Too much lift?

[REVAN]

Scuba *with* fairly buoyant exposure suits predate BC's by many years. Rolling up your "1/4" wetsuit (with duck tail) and determining exactly how much lead was needed to sink it was common practice.

I'll add that when you do this exercise, it is important to take care to not have air trapped in the folds of the suit. But knowing the increments of buoyancy for your various pieces of gear is fairly important to being a competent diver.

Examples are good to run through to provide perspective for those who may not sit down and go through the calculations themselves. Here is my example:

My 6mm wetsuit is about 15 pounds buoyant at the surface. Suits don't fully compress according to Boyle's Law, because the neoprene has structure to it and will provide a some resistance to compression. However, Boyle's Law will be a conservative estimate, slightly over-predicting the true variations in buoyancy as a function of depth. At 15 feet, my 6mm suit will have compressed to about 2/3rd its original thickness and the 15 pounds buoyancy at the surface will now be about 10 pounds buoyancy at 5 meters. If I dive to 40 meters, the suit will compress to about 1/5th, reducing the buoyancy to about 3 pounds.

If I plan to be neutral at the safety stop, not the surface, the maximum buoyancy variations due to suit compression I can expect to encounter during a dive at recreational depths is on the order of 7 pounds. This is for a freedive style wetsuit with farmer-john pants and beavertail + built-in hood, which provides 12mm of neoprene on my body core. That's a lot of wetsuit.

If I'm using a 17 pound lift wing with this 6 mm suit, I've got 10 pounds left over for breathing air before maxing out the BC's capability. That will accommodate a 120 ft^3 air tank. If I predict off a little bit, I've still got another 6 pounds of structured breathing enabled buoyancy control that I can tap into for a reserve. If I screw up worse than that, the options are to drop lead or swim out. It's not like there are no other options than to have massive amounts of reserve BC buoyancy, or die.

Suit buoyancy is not just a function of thickness. The size of the suit also matters. If I weighed 270 pounds instead of 170, that 6mm suit would probably have 23 pounds surface buoyancy instead of 15. This is why a 17 pound wing may work perfectly fine for me, but may be inadequate for someone else. Each diver needs to look at their situation to determine what will and will not work for them.

 

[The Chairman]

I'd suggest they create at least as many problems as they solve.

Again, we disagree. What problems do they really create?

The physics remain the same worldwide. Buoyant suits require ballast, and BC's encourage the use of too much ballast.

It's my humble opinion that over-weighting is a training issue. I have learned over my 48 years of diving that simply throwing gear at a training issue never really resolves it. Education is how I choose to resolve training issues.

If backpackers started attaching huge helium balloons to their packs I'd venture a guess they'd be less focused on lightweight gear too.

I have the same backpack I used as a kid. It's a magnesium frame with the body made from nylon that I bought it in 1973 from Sears and Roebuck and it's lighter than 99% of the packs on the market today. People tell me it's too big for a day hike and it probably is. I just don't put a week's worth of food and clothes into it for a day hike. But dayum, I can stuff a week's worth of crap in there if I have to. What I learned in the Boy Scouts along with hundreds of miles on Appalachian trails, the Rockies and even here in Florida taught me to pack exceedingly light. My pack isn't the problem. It's never been the problem. I know my limits and pack accordingly. Buying a newer, lighter pack might be great for the manufacturer, but it won't change the way that I pack. It might be great marketing for them to suggest that a pack that only holds 18 pounds is the best thing going... until I have to carry 20 or 30. Please, I'm not suggesting that your motives are anything but pure. I'm simply making a point that the BC in their closet is probably just fine.

To put it in perspective: a great diver will always use the least amount of weight they need, no matter what BC they have on their back. It's a part of their training and their culture. They will only put in the amount of air they need to be neutral. Large BCs don't cause accidents. Poor training causes accidents. In my estimation, the very best BC is the one on my back at the moment. My BC doesn't dictate the way I dive: I do.

