What pressure to end a dive? A debate!

[amascuba]

The rule of "Be back at 500psi" is just laziness for a industry that doesn't teach proper gas management. I don't think there much of a debate of that. People can argue that they were taught gas management in in their tech classes or by some odd instance an instructor actually covered it in detail in a recreational class, but this thread seems to be based on recreational divers and I have not read any course standards from any WRSTC compliant agency that covers gas management.

Check out Lamont's Rock Bottom and Gas Management for Recreational Divers Reference paper.

 

[String]

First stages typically deliver gas at 10bar above ambient. However that doesnt mean they stop giving gas when the pressure is lower than that, merely that the IP is lowered before eventually stopping.
I can quite easily imagine 2-3 bar being the point the valve will no longer open and thats well inside the gauge and human read error on most SPGs.

 

[HowardE]

The rule of "Be back at 500psi" is just laziness for a industry that doesn't teach proper gas management.

I agree with this... it has more to do with an aribtrary "be back at the boat" number. It doesn't take into account someone's tank size or service pressure of the tank either.

 

[fisherdvm]

Please tell me that you did not really mean "Decompression" in the above post??

I think it means exactly what I meant, the safety stop which many of us use as a safety margin, just in case you don't trust your no-deco table. Otherwise, you would bolt to the surface if you trusted your computer, wouldn't you??

 

[fisherdvm]

The reason for the red zone on the gauge isnt due to low gas - its the tolerance limit of the gauge. Below that figure the reading is not as accurate regarding gas pressure so shouldnt be relied upon.

Nice to know that. So it is kinda like some gas tank having 1/4 full when it is empty, and some car has a gauze that reads 1/8 full when it actually isn't??

 

[Doc Intrepid]

Hi Cody,

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?

You are correct. The diver would be safer to calculate their 'end pressure' as a percentage of their total gas available. I am likely too conservative a diver for many recreational situations, but I agree with Nemrod when he said

"The INTENT of the rule of thirds 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."

"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!"

Again IMHO, you are dead on. You need to gauge your gas planning based on the circumstances and the environment, not use the same yardstick for every dive in every location. Moreover, remember that 'issues' often arise at the end of dives when divers who fail to plan suffer OOAs, divers get cold, etc. You may need that gas to be able to perform rescues at the end of dives. It isn't necessarily 'wasted gas' because you plan to get back to the boat with 700 psi (as an example)...

 

[Diver Dennis]

Th amount of air I have when getting back on the boat is regulated more by time. I always plan with the DM when I'm solo for a maximum 60 minute dive so other people on the boat are not waiting. If I'm on a second boat pick up plan or doing a shore dive and taking pictures in 15' of water or less, I will come up with 100psi. This is in a warm water, good vis situation.

I'm not advocating this for anyone, just telling you what I do.

 

[MikeFerrara]

First stages typically deliver gas at 10bar above ambient. However that doesnt mean they stop giving gas when the pressure is lower than that, merely that the IP is lowered before eventually stopping.
I can quite easily imagine 2-3 bar being the point the valve will no longer open and thats well inside the gauge and human read error on most SPGs.

The first stage valve is normally open. It's the intermediate chamber pressure rising that closes the valve. That means that when tank pressure gets too low, the valve will no longer close and will be open all the time. You'll be able to breath until the tank pressure falls to near ambient pressure and then the tank is empty for all practical purposes.

 

[String]

So in reality 3-4 bar or less for most diving.

(which also agrees with my breathing on a zero gauge).

 

[HowardE]

Th amount of air I have when getting back on the boat is regulated more by time. I always plan with the DM when I'm solo for a maximum 60 minute dive so other people on the boat are not waiting. If I'm on a second boat pick up plan or doing a shore dive and taking pictures in 15' of water or less, I will come up with 100psi. This is in a warm water, good vis situation.

I'm not advocating this for anyone, just telling you what I do.

i migh (and have) do this (100psi) as well, when I dive from my beach in Ft Lauderdale. Its only 200 yards to the reef from shore, with a MAX depth of 15 FSW. I wouldn't advocate it to anyone either, but I'm in MY comfort zone there. I wouldn't do that from a boat (probably because the boat crew would freak out if they saw my SPG)