I noticed a bull shark coming up at me and had to dive down a bit to make her break off. I believe I made it to the surface with around 300 psi in the tank.

 

[oldschoolto]

I returned a tank to the dive shop at pennekamp in key largo with 200 psi.. The guy had a fit and told me that it is very dangerous to breath a tank that low... He also said " I should charge for a visual inspection to make sure no water got in the tank " I just started laughing after he returned the credit card to me...

Jim...

 

[diver 85]

Is it pretty universal and standard to expect everyone to "get back on the boat with 500psi"

Depends on where you dive---ie the Fling, Flower Gardens regular live aboard dive boat(btw we are diving 125 miles offshore in the GoM if you're not familiar with the Flower Gardens) wants you back on board with enuff air in a tank to be able to take another breath.............hmmm, ........well---------they don't 'penalize' you for being (that)low........

 

[AdivingBel]

Last time I was on the Vandy with Captain's Corner they said the boat was leaving the site @ 12:30 and that I should be on it. Didn't say anything about tank pressure... The rules are relative to where you're standing at the time. Best to have your own rules and make them good ones then adjust them to fit with the boat's rules if necessary.

 

[PfcAJ]

I think "being back on the boat with 500psi" isn't terrible. For 99% of rec dives that go on across the world, its reasonable.

The problem is that divers *in general* don't know how to ensure they leave the bottom with enough gas to accomplish this. 500psi is generally what you're left with if you start your ascent when hitting rock bottom if you're diving a single al80.

If you've got great big huge tanks, I still think 500 is a decent value to be back with as the SPG isn't super accurate at low pressures.

 

[KeithG]

the true answer depends upon whose tank you are diving:

as a rec diver i dive their tanks and their rules apply. their rule is 500 psi back on the boat / dock. not at the safety stop, not at the ladder. 500 psi out of the water. regardless of the size of tank. their op, their rules. end of story.

if i was diving my own tank under different conditions then i might worry about some other rule / calculaton. based upon the types of dives i do, i routinely return with over 1000 psi, and lots of ndl to boot. diving shallow makes life simple.

 

[Bob DBF]

I am really not that knowledgeable about regulators, but some of the information provided here makes no sense to me. drain your tank down to a very low pressure, hook a reg up and breath!

I think you will find that you can continue to sip air slooowly from the tank for a while. I don't believe any of this 130 psi tank pressure means you can not get air from the reg.. Try it yourself. Press the purge instead of sucking and you will get air, without effort for quite a while.

I think that the 130 psi came from the IP of an unbalanced first, which you can suck down a little lower, but you would feel it all happening well before that. With a balanced first, you can breathe it down to the "balance pressure". Same deal with the second stage, but like you said, press the purge and you will get it all.

I, personally, would rather hit the surface with some air to play with, but I'm not a fanatic. More important to me is that I'm neutrally buoyant on the surface with an empty tank and an empty BC, so I'm not going back down without air which would be quite inconvenient.


Bob

 

[gfaith]

Rock bottom is all about buddy diving. You can easily turn a 100 foot dive at 1000 psi even with an elevated 1.0 CF/min, but if you or your buddy have a total loss of gas at depth (100 ft) you better have 1500 psi in both tanks. Never know which buddy it will be. Both diving AL 80s..
Just my take on it.