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Lobzilla
Contributor
Exactly.
ANSWER TWO:
We are loosing the weight of the breathing gas during the dive and need to account for that. Our buoyancy at the beginning of the dive should be neutral minus gas weight.
100 cuft of air or Nitrox weigh about 8 lbs. 77 cuft (AL80 tank) of air or Nitrox weigh about 6 lbs. Close enough and easy to remember.
If we can achieve neutral buoyancy with a full tank and empty(!) BC by adding or dropping weight then we simply add 8 lbs more lead for every 100 cuft of air/Nitrox and go diving.
QUESTION THREE
How do we check that we have enough negative buoyancy (equal to gas weight) if we sink with empty BC and no lead at all? This may happen with certain steel tanks and thin exposure protection.
ANSWER:
The volume of the average full breath is about 1 gal, which creates about 8 lbs of positive buoyancy. So, when diving a 100 cuft steel tank I put one breath into my empty BC/wing via the oral inflator. If I am neutrally buoyant or still negative, then I go diving. If the 8 lbs of added positive buoyancy make me float I add lead weight to achieve neutral buoyancy.
Safety Note: With heavy steel tanks we have to be concerned about BC/wing failure. By determining how many breaths we have to add to our BC to achieve neutral buoyancy we can get an idea of how negative the steel tank makes us. Also, we need to remember that a wetsuit will loose buoyancy due to compression as we descend. Therefore, I never dive very negative tanks like LP104 doubles without a drysuit that will serve as a backup BC.
ANSWER TWO:
We are loosing the weight of the breathing gas during the dive and need to account for that. Our buoyancy at the beginning of the dive should be neutral minus gas weight.
100 cuft of air or Nitrox weigh about 8 lbs. 77 cuft (AL80 tank) of air or Nitrox weigh about 6 lbs. Close enough and easy to remember.
If we can achieve neutral buoyancy with a full tank and empty(!) BC by adding or dropping weight then we simply add 8 lbs more lead for every 100 cuft of air/Nitrox and go diving.
QUESTION THREE
How do we check that we have enough negative buoyancy (equal to gas weight) if we sink with empty BC and no lead at all? This may happen with certain steel tanks and thin exposure protection.
ANSWER:
The volume of the average full breath is about 1 gal, which creates about 8 lbs of positive buoyancy. So, when diving a 100 cuft steel tank I put one breath into my empty BC/wing via the oral inflator. If I am neutrally buoyant or still negative, then I go diving. If the 8 lbs of added positive buoyancy make me float I add lead weight to achieve neutral buoyancy.
Safety Note: With heavy steel tanks we have to be concerned about BC/wing failure. By determining how many breaths we have to add to our BC to achieve neutral buoyancy we can get an idea of how negative the steel tank makes us. Also, we need to remember that a wetsuit will loose buoyancy due to compression as we descend. Therefore, I never dive very negative tanks like LP104 doubles without a drysuit that will serve as a backup BC.
Last edited:
Lobzilla
Contributor
@divinh
I have approached weighting the same way you did by measuring the buoyancy of every component in a pool and doing the math. The problem was the ever growing list of items. I now have one wetsuit, two different drysuits with three different undergarments, three different tank sets, two different regulator sets, two different light canisters, aluminum backplate, steel backplate, etc.
At some point I knew which combinations work well after using the pre-dive weighting method described above.
No matter how you approach the problem, remember that we should always try to reduce the air needed in your BC at the beginning of the dive to the amount that creates buoyancy equivalent to your gas weight. Any more air in our BC (to compensate for excess weight) and our buoyancy control will suffer.
I have approached weighting the same way you did by measuring the buoyancy of every component in a pool and doing the math. The problem was the ever growing list of items. I now have one wetsuit, two different drysuits with three different undergarments, three different tank sets, two different regulator sets, two different light canisters, aluminum backplate, steel backplate, etc.
At some point I knew which combinations work well after using the pre-dive weighting method described above.
No matter how you approach the problem, remember that we should always try to reduce the air needed in your BC at the beginning of the dive to the amount that creates buoyancy equivalent to your gas weight. Any more air in our BC (to compensate for excess weight) and our buoyancy control will suffer.
Last edited:
Lobzilla
Contributor
@Bob DBF,
I enjoy 'old school' scuba diving without neoprene and BC if the water is warm enough and I don't have to go far. My favorite pool and shallow water rig consists of a vintage plastic back plate, holding a Jumbo D medical cylinder with scuba valve that feeds a light regulator set. I am a sinker so that balances out just perfectly.
For single AL80 & 3/2 wetsuit: 2 lbs. of weight on belt & 2 lbs. on cylinder. You can't put on 3 lbs. on your cylinder because weight pouch would stickout, you have to use 2 pouches on the sides. In addition, you have to be able to don your BCD in-water, meaning, you have to be neutral on the surface first. Mathematically, it's like 5 lbs. min, for a 3/2 80 but depends on gear weight & body composition in terms of BF.
