Equipment for Adv Nitrox and Deco Proc

[Meng_Tze]

I'm confused. The idea of Rock Bottom ONLY refers to the gas reserves that have to be maintained to get you and a buddy to the surface (for NDL diving) or to the next gas switch (for staged decompression diving). The management of the usable gas is determined by the type and conditions of the proposed dive.

Doesn't every agency teach you to maintain enough gas to get you and one other diver to the gas switch?

Some agencies indeed teach that: usable gas= (total volume-RB)*rule of x.

Where X can be rule of 1/3, or 1/2 or something else depending on the situation and dive. I don't believe any agencies outside the ones that teach fundies or one of it's derivatives teach this.

 

[Divin'Papaw]

Some agencies indeed teach that: usable gas= (total volume-RB)*rule of x.

Where X can be rule of 1/3, or 1/2 or something else depending on the situation and dive. I don't believe any agencies outside the ones that teach fundies or one of it's derivatives teach this.

I was taught rock bottom based gas planning by Duane Johnson (ppo2_diver) in his PADI Wreck Specialty course. Duane is a DIR proponent and includes many of the DIR concepts in his courses, both PADI and TDI. In my TDI courses I've been taught basic 'rule of X' (typically thirds) based gas planning, not rock bottom based thirds. I do understand both however and feel completely equipment to use whichever algorithm I feel best fits the specific dive being planned.

 

[Flightlead]

Really? That seems like overkill for drifting deco. TDI standards require only >=25 lbs. I've been more than happy with a 50 lb bag.

my 50lb bag has toppled over and dumped some air making it much less buoyant. my team mates 100lbs don't topple over with the same amount of air in them. since they pack the same and cost the same, why not get 100lb?

 

[Flightlead]

Dave, excuse my ignorance (I'm not open water tech trained), but don't you still have to figure some kind of rock bottom type reserve for extended range diving? If you use your backgas as your bottom mix, don't you still have to make sure you have enough to get you and a buddy up to your first gas switch?

dsat has you carry 1/3 additional above your planned volume for all gasses

 

[Web Monkey]

my 50lb bag has toppled over and dumped some air making it much less buoyant. my team mates 100lbs don't topple over with the same amount of air in them. since they pack the same and cost the same, why not get 100lb?

You can get bags that contain an internal valve (a flap, actually) to keep the air in.

The OMS and Carter SMBs do this. I assume others do too. These are SMBs (long sausage-type things) not lift bags (more or less square shape), so if you need an actual lift bag and not an SMB, please ignore this.

Terry

 

[Divin'Papaw]

my 50lb bag has toppled over and dumped some air making it much less buoyant. my team mates 100lbs don't topple over with the same amount of air in them. since they pack the same and cost the same, why not get 100lb?

Good point. Hadn't considered that.

 

[tstormdiver]

Having said all that is anyone aware of any case of a doubles divers being rendered completely out of gas?

My instructor has told me that he once cracked his manifold (primary side) in a cave. He isolated his tanks, breathed off of the leaking tank until it was empty, while making his way back out, then went to the second tank & breathed it down all the way & still had about 75- 100ft to go to get to his stage bottle. He said it was one of the times he didn't think he was going to make it out alive. That's the only case I've heard of that. But it was also technical diving, not open water.

 

[wedivebc]

My instructor has told me that he once cracked his manifold (primary side) in a cave. He isolated his tanks, breathed off of the leaking tank until it was empty, while making his way back out, then went to the second tank & breathed it down all the way & still had about 75- 100ft to go to get to his stage bottle. He said it was one of the times he didn't think he was going to make it out alive. That's the only case I've heard of that. But it was also technical diving, not open water.

Right but my point is that he had enough of his own gas to get mostly back. If he had been with a buddy your instructor would not have had an issue.

 

[ucfdiver]

Right but my point is that he had enough of his own gas to get mostly back. If he had been with a buddy your instructor would not have had an issue.

I think most people who solo dive in a cave at least carry a buddy bottle, so I'd say this is a rare case. Maybe I'm wrong, but that's just my personal observation, certainly not from personal experience as I don't think I'll ever solo cave dive.

 

[Meng_Tze]

my 50lb bag has toppled over and dumped some air making it much less buoyant. my team mates 100lbs don't topple over with the same amount of air in them. since they pack the same and cost the same, why not get 100lb?

If you keep tension on an open bag, it will not topple..... I believe the requirement is 50lbs (which is a LOT).