Gas Math - Help

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What happens if you hit deco obligation on a dive that you share air on the exit?....Takes over 2/3 to get you out AND do deco. Other factors play a role too that I'm not thinking of, I'm sure.

I'm referring strictly to no-deco dives....obviously if deco is anticipated you would factor that into your dive plan.
 
What happens if you hit deco obligation on a dive that you share air on the exit?....Takes over 2/3 to get you out AND do deco. Other factors play a role too that I'm not thinking of, I'm sure.

you blow off the stop and call your friend 'DAN' to come pick you up...
 
you blow off the stop and call your friend 'DAN' to come pick you up...

Why in the world would you blow off a stop and risk getting bent so you could surface with air in your tank? 100ft for 28 mins is a deco dive, and could be done on a 100 but not with a practical margin of safety. Using the OP's consumption rate, this dive needs about 80cu/ft to complete, if nothing went wrong and tables were followed.
Safe Dives
Charlie
 
Rule of 1/3 is used when you HAVE to come back to the point of origin, like a hard overhead; caves, wrecks.

In the ocean you can always go up to the surface so you do not have to use rule of 1/3 if you do not have to. You could use 1/2 if it would be NICE to come back to the point of entry. Or just a set margin (like Rock bottom) to ensure you and a buddy get up to the surface.

Now, in the ocean you can also have virtual overheads and you adjust gas requirements to that. The OP is a new diver and as such we can safely assume he/she is not doing deco/nitrox diving.

Too often you see divers use these gas rules without it being needed for the dive.

Always have enough gas to make it back to the surface, but try to do that with gas planning and thinking about it. Blindly applying ' gas rules' could end you up with not enough gas. You should never hit a deco obligation by accident. If you do, you are cutting things too close.

I think OP is on the right path by thinking about the gas needs. I just do not see why 1/3 is needed to go up........
 
Always have enough gas to make it back to the surface, .....
... while sharing air with your buddy.

I prefer to state the most basic gas planning rule as
"always have enough to safely abort the dive while you and your buddy are sharing air".

You just have to think a bit about what is required to safely abort the dive.
Sometimes it means you need to swim back out of a cave or wreck. 1/3 to go in, 2/3 reserved for swimming out while both divers are using the same tank on the way out. This is the original "thirds" rule. Note that there isn't any margin for increased SAC while swiming out while sharing air. Contrary to popular belief, the rule of thirds is not at all conservative if one truly needs to get back out in order to be safe.

Sometimes you would LIKE to swim back under a shipping channel, or go back to a certain exit point, but you are willing to accept a bit more risk by surfacing early if you need to share air. For this a 1/2 + few hundred psi is reasonable (Assuming you don't need to fight a current on the way back)

Sometimes simply going directly to the surface is a safe abort. For this one, rock bottom calculations are sufficient.

Applying a rule of thirds is ridiculous when it isn't needed. Misapplying the rule of thirds to mean turnpoint is 1/3 of total gas is truly ridiculous.
 
Rule of 1/3 is used when you HAVE to come back to the point of origin, like a hard overhead; caves, wrecks.

In the ocean you can always go up to the surface so you do not have to use rule of 1/3 if you do not have to. You could use 1/2 if it would be NICE to come back to the point of entry. Or just a set margin (like Rock bottom) to ensure you and a buddy get up to the surface.

Now, in the ocean you can also have virtual overheads and you adjust gas requirements to that. The OP is a new diver and as such we can safely assume he/she is not doing deco/nitrox diving.

Too often you see divers use these gas rules without it being needed for the dive.

Always have enough gas to make it back to the surface, but try to do that with gas planning and thinking about it. Blindly applying ' gas rules' could end you up with not enough gas. You should never hit a deco obligation by accident. If you do, you are cutting things too close.

I think OP is on the right path by thinking about the gas needs. I just do not see why 1/3 is needed to go up........

Agree...."By the book" rule of 1/3's is used for overhead evironments and deco diving. Half + 2 is used for open ocean/no deco diving.
 
I think this is getting confusing, and the original poster has less than 25 dives.

First off, let me commend the OP for doing this planning. Most divers don't bother.

Second, let's talk briefly about gas planning. Dives basically come in three types. In the first, you go down and you come up wherever you want and the boat picks you up there (live boat diving). In that case, you calculate your rock bottom, or minimum gas, subtract it from the gas you begin with, and you can use what's left. That's an "all available air" dive.

When you're shore diving, or diving off an anchored boat where you would PREFER to get back to your starting point, but it's not a disaster if you don't, you subtract your minimum gas and diving what's left by two. You use half on the outbound leg, and half coming back.

When you MUST get back to your starting point (overhead environments, diving off an anchored boat in high current or without a chase boat, and the like) you remove your RB and divide what's left into thirds. The reason for this is that, if your buddy has a gas emergency at your furthest point out, you need to be able to swim him back to the starting point and then get him to the surface.

Some technical systems do not reserve RB to begin with, feeling that dividing the gas into thirds is conservative enough (and this may well be true if you are diving teams of three).

No novice diver should be doing a dive that requires gas planning using the rule of thirds, in my opinion.

For a dive off a live boat to 100 feet, I would use 40 cu ft as rock bottom, which in an LP 100 would be just about 1000 psi. If it were a square profile dive, I'd start my ascent at 1000 psi, which actually works out just about the thirds you calculated in your original post.
 
Why in the world would you blow off a stop and risk getting bent so you could surface with air in your tank? 100ft for 28 mins is a deco dive, and could be done on a 100 but not with a practical margin of safety. Using the OP's consumption rate, this dive needs about 80cu/ft to complete, if nothing went wrong and tables were followed.
Safe Dives
Charlie

Sorry, perhaps I misunderstood. I thought he was asking what to do if he/she was buddy breathing and they were both OOA AND had a deco obligation. Of course, if you have the air, make the stop... Of course, you should have the air, because you've planned properly.
 
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