Ascending/Decending/Buoyancy

[BlueTrin]

@steinbil , I think people agree with what you said but if you follow the thread many people pointed out that the OP is a new diver and that we are in the basic forum.

We agree that if you have two stages and dive with twin 18L you may need a bit more gas or even a bigger wing but I don’t think that’s what @Jfinch is asking

I know you mentioned you took OW in twinset. I think most bsac divers will have single cylinders for a while and are likely to start with a single 12 steel (or maybe a 15l)

 

[John C. Ratliff]

Not necessarily. The wing might also hold more air than you need, maybe to float a heavy rig. The amount of air you need to dump to descend is irrelevant – imagine having a giant balloon attached to you on the surface, you might still be balanced without it. The amount of air you need to float, however, is relevant.


The reason you "need" the wing is to offset the weight of the gas, not the tanks. So bigger volume of gas (big doubles) = you must be overweighted at the beginning of the dive, otherwise you would be severely underweighted when you're low on gas. Yes, the drysuit can also compensate for that. But a BCD/wing is easier to control and you have redundancy in the drysuit. I assume that diving with doubles before the time of BCDs meant one less layer of protection against possible issues - you would need to use/inflate the drysuit or breathe down the tanks or dump gas from the tanks to be able to comfortably ascend. (emphasis added, jcr)

Okay, I'm not used to the new, high pressure 12 liter steel scuba tanks. Here's one quote about the change in buoyancy in these tanks from a very interesting article published by the U.S. National Institutes of Health, National Library of Medicine, Diving Buoyancy:

Scuba tanks vary in their buoyancy based on their material. An Aluminum 80 cu ft (12 L) tank is 1.6 pounds negative when full (0.72 kg) and 2.8 pounds positive with 500 psi (1.28 kg at 50 bar). A 12 L steel tank is about 9 pounds (4 kg) negative full and three pounds (1.36 kg) negative empty. (emphasis added, jcr)

If wearing 12 liter steel doubles, then the buoyancy change is 6 pounds per tank, or 12 pounds of weight as the air is used. In this case, I can see where the BCD is required by the diver. However, the change for an AL 80 is only 4.6 pounds. Here's what the above source states:

Neutral Buoyancy Summation

The goal of these combined systems is to obtain neutral buoyancy which maximizes the diver’s trim. Trim is the diver's attitude in the water, in terms of balance with the direction of motion. Accurately controlled trim decreases swimming effort, as it reduces the sectional area of the diver as they pass through the water. A slight head down trim is the best position to reduce down thrust during finning, and this reduces silting and fin impact with the bottom. It also allows the diver to look down comfortably. A stable horizontal trim requires that the diver's center of gravity to be directly below the center of buoyancy (the centroid).

On the surface with a full tank of air, a diver can adjust their lead so that when their BCD is fully deflated and they have a half breath of air, they float at eye level in the water. This is the standard for initial assessment of neutral buoyancy. As they make their descent, their neoprene, and the residual air in their BCD is further compressed. They may then need to add air via filling their lungs or adding air to their BCD to once again achieve neutral buoyancy at depth. On ascent, the release of BCD air and a long slow exhalation is needed as the air expands (according to Boyle’s law) to control buoyancy, lung inflation, and rate of ascent.

Other advantages of neutral buoyancy maintenance include reduced energy expenditure, reduced decompression stress, better gas management, and a sense of freedom and weightlessness without free fall. The diver can hover upside down and peer into crevices and holes without fear of striking the bottom. They can lie on their back in the water column without sinking to spy surface marine creatures and boats. Most importantly they can relax, and focus on work or simple enjoyment while minimizing environmental impact. [1]

There is only one reason to wear double 12 liter tanks, and that is to have a lot of air for deep or cave or under ice diving; the deep diving is usually well below 100 feet. For this a diver is considered a technical diver, as most likely the diver will require decompression before surfacing.

This is a "Basic Scuba" forum; technical diving is not the rule here, and discussing the needs of a technical diver in a basic forum can lead to a basic diver using technical techniques inappropriately. Keep this in mind when you post here.

SeaRat

 

[wetb4igetinthewater]

There is only one reason to wear double 12 liter tanks, and that is to have a lot of air for deep or cave or under ice diving; the deep diving is usually well below 100 feet. For this a diver is considered a technical diver, as most likely the diver will require decompression before surfacing.

There is another reason. You are lazy like me and don't want to swap out cylinders in between dives.

 

[John C. Ratliff]

There is another reason. You are lazy like me and don't want to swap out cylinders in between dives.

That only works if you are on a boat or close shore dives, and not, like me, having to walk several hundred yards in gear to your car.

SeaRat

 

[boulderjohn]

When diving with heavy steel tanks, I have to dump a lot of air to break contact with the surface. Then, immediately start adding back to control the descent. Once those heavy tanks start down, they accelerate faster than AL tanks.