 

[cool_hardware52]

Again, we disagree. What problems do they really create?

Oversized BC permit gross over weighting. It's really that simple.

It's my humble opinion that over-weighting is a training issue. I have learned over my 48 years of diving that simply throwing gear at a training issue never really resolves it. Education is how I choose to resolve training issues.

If there were no BC's or more to the point no BC's with huge, un needed capacity, the training issue would be resolved instantly. Send a diver into the water with 12-14-16-18 lbs more than they need with a BC sized for a proper weighting and the weighting issue is immediately exposed.

The fact remains that far too many deaths play out like this; A grossly over weighted diver swims out using his huge BC, descends, panics, kicks to the surface, doesn't ditch ballast, doesn't add gas to his BC, fatigues, and is found dead on the bottom with their weight belt on and gas in their cylinder.

This entire sequence started with the gross over weighting, which the huge BC's enable.

You might just want to check with the folks doing the body recoveries.

PM me if you'd like a contact.

Tobin

 

[cool_hardware52]

I'll add that when you do this exercise, it is important to take care to not have air trapped in the folds of the suit. But knowing the increments of buoyancy for your various pieces of gear is fairly important to being a competent diver.

Examples are good to run through to provide perspective for those who may not sit down and go through the calculations themselves. Here is my example:

My 6mm wetsuit is about 15 pounds buoyant at the surface. Suits don't fully compress according to Boyle's Law, because the neoprene has structure to it and will provide a some resistance to compression. However, Boyle's Law will be a conservative estimate, slightly over-predicting the true variations in buoyancy as a function of depth. At 15 feet, my 6mm suit will have compressed to about 2/3rd its original thickness and the 15 pounds buoyancy at the surface will now be about 10 pounds buoyancy at 5 meters. If I dive to 40 meters, the suit will compress to about 1/5th, reducing the buoyancy to about 3 pounds.

If I plan to be neutral at the safety stop, not the surface, the maximum buoyancy variations due to suit compression I can expect to encounter during a dive at recreational depths is on the order of 7 pounds. This is for a freedive style wetsuit with farmer-john pants and beavertail + built-in hood, which provides 12mm of neoprene on my body core. That's a lot of wetsuit.

If I'm using a 17 pound lift wing with this 6 mm suit, I've got 10 pounds left over for breathing air before maxing out the BC's capability. That will accommodate a 120 ft^3 air tank. If I predict off a little bit, I've still got another 6 pounds of structured breathing enabled buoyancy control that I can tap into for a reserve. If I screw up worse than that, the options are to drop lead or swim out. It's not like there are no other options than to have massive amounts of reserve BC buoyancy, or die.

Suit buoyancy is not just a function of thickness. The size of the suit also matters. If I weighed 270 pounds instead of 170, that 6mm suit would probably have 23 pounds surface buoyancy instead of 15. This is why a 17 pound wing may work perfectly fine for me, but may be inadequate for someone else. Each diver needs to look at their situation to determine what will and will not work for them.

Exactly why I recommend divers test their own suit. Wetsuits vary widely, the size of the suit, the type of neoprene, etc.

Your numbers closely match my experience, with the typical 6-7mm suit losing ~6-8 lbs at 15 ft. That's just about the weight of gas in a typical single cylinder. This is why I have long recommended for divers in thick suits using normal sized single cylinders initially adjust their weighting so they are eyelevel at the surface with no gas in their wing with a full cylinder. I recommend they conduct an easy no stop dive and verify they can hold a shallow stop with a near empty tank. If they are little light they can either correct that via breathing or simply swim down a little and let their suit compress. The adjust a pound or two for the next dive.

The key number for cold water wing selection is the buoyancy of the diver's exposure suit. Maybe 1 diver in 40 will know what that actually is. It simply is not taught currently. Why bother when virtually every BC has super jumbo capacity........

