What pressure to end a dive? A debate!

[Codyjp]

It seems that the general rule for recreational diving is to return to the surface with no less than 500psi remaining in your tank. Now, here is my question. If you are diving with a low pressure tank at 500psi you are returning with a whole lot more air then if you are returning with 500psi in a HP tank.

Examples: All considering 80cf tanks

@2640 500psi=19% air remaining

@ 3000psi 500psi=16.6% air remaining

@ 3442 500psi=14.5% air remaining -- oops, that’s about 25% less than on a LP tank!

Now, assuming a dive with a 3442 tank you were able to dive for 40 minutes until you are down to 500psi. With that 500psi remaining you will be able to continue diving for another 6.8 minutes under the same conditions. 6.8-3 minutes for deco stop leaves you with 3.8 minutes to dive. Better not have any problems!

With a 2640psi tank with the same scenario you would be left with 9.3 minutes or 6.3 after the deco stop. Nearly double the time from the above scenario.

Another factor that will be of benefit to you is that as you ascend you will use air at a slower rate, but that is too difficult for me to calculate!

So my question is; wouldn’t a diver be safer if he/she were to calculate their surfacing pressure as a percentage of their total air available? There are many other factors that will affect this as well (that should satisfy the cave/tech/deep divers). I understand that the cave diver standard is 1/3 of your pressure remaining when back at the surface. Maybe the rec diver should consider something similar to this? When diving a reef at 30’ maybe 33% of your air remaining is a bit excessive, but under many circumstances 500psi is not an acceptable rule of thumb either!

What do you think?

 

[String]

"Back with 500psi" or "50 bar" isnt gas managment or planning at all. Its basically a simple rule employed mainly by dive operators to give some sort of sanity.

What you mentioned above is planning for 1 diver...What if a buddy needs air? What about elevated breathing rates? What if you need to return to a shot line prior to surfacing? Countless things to consider and things that the "back with 500" doesnt attempt to address.

Plenty of threads on here before about it, search for "rule of thirds", "rock bottom" and similar topics should yield a lot.

Another factor that will be of benefit to you is that as you ascend you will use air at a slower rate, but that is too difficult for me to calculate!

Thats actually very simple if you think about it - you know the ascent rate therefore time it takes, you can therefore work out an average depth for that segment.

 

[rongoodman]

One of the nice things about Rock Bottom calculations is that they're usually done in cubic feet, which you can then convert to pressures for your particular tank. However you do it, you need to have enough air at any given depth to get two stessed divers back to the surface (which might or might not be a direct ascent).

 

[spectrum]

So my question is; wouldn’t a diver be safer if he/she were to calculate their surfacing pressure as a percentage of their total air available?

As you said the 500 PSI rule is just a shot in the dark. A diver should always be planning based on the actual gas volumes, regardless of the pressure required to stuff it into a given cylinder.

Pete

 

[TheHobster]

As was mentioned, leave the bottom with enough for you and your buddy - the vast majority of your dives you will exit with alot of air, especially as your air consumpotion improves, or if you frequently dive with newbies - safety is always good

that 500 psi rule is an increase from the 300 psi rule when I learned in the early 70's - the tech/cave rule of thirds is explained as 1/3 in, 1/3 out, 1/3 for my buddy - surfacing with 1/3 of your gas is expected

Wait'll your first OOA; you'll appreciate how much you have after that!

 

[Web Monkey]

It seems that the general rule for recreational diving is to return to the surface with no less than 500psi remaining in your tank. Now, here is my question. If you are diving with a low pressure tank at 500psi you are returning with a whole lot more air then if you are returning with 500psi in a HP tank.

500 PSI is just enough air so that with a crappy SPG and a diver who isn't watching too closely, they probably won't have to VIP or clean the tank when it comes back. As noted, it is not, however, gas planning.

However, if the boat guy is scary enough, telling divers that they have to be back on the boat with 500 PSI will probbaly make them start to surface when they hit 1000, which is generally in the right neighborhood with an AL 80 on a not too deep recreational dive.

Terry

 

[Cacia]

However, if the boat guy is scary enough,

lol...yes, exactly right. Like telling the kids to be home before dark.

 

[RoyN]

When my tanks hit 700, thats the time to go back in. They're neutral 80 aluminum tanks and can hold up to 3300. Hmm, wonder if I could pump nitrox and use it as a nitrox tank since I got certified for it .

 

[kidspot]

I'd suggest sending a PM to NWGratefulDiver and asking for his gas management paper - he's been very kind sharing it with many here on SB and it's an excellent paper on the subject. Plus it's very well written and easy to understand.

 

[Nemrod]

Oh no, here we go again.

The rule is and always has been though rarely fully employed and often payed lip service only is The Rule of Thirds.

This rule is actually a guide line to which the requirments of the dive can be accomodated. Many people who use the rule will not employ it to it's fullest extent for shallow, easy dives but will strictly employ it for more demanding situations. As has been pointed out in another thread--what about a drift dive where there is no out and back and the boat follows you? Obviously this rule needs to be applied with a bit of common sense. The INTENT of the rule is that you should PLAN your dive to have enough air to accomplish the dive and have an adequate reserve. The reserve is used only in emergency, therefore you will be back on the boat or beach or wherever with that reserve intact. Planning a dive includes turn pressures etc and forseeable what ifs.

The 500 psi resort rule is better than nothing. That means back on the boat with 500 psi. OK, assuming an anchored boat dive in mild current as an example, 60 feet or so, warm and clear water, no hazards of undue danger, a diver may using the common 3,000 psi aluminum 80 go say 1400 out (up current) or even a bit more since you will drift back, 1100 back and up and safety stop leaving 500 psi back on the boat. If your using a LP tank then you still divide by thirds or whatever of that nature to produce the same result but the pressures will be lower.

Yes, using 500 psi or (X) pressure "back on the boat" will cheat you if your held to that and your diving LP steels and everybody other than you is on aluminum 3,000 psi tanks. Usually dive ops realize this and allow you to adjust for it--usually.

N