With AL tanks, I can just go head down. That’s harder to do with steel doubles because the mass of the tanks wants to fight big changes in direction.

Aluminum tanks are more buoyant than steel tanks, so to be properly weighted when using aluminum tanks, you will need to carry more weight. Once you have added that extra weight, there should be no difference. It sounds as if when you use steel tanks, you are a bit overweighted, and when you use aluminum tanks, you are a bit underweighted. That is overweighting and underweighting depends your entire package, not just the tanks.

 

[John C. Ratliff]

Not necessarily. The wing might also hold more air than you need, maybe to float a heavy rig. The amount of air you need to dump to descend is irrelevant – imagine having a giant balloon attached to you on the surface, you might still be balanced without it. The amount of air you need to float, however, is relevant.


The reason you "need" the wing is to offset the weight of the gas, not the tanks. So bigger volume of gas (big doubles) = you must be overweighted at the beginning of the dive, otherwise you would be severely underweighted when you're low on gas. Yes, the drysuit can also compensate for that. But a BCD/wing is easier to control and you have redundancy in the drysuit. I assume that diving with doubles before the time of BCDs meant one less layer of protection against possible issues - you would need to use/inflate the drysuit or breathe down the tanks or dump gas from the tanks to be able to comfortably ascend.

No, not necessarily. The reason is that the largest doubles we were using were twin steel 72s, with a normal pressure of 2250 psig (2475 psig if they still had the "+" rating). These tanks were not unreasonably heavy in the water. The photo below has two U.S. Air Force Pararescuemen diving twin 72s without any BCD (we didn't have them in 1968), in Okinawa waters. Note, no wetsuit too. These doubles were made to be pretty much neutral in salt water when empty. That cannot be said today, where these heavy, huge tanks are meant to hold as much air as possible at as high a pressure as possible.

SeaRat

 

[Doc Harry]

Once I get my weight dialed in, I use my breath to control my depth.

I dump my BC to descend from the surface, and then I add air as I approach the bottom (because I'm the heaviest I will be for the entire dive).

From there, I spend the rest of the dive dumping small amounts of air from my BC as I consume air and my aluminum tank gets lighter. Depth is all breath control. I never add more air to my BC unless I descend significantly deeper.

Halfway through the dive my BC is empty - all the air has been dumped. Its all breath control for the rest of the dive, through the safety stop and to the surface. That's the benefit of proper weighting.

 

[steinbil]

Once I get my weight dialed in, I use my breath to control my depth.

I dump my BC to descend from the surface, and then I add air as I approach the bottom (because I'm the heaviest I will be for the entire dive).

From there, I spend the rest of the dive dumping small amounts of air from my BC as I consume air and my aluminum tank gets lighter. Depth is all breath control. I never add more air to my BC unless I descend significantly deeper.

Halfway through the dive my BC is empty - all the air has been dumped. Its all breath control for the rest of the dive, through the safety stop and to the surface. That's the benefit of proper weighting.

Unless you're talking about very square profiles, why wouldn't you adjust your BC to different depths so you don't have to modify your breathing all the time - it seems to me that an exaggerated dependence on adjustment of the breathing cycle could lead to inefficient ventilation. Also, if you're neutral half way through the dive with an empty BC, wouldn't that restrict your ventilation on the last part of the dive? Seems unnecessary to me, even if it might work.

 

[Doc Harry]

Unless you're talking about very square profiles, why wouldn't you adjust your BC to different depths so you don't have to modify your breathing all the time - it seems to me that an exaggerated dependence on adjustment of the breathing cycle could lead to inefficient ventilation.

I don't need to adjust my BC and I don't need exaggerated breathing cycles when I'm neutrally buoyant. It takes no effort really, I don't even think about it.

What you're talking about is someone who is overweighted. Or perhaps someone with a thick wetsuit.

 

[steinbil]

I don't need to adjust my BC and I don't need exaggerated breathing cycles when I'm neutrally buoyant. It takes no effort really, I don't even think about it.

What you're talking about is someone who is overweighted. Or perhaps someone with a thick wetsuit.

My point is this:
If you're neutrally buoyant at any point in your breathing cycle (let's say half-empty) at 30m/100ft, you will not be neutral at 15m/50ft with the same amount of gas in your wing. Which means you have to change your breathing cadence significantly to compensate, unless you vent gas from the BC.

Now maybe in practice it is less of a problem, because you probably won't do a lot of yo-yo profiles, and when you vent as you use gas, you will probably also vent to compensate for going shallower towards the end of the dive. So it ends up working out ok for you.

However, I still think it's a sub-optimal approach, as there is no need to alter your breathing cycles if you just make small adjustments to the BC as you go - which is also what @oya recommended in his blog about buoyancy, and what I've heard from other instructors. And also as advice for newer divers, I think it can lead to problems when they don't use their BC enough to get buoyant, but rely to heavily on their breath, especially if they're overweighted, but also if they're not.