Low air limit?

[MikeS]

or not taught as it were, by PADI has always bugged me. For clarification, I’ve finished all but the ongoing internship of DM training and have yet to get past the mystical get back on the boat with 500 PSI. Never in any of the PADI documentation, I’ve read have I seen how you plan to get there. Granted there are many variables making dive planning difficult to teach, but it seems any plan is better than no plan. As a result, I’m guilty of being Internet (Scuba Board) trained.

I’m not even a DM yet let alone an Instructor, so you are cautioned to take this with a grain of salt, but if I were to teach basic dive planning it would go something like this.

Rule # 1 – There are two primary factors that limit your dive time, air supply and nitrogen loading. Both must be tracked independently; you should be safely out of the water before ether limit is reached.

Basic Air Rule – Always maintain enough air to get you and your buddy back to the exit point and safely to the surface at a safe rate of ascent 30 f/min with a safety stop.

Basic Nitrogen Loading Rule – Always maintain sufficient remaining NDL time left to get back to the exit point and start a safe ascent.

Practical Application of air management

gbrandon once bubbled...
I finished my AOWD class this weekend and on the 3 dives we made deep (74', 78' and 83' …

If you could ascend directly from 83’ to the surface, you should have started the ascent with at least enough air to get yourself and you buddy to the surface safely. With a SAC 1.0, and ascent rate, of 30 f/min and a 3 minute safety stop at 15 feet that would require about 22 cubic feet of air. With an aluminum 80 that’s about 840 psi.

So a good rule of thumb with for diving with an aluminum 80, only where you can ascend directly to the surface, is you should always have a minimum of 10 psi per foot of depth, when below 60 feet.

gbrandon once bubbled...
My computer is set to alarm at 600psi, but on all 3 dives I was under that when I started my ascent, and quite a bit below that by the time I got to the surface (and only 1 did I do a safety stop, and even then it was only a minute long (it wasnt a REQUIRED safety stop).

I don’t believe in using the low air alarm but suggest that having the alarm set to 600 psi is only appropriate for a depth of 60 feet or less.

Kayla once bubbled...
always try to surface with 500 PSI in my tank, just to be on the safe side, I figure, better safe then sorry!


Kayla

IMO the amount of air you have at the end of the dive is moot, other than you don’t want to completely drain the tank. What’s important is that you always have sufficient air to safely get you and your buddy safely to the exit point and surface safely.

All comments, critizizm, and rock throwing is welcome.

Mike

 

[gbrandon]

Im listening to what you guys are saying, and they are very good points. I too believe in whoever mentioned that it doesent matter what you have when you get to the surface, as long as you have enough to get there and do a safety stop if possible. (although there is a fine line between that and having enough air to respond to any emergency situation) I was also wrong to trust my instructor and not give the signal to end the dive and ascend. I shouldnt be depending on him, and should be making my own decisions. Thanks for the input.

 

[MikeS]

gbrandon once bubbled...
I too believe in whoever mentioned that it doesent matter what you have when you get to the surface, as long as you have enough to get there and do a safety stop if possible

Perhaps subtle, yet important distinction; not simply enough to get to the surface, but enough air to get yourself and your buddy to the surface safely. It sounds as if you are relying on your instructor. At some point you’ll want to venture out with a buddy, at that point your buddy should be able to count on you to manage your air in a way that ensures you have enough at all times to get you both to the surface safely. That’s part of being a good buddy.

Mike

 

[pt40fathoms]

On average as a general rule, I follow the rule of thirds. Simply put when my guage reads 900 to 1000 psi, I start to make my way back. I have come out of the water with 800 psi in my tank, and I have come out with 300 psi. At no time did I rush to get out just because my guage said I was at 500 psi, I continued my safety stop at 15 feet and relaxed. I have even sucked the air down to 175 psi hovering at 5 to 10 feet, while I played with a very friendly school of Bass just off the edge of the shore last year. The 500 psi limit is a guide that you should aim for and practice to achieve on every recreational dive.

 

[MikeS]

pt40fathoms once bubbled...
On average as a general rule, I follow the rule of thirds. Simply put when my guage reads 900 to 1000 psi, I start to make my way back.

Pt,

I thought that the "rule of thirds" was, one third out, one third back, and one third for contingency. Following that, assuming and a working pressure of 3,000 psi, shouldn't you be back on the boat with 1,000 psi? How does depth figure into the equation?

Mike

 

[mchiapetto]

Here is a link to another thread on this subject:

http://www.scubaboard.com/showthread.php?s=&threadid=16138

 

[sheck33]

MikeS once bubbled...


Pt,

I thought that the "rule of thirds" was, one third out, one third back, and one third for contingency. Following that, assuming and a working pressure of 3,000 psi, shouldn't you be back on the boat with 1,000 psi? How does depth figure into the equation?

Mike

Depth is not used explicitely unless you want to get a certain time at the certain depth and plan everything in detail. But of course the deeper you are the sooner you will hit your 2/3 turnaround point.

 

[pt40fathoms]

MikeS once bubbled...


Pt,

I thought that the "rule of thirds" was, one third out, one third back, and one third for contingency. Following that, assuming and a working pressure of 3,000 psi, shouldn't you be back on the boat with 1,000 psi? How does depth figure into the equation?

Mike

You know, I was wondering about my post, and contemplating on if I should have added a little more detail in it. Mainly about context. The scenario I was thinking about when I wrote that was recreational diving. You know, off of a shore or boat, no deeper than 130 feet, no overhead environment. In short, a low stress limited fuss goofing around in (under) the water. Camera in hand, or just practicing a few drills for the "game day" dives.

But as for diving deep and more importantly in an overhead environment. You are right. However I would also be carrying more "stuff", like staging bottles, redundant air and so on.

 

[Charlie99]

MikeS once bubbled...
Perhaps subtle, yet important distinction; not simply enough to get to the surface, but enough air to get yourself and your buddy to the surface safely.

I use another subtle variation: My rock bottom is enough air for me and my buddy to do what we need to be reasonably safe; and enough air for what I want to do.

I define the need as a minute at depth (to resolve some sort of problem) and 30fpm direct ascent while sharing air at 2cu ft/min total. I calculate for 30fpm, because I'll be shooting for something less than 60fpm, but may very well end up at half that rate.

What I want is a 20fpm ascent with a couple of 1-2 minute deep stops, and 3 or more minute safety stop, but calculated at 0.5cu ft/min, which is a bit higher than what I actually use.

This makes my rock bottom 1100psi at 110', and 720 psi at 80', using AL80. So after a lot of math, I end up back at the 100 psi per 10' of depth rule -- but now I have a reason for it other than "PADI says".

In reality, unless I'm on a wreck, it is totally a non-issue since I end doing a gradual multi-level profile.

 

[docmartin]

i was a little worried about your remarks regarding safety stops. strictly speaking no safety stop is REQUIRED. however, all research out there suggests it's a really good idea not to skip it. why run an increased risk of dcs? you should never manage your air so poorly that you'd be forced to skip the safety stop. i know as a beginner when your air consumption is higher than everyone else's you don't want to be the first one going up. therefore, beginners will often run their air dangerously low just to stay with the group or their more experienced buddy. you should not feel embarassed about being safety conscious. and a buddy who continues on diving when you show him your air is low deserves to be harassed by titan triggers and remoras for the rest of his diving life.