Weighting question

[FishWatcher747]

I have a question about multiple standard tank diving.

So the weight of compressed gas in a standard cylinder is apparently 5.9 pounds. But one doesn't plan to breath all of the air in the tank before the 15 foot safety stop. So let us say we only intend to breath 4.5 pounds and be neutral at the 15 foot safety stop. To do so we must start the dive negative 4.5 pounds.

Now let us say we are diving standard cylinders but we take 2 (doubles or sidemount). Again we intend to extend the dive and breath 4.5 pounds of each cylinder. We must start the dive 9 pounds negative.

Now let us say we are taking doubles plus 2 standard stage bottles. We intend to breath 4.5 pounds out of each cylinder. We need to start the dive 18 pounds negative.

Now we are setting some open circuit depth record dive and taking 20 standard cylinders. We need to start the dive 90 pounds negative.

Is this correct?

If it is correct isn't buoyancy harder because there is a progressive increase in gas in the wing/drysuit compared to single cylinder in every aspect of the dive before the safety stop?

 

[johndiver999]

I try to have enough lead to be neutral at zero feet NOT 15,

 

[CuzzA]

Yes, the more over or under weight you are the more difficult it is to control buoyancy.

I'm not sure what you mean by "setting open circuit records." People who do that are often tethered to a line and stages are placed on the line. Same deal with expedition cave diving, stages are often pre placed rather than carried all at once.

That said, I think once a diver decides he's going to go much further beyond technical diving using doubles and maybe slinging a bottle or two, he or she will move on to closed circuit for the reason of having to deal with multiple tanks, and the cost of helium.

 

[Jacob BV]

You actually need to account for all the gas you carry. So in your example you'd need to start the dive at least 5.9 pounds negative for a single. In metric it is roughly 0.0012kg/L of gas. So in a twin 12s that is 210bar *24L*0.0012 ~=6kg.

You need to be at least this negative at the start of a dive if you want to be able to use all that gas in an emergency. Yes you don't plan to use it all but you need it as an option. If you were only 4kg negative at the start and your buddy run out of air you would quickly start to struggle holding stops.

With regards to the record breaking stuff they are such a niche case they may do things very differently so to avoid being impossibly heavy (especially as they have safety divers and stuff).

 

[J-Vo]

Technical divers weight themselves neutral with empty primary tanks. Everything else is extra detachable weight. If in emergency you use all of a deco/stage gas and it's making you lite, you simply ditch it.

 

[FishWatcher747]

You actually need to account for all the gas you carry. So in your example you'd need to start the dive at least 5.9 pounds negative for a single. In metric it is roughly 0.0012kg/L of gas. So in a twin 12s that is 210bar *24L*0.0012 ~=6kg.

You need to be at least this negative at the start of a dive if you want to be able to use all that gas in an emergency. Yes you don't plan to use it all but you need it as an option. If you were only 4kg negative at the start and your buddy run out of air you would quickly start to struggle holding stops.

This is exactly the explanation I was seeking.

For example if one dives with a pony bottle like I do and never intends to use it one still needs to account for the weight of that gas in an emergency ascent when all of the gas in the primary tank is depleted for some emergency reason.

Otherwise it is going to be difficult to hold a safety stop.

 

[FishWatcher747]

Technical divers weight themselves neutral with empty primary tanks. Everything else is extra detachable weight. If in emergency you use all of a deco/stage gas and it's making you lite, you simply ditch it.

I did not know ditching stage bottles was a thing. Are there real life examples where this has happened?

 

[inquis]

isn't buoyancy harder because there is a progressive increase in gas in the wing/drysuit

In theory yes, but practically no. The reason to carry all that is to go deep, and the buoyancy change per unit of depth is significantly less when deep. By the time you're shallow, much of the "excess" gas weight is gone. Normally not all, of course, but hopefully someone in that position is skilled enough to deal.

That said, I have seen some buoyancy struggles following a break, until the rust has been knocked off. That's one big reason to ease back into things.

 

[inquis]

I did not know ditching stage bottles was a thing. Are there real life examples where this has happened?

 

[xan]

I did not know ditching stage bottles was a thing. Are there real life examples where this has happened?

ISE teaches to”throw” empty stages when you’d end up with more than one by the end of the dive.