Buoyancy question

[Imla]

I don't know the difference between 3 and 5mm suits... But I put on 2 pounds when i go from boardshorts to 3mm full suit. So it doesn't sound too off.

 

[herman]

Will, a couple of more things to take into consideration, the material of the wetsuit can make a considerable difference. Different brands and qualities of neoprene have vastly different bouyancy characteristics. If one is more dense than the other, it will require less lead to offset it. Also, there may be so additional lead requirements due to the BC itself, some have 4 or more pounds of inhearant bouyancy. I would suggest doing some good bouyancy test of the wetsuits and the BC.

A noob here. A bit off topic question. How does one hold buoyancy without a wing or BC? Seems like unless you get weights exactly for neutral buoyancy, you will spend time fighting (to a small or a large degree) to stay at a certain depth as your tanks shift weight. Thus, how would you make adjustments to buoyancy though out the dive? ...by picking up some stones at the bottom?
...or do you overweight yourself and then constantly adjust your depth as you sink bit by bit?

PS: What is a "horse collar" BC configuration?

Actually, no BC diving is quite easy once you get your weighting correct. If you are diving a thick wetsuit and/or a poor quality ones things get more difficult, as will large capacity tanks. If however, you are diving 80 cf or so of gas and no or thin/high quality wetsuits it easy. The shift in bouyancy of 80 cf of gas is around 4 lbs, properly weighted for no BC, thin wetsuit diving you would start a dive about 2 lbs negative, as you reach 1/2 tank you will be dead on neutral and about 2 lbs positive at the end of the dive. If you are wearing a thin or high quality wetsuit you may have to vary the weights slightly but not that much. The majority of shift in a wetsuit is within the first 30 ft so it is not uncommon for us when diving a wetsuit to slightly underweight at the surface and just swim the rig down the first 15 feet or so. Being 2 lbs above or below neutral is not a big deal the handle. Most divers are way overweighted but have no clue they are. If you are diving 80 cf or less tanks with thin or no wetsuits and you touch your BC more than once or twice after leaving the surface until the time you return to the surface, you are overweighted. I encourage every diver to learn the no BC technique even if they never remove the BC (just dump ALL of the air at the beginning and don't touch it until you are back on the surface) because unitl they do, they never actually understand it is a bouyancy COMPENSATOR, not an "add air until I don't sink because I have no clue how to weight myself" compensator.

 

[miketsp]

..snip..
I am just wondering if this marginal, 2 pound difference in buoyancy between 3mm and 5mm suits is typical or if, as usual, I am an oddball.

It goes against both the laws of physics and my experience.
You can run some simple calculations if you do it in metric.
My body area is very close to 2 sq meters (you can find formulas on the Internet with varying degrees of accuracy) so if the neoprene didn't weigh anything then I would be displacing 2kg of water (assuming density of 1000kg/cu meter - not exactly right for sea water but close enough for a first approximation) for every extra mm of neoprene.
So the first question is, did you also go to a thicker hood, boots and gloves? If not then you can reduce the shift proportionally.
Next issue is the density of the neoprene you bought. There are a LOT of different qualities on the market from the very soft stuff normally used in surfing suits to good quality diving suits, but a typical range nowadays would be 160 - 300 kg/cu meter. (This doesn't cover the very hard rubber/neoprene suits we used 40 years ago before BCs were common). If we take an average value of 250kg/cu meter then we expect a net result of 1.5kg per additional mm of neoprene.
So at least 3kg or almost 7lbs to go from 3mm to 5mm.

This is not far off my personal variation. I add 4kg to go from my tropical suit (3mm long, 2mm hood, no gloves) to my temperate suit (5mm long, 5mm hood, 3mm gloves) and a further 3kg to go to my semi-dry 7mm with a 2mm vest on my torso.

 

[Craig66]

My personal variation has been: 2 pounds to go from a 3 mm to 5 mm but 3 pounds to go from a 5 mm to a 7 mm


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[Paladin]

From the varied responses seen so far, it would seem that there is no true "normal" in this area. One just has to go with what works.

I can live with that.

 

[DivemasterDennis]

Exactly, Paladin. The variable factors in how much weight you will add going from a 3 mil to 5mil full wetsuit include: 1) manufacturer; 2) age of the suits (all lose some buoyancy over time); 3)water conditions ( salt vs. fresh). Other weighting factors do not impact your question.
As to the other question raised in the thread about maintaining neutral buoyancy with no BCD or wing, watch some old "Sea Hunt" episodes and you will find that there is a lot of movement with both hands and arms, and a lot of of "bottom sitting and creeping) by our predecessors.
DivemasterDennis

 

[fjpatrum]

Just to add more data, I add about 3 pounds when I go to my 5mm. I think herman's got the right answer, it depends so much upon manufacturer and material (and size) that there's no real way to say "normal" except to say "normal for me in this particular configuration".

If you're not having any extra buoyancy issues, no need to worry, right?

 

[herman]

Exactly, Paladin. The variable factors in how much weight you will add going from a 3 mil to 5mil full wetsuit include: 1) manufacturer; 2) age of the suits (all lose some buoyancy over time); 3)water conditions ( salt vs. fresh). Other weighting factors do not impact your question.
As to the other question raised in the thread about maintaining neutral buoyancy with no BCD or wing, watch some old "Sea Hunt" episodes and you will find that there is a lot of movement with both hands and arms, and a lot of of "bottom sitting and creeping) by our predecessors.
DivemasterDennis

That was 50s TV, not real life....I dive with a good number of no BC divers and that is the way I prefer to dive unless I am with a class or on a boat that requires otherwise. Even then, my BC has no air in it and is there more for show than use. Those of us who dive sans BC often for the most part have better skills and control than most divers with BCs. If you are diving with little to no wetsuit and actually need a BC, your skills need a lot of improvement.

 

[Texasguy]

Do you use slightly more air if you have to inhale more to control the buoyancy?

 

[KD8NPB]

I weight myself so at the beginning of my dive, a full exhale and a 3 second wait is what it takes for me to sink and gain momentum on my descend. Most of my friends are weighted heavier so if they were to breath in, they'd still be sinking.
If I were to breath in within those 3 seconds, I would pop back up to the surface. I fix this by back kicking to 3 ft, then bending over to a head down position. I'm kinda an oddball too.

I'm the same way when I'm diving my Doublehose + LP72 + Nemrod snugpack. Henderson Thermocline 7mm + negative fins + 12 lb, I'd be positive at the surface. To descend, I would have to partially exhale and hold it that way until I got to about 5 ft, then I could finally take a breath. When ascending with ~400 psi left, I am neutrally buoyant at 15-20 with a "middle of the road" breath. For my final ascent from my 3 min safety stop, I hold my breath completely so as not to shoot up like a cork. It's nice, because at the end of the dive, it takes quite a bit of effort for me to submerge again...so I float without a BC!

Do you use slightly more air if you have to inhale more to control the buoyancy?

I average about -0.05 less cfm when I dive without a BC.