Rock bottom, 500 PSI, or something else?

[DanielRJones]

After converting from bar to psi i have ended up with the following.

I am looking to be at the surface at 50 bar or 725 psi from a dive. If i am on a shallow reef of 5-8m i will go down to 30 bar so 450 psi.

On a dive 25+m i am starting my assent at 90 bar 1350psi

 

[DogDiver]

If I’m on the bottom 28 feet below the dive boat looking at critters. What difference does it make if I hit the boat ladder with 100psi or 700psi? None..... I will assess my own risks and as my gas is for me and only me this would be my decision. Oh, been diving for 52 years and have never run out of air. Nore has any team member I’m diving with run out of air. Been real low a few times though.

 

[2airishuman]

@2airishuman have you ever dove on any wrecks with any current? If the OOA happens on the wrong side of the wreck, the last thing most divers should be doing is making a blue water ascent. You have to get to the up-line so you don't end up in the middle of nowhere

Yes, also, shore dives with poor surface conditions. I've been upbraided on these forums for comparing such dives to technical dives.

Nomenclature aside, I think that the appropriate approach to gas planning for these dives is rule of thirds. I don't believe that a "rock bottom" approach can be made to apply.

If I’m on the bottom 28 feet below the dive boat looking at critters. What difference does it make if I hit the boat ladder with 100psi or 700psi? None..... I will assess my own risks and as my gas is for me and only me this would be my decision. Oh, been diving for 52 years and have never run out of air. Nore has any team member I’m diving with run out of air. Been real low a few times though.

I have made a decision that I will always carry with me, sufficient air to rescue an OOA diver, unless I am diving solo in a location where the chances of me encountering another diver are near zero.

 

[BertStevens]

We are in the Basic forum. So I think it’s important to point out that recreational, non-technical diving is no stop diving. In an out of gas situation, you do not have to make a stop. If you have the gas, then a safety stop is advisable. But it’s not required!

 

[2airishuman]

We are in the Basic forum. So I think it’s important to point out that recreational, non-technical diving is no stop diving. In an out of gas situation, you do not have to make a stop. If you have the gas, then a safety stop is advisable. But it’s not required!

The PADI RDP tables distinguish between optional and mandatory safety stops. I suppose the real question is whether it is a wise practice to conduct gas planning on the assumption that the safety stop will be skipped. For the deeper dives where gas planning is a bigger deal, the contribution of the gas used at the safety stop to the total gas required is usually small, so it may not matter much.

 

[boat sju]

I tried to find the stuff that Lamont posted years ago but can't link to his website. Anyone have a copy of his calculations/explanation?

 

[rongoodman]

Yes, also, shore dives with poor surface conditions. I've been upbraided on these forums for comparing such dives to technical dives.

Nomenclature aside, I think that the appropriate approach to gas planning for these dives is rule of thirds. I don't believe that a "rock bottom" approach can be made to apply.

Actually, you need both. Thirds should get you and your OOG buddy back to the line, but you still have to get the two of you to the surface. Thus, the idea of thirds of usable gas.

 

[boat sju]

Actually, you need both. Thirds should get you and your OOG buddy back to the line, but you still have to get the two of you to the surface. Thus, the idea of thirds of usable gas.

Isn't "rule of thirds" actually a cave diving concept that requires more planning behind it than most recreational divers realize?

 

[seeker242]

So, how do you conduct your gas planning for deeper recreational dives?

The standard GUE min gas calculation. Because I enjoy kool-aid.

2 divers X 0.75 ft3 SAC X time needed to ascend x average ATA. = ft3

So for 100' it would be 2 x 0.75 x 11 x 2.5 = 41.25 ft3, rounded up to 42 ft3

(Time needed to ascend = 10ft/min + 1 min)

Then you would convert the ft3 to psi according to what tank you are using. (Xft3 ÷ tank factor x 100 = psi)

So an AL80 @100' = 42 ft3 / 2.5 X 100 = 1660, round up to 1700 psi.

HP100 = 42 ft3 / 3 X 100 = 1383, round up to 1400 psi.

I've never dove with someone, beside other GUE divers, that reserve that much...Which is why I got an AL40 pony.

 

[TMHeimer]

A lot of figures have been brought up. Also a couple pointing out that diving solo vs. considering a OOG buddy differs a lot. And of course other factors on how fast you use your gas in different situations (current, etc.), even when ascending.
I learned from my Deep instructor (a CD) that you begin your ascent allowing 100 psi for every 10 feet of depth...To ascend from 100' start it with 1000 psi. That would seem to violate the rock bottom approach.