Tobin

 

[TrimixToo]

Lessee....about 08# per CF for air, right? So ignoring gas composition for the moment, 16-20# of gas on my back...6.5# or so per extra bottom gas or deco bottle (up to 4 of those)...about 20# of bouyancy in my suit with minimal air in it (as tested in fresh water, suit alone with a snorkel, half a lungful of air).... Sometimes big wings are good to cover the contingencies if you want to bring everything up that you brought down and the suit floods completely at just the wrong time. Also, one might wish for the operational simplicity of "hold inflator button until neutral" in place of "shed gear until neutral while holding inflator button" as you're headed for the bottom and want to be going the other way, Right Now.

I think they have their place. Like any other tool they can of course be misused, of course.

From this and prior discussions elsewhere, I suspect we might have to agree to disagree on this. Everyone makes their choices based on their own risk assessments. You're happy with yours, I'm happy with mine. As long as they are reasoned choices and we understand their implications I think that's fine.

John

 

[cool_hardware52]

Lessee....about 08# per CF for air, right? So ignoring gas composition for the moment, 16-20# of gas on my back...6.5# or so per extra bottom gas or deco bottle (up to 4 of those)...about 20# of bouyancy in my suit with minimal air in it (as tested in fresh water, suit alone with a snorkel, half a lungful of air).... Sometimes big wings are good to cover the contingencies if you want to bring everything up that you brought down and the suit floods completely at just the wrong time. Also, one might wish for the operational simplicity of "hold inflator button until neutral" in place of "shed gear until neutral while holding inflator button" as you're headed for the bottom and want to be going the other way, Right Now.

LOL.

1) Nobody in this thread has been discussing tech dives with multiple bottles.

2) The typical tech diver will end the dive with about 1/2 the gas he started with if the dive is uneventful. If events require the "other" 1/2 of the gas be used the least of my concerns is having enough ballast to keep empty stage or deco bottles underwater.

I'm ballasted to be able to hold a shallow stop with empty back gas tanks.

The whole "Well I dive with a bouquet of stage bottles so I need a 94 lbs wing" is hilarious.

Tobin

 

[REVAN]

Lessee....about 08# per CF for air, right? So ignoring gas composition for the moment, 16-20# of gas on my back...6.5# or so per extra bottom gas or deco bottle (up to 4 of those)...about 20# of bouyancy in my suit with minimal air in it (as tested in fresh water, suit alone with a snorkel, half a lungful of air)....

Deco gas, bottom gas, air: How many tech divers pay attention to the weight of the gas in the bottle vs. simply allocating 6.5 pounds for a standard tank and 10 pounds for a big one? Nitrogen at atomic weight 14 being replaced with helium at atomic weight 4 will impact the mass of the gas in the scuba cylinder. There may be some out there that pay attention to these differences, but I get the feeling the MO for many is to just overallocate with familiar numbers and use the BC to make up for the errors. This could be a direct extension of methods learned from basic open water scuba training and practices. i.e. - poor weight tracking made okay by use of a larger than necessary BC.

Oh, and by the way, no one was talking about tech diving here.

PS - Also, I don't know if your first question was a typo or a serious question. Air is quite a bit lighter than 8# cubic foot at standard pressure and temperature; about 0.075 actually.

 

[cool_hardware52]

Oh, and by the way, no one was talking about tech diving here.

Ya, just all those single tank recreational limit divers with multiple stage and deco bottles. Got it.

Tobin

 

[stuartv]

PS - Also, I don't know if your first question was a typo or a serious question. Air is quite a bit lighter than 8# cubic foot at standard pressure and temperature; about 0.075 actually.

Oddly, I had this feeling that he just typoed by leaving out a decimal point and actually meant 0.08 # /ft3. Maybe I was assuming too much...

 

[TrimixToo]

Oddly, I had this feeling that he just typoed by leaving out a decimal point and actually meant 0.08 # /ft3. Maybe I was assuming too much...

That was the case. No clue where the leading decimal point got off to.

I went back and reread the entire thread, and I did get rather off-topic. Sorry, everyone. If someone wants, we can have a tech-related discussion in another thread.

John