I use, as little, as 2 lbs. of lead & add the rest of the weight in gear to BCD. I bypass using tank pouches, with a 3/2 & use pounders for adjusting trim. ... 10 lbs. of weight is overkill for a 3/2 AL80. It also depends on how much neoprene you have on, it should be in the 1-2 mm range because 3/2s are summer/spring suits. Make sure all the air is out of your BCD, pools are shallow & air gets trapped in different bladders.
An easier way of doing it is, suck down all of your air till you get down to 500 psi in your pool & weigh till you can go down to the bottom, with no air in your BCD. Literally, dump all of your air out & try to go down & see if you are neutral. Start test, at like 700-1000 psi & see how it is, with 500 psi left. Weigh for 500 psi remaining, with all the air, out of your BCD.
I f you are weighing on the surface, likely, with a full cylinder, it's the basic OW way. Change it to the tech way, with 500 psi left. There's math to it but it isn't perfect because suits loose buoyancy & body fat.
You should have (2) tank weight pouches on your band & if you use bands, pick the lower one first, nearest your weight belt. If your legs sink, move the weight pouches to the upper band & see how that feels. Mine are in the upper. It's only 2.8 lbs (Roughly 3 lbs.) on your cylinder for 500 psi. Use only 2 lbs., 1 lbs. on each side. The other 2 lbs. if you need it, goes on your weight belt hips. You then adjust trim, by moving the weights forward or to the rear. 4 lbs. should be good for you, since you likely carry some gear.
I would still use a weight belt & slap-on a pair of 1 lbs. weights to belt, to fine tune trim issues. If you put all of your weight on your tank band, you might not be able to fine-tune trim, during your dive, eventhough you can still achieve a good position. Plus, "you" have to be neutral & have to stay down as well. Ex.? Doffing BCD underwater in an emergency. If you takeoff your BCD U/W, your body will rise above the BCD. Also, releasing weights can expedite an ER ascent. Straight up, I wouldn't put all of your weight on the cylinder, behind you because of trim, self neutrality & ER ascent reasons. It's one of those things, that you benefit from spreading weight.
I agree but no one answered, that the weighing should be done, at 500 psi & end of dive. No one told him, he needed to stay down or how to fine-tune trim, with use of 1 lbs. weights on belt or how to spread weight. So, I pitched-in.
I use, as little, as 2 lbs. of lead & add the rest of the weight in gear to BCD. I bypass using tank pouches, with a 3/2 & use pounders for adjusting trim. ... 10 lbs. of weight is overkill for a 3/2 AL80. It also depends on how much neoprene you have on, it should be in the 1-2 mm range because 3/2s are summer/spring suits. Make sure all the air is out of your BCD, pools are shallow & air gets trapped in different bladders.
An easier way of doing it is, suck down all of your air till you get down to 500 psi in your pool & weigh till you can go down to the bottom, with no air in your BCD. Literally, dump all of your air out & try to go down & see if you are neutral. Start test, at like 700-1000 psi & see how it is, with 500 psi left. Weigh for 500 psi remaining, with all the air, out of your BCD.
I f you are weighing on the surface, likely, with a full cylinder, it's the basic OW way. Change it to the tech way, with 500 psi left. There's math to it but it isn't perfect because suits loose buoyancy & body fat.
You should have (2) tank weight pouches on your band & if you use bands, pick the lower one first, nearest your weight belt. If your legs sink, move the weight pouches to the upper band & see how that feels. Mine are in the upper. It's only 2.8 lbs (Roughly 3 lbs.) on your cylinder for 500 psi. Use only 2 lbs., 1 lbs. on each side. The other 2 lbs. if you need it, goes on your weight belt hips. You then adjust trim, by moving the weights forward or to the rear. 4 lbs. should be good for you, since you likely carry some gear.
I would still use a weight belt & slap-on a pair of 1 lbs. weights to belt, to fine tune trim issues. If you put all of your weight on your tank band, you might not be able to fine-tune trim, during your dive, eventhough you can still achieve a good position. Plus, "you" have to be neutral & have to stay down as well. Ex.? Doffing BCD underwater in an emergency. If you takeoff your BCD U/W, your body will rise above the BCD. Also, releasing weights can expedite an ER ascent. Straight up, I wouldn't put all of your weight on the cylinder, behind you because of trim, self neutrality & ER ascent reasons. It's one of those things, that you benefit from spreading weight.
I agree but no one answered, that the weighing should be done, at 500 psi & end of dive. No one told him, he needed to stay down or how to fine-tune trim, with use of 1 lbs. weights on belt or how to spread weight. So, I pitched-in.
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