Weight and Buoyancy

[mccabejc]

Recently certified, going to take my Advanced class starting this weekend, including Peak Performance Buoyancy. I preparation, I donned all equipment, got in my swimming pool, and determined that with exactly 20 lbs I was neutrally buoyant in shallow fresh water. I could ascend and descend with an empty BC merely by inhaling and exhaling. So based on what others have said, I figure that when going to the ocean I'll need another 4-6 lbs. I have a 7mm full suit, hood, gloves, fins, everything. I'm 170 lbs, slender, pretty average, with an AL80 tank. When diving in Catalina a few weeks ago, I learned that 29 lbs was too much, and during my cert dives I used 28, but that seemed a little much.

Does anyone have and idea whether I'm far off in figuring 24-26 lbs? I get the sense that some use substantially less, though I'm not sure if that's due to steel tanks.

 

[ScubaScott]

Recently certified, going to take my Advanced class starting this weekend, including Peak Performance Buoyancy. I preparation, I donned all equipment, got in my swimming pool, and determined that with exactly 20 lbs I was neutrally buoyant in shallow fresh water. I could ascend and descend with an empty BC merely by inhaling and exhaling. So based on what others have said, I figure that when going to the ocean I'll need another 4-6 lbs. I have a 7mm full suit, hood, gloves, fins, everything. I'm 170 lbs, slender, pretty average, with an AL80 tank. When diving in Catalina a few weeks ago, I learned that 29 lbs was too much, and during my cert dives I used 28, but that seemed a little much.

Does anyone have and idea whether I'm far off in figuring 24-26 lbs? I get the sense that some use substantially less, though I'm not sure if that's due to steel tanks.

Your likely right in the ballpark. When I was diving wet in a 7mm with Al tank, I needed 30#. + or - 1-2lbs should not throw you out of whack , and your instructor should be able to help you nail it down this weekend....

SS

 

[mech]

How full was your aluminum 80 when you did your test?
You will need to add some extra if the tank was full.

 

[MXGratefulDiver]

Archemides Principle ... the weight you'll need equals the weight of the water you displace. This assumes that in fresh water you're perfectly weighted ... but it will get you in the ballpark pretty close.

You weigh 170 lbs. Let's say your kit (including tank, weights, and everything else) is another 70 lbs. That's 240 lbs.

Assuming this is your "perfect weight" to achieve neutral buoyancy, you can say you displace 240 lbs. worth of fresh water. At 62.4 lbs per cubic foot that's 3.846 cubic feet.

Since you'll displace the same amount of salt water, which weighs 64.0 lbs per cubic foot ... 1.6 lbs per cubic foot more than fresh water ... you'll need 1.6 x 3.846 lbs more weight ... 6.154 lbs. I think it's safe to round it off to an even 6 lbs.

To get a more precise figure, weigh your kit and substitute that number for the assumed 70 lbs in the above example.

... Bob (Grateful Diver)

 

[mccabejc]

Excellent Bob. Thanks.

Which all means that with around 26 lbs. I should be neutral at the surface. Now should I knock off a few pounds to allow for the decrease in buoyancy at depth ?

 

[MXGratefulDiver]

Excellent Bob. Thanks.

Which all means that with around 26 lbs. I should be neutral at the surface. Now should I knock off a few pounds to allow for the decrease in buoyancy at depth ?

No ... remember that what goes down must come up ... and as your cylinder gets empty it will become increasingly buoyant. You'll want enough weight to stay down during your safety stop. Too little weight and that three minutes will seem like an eternity as you struggle to not come up prematurely.

You said you're a new diver. I'd say stick with a comfortable weight and work on proper breathing techniques and relaxing for a few dives. Then you can start to "shed" weight ... in 1 to 2 lb. increments ... until you find your ideal weight.

Do a weight check every 20-30 dives thereafter ... or any time you change a major piece of your kit (like getting a different cylinder or BCD).

... Bob (Grateful Diver)

 

[Charlie99]

As Mech noted, don't forget to account for whatever air was in your AL80 during your pool checks. Adding 1 pound of lead for every 500psi in your tank is pretty close, and allows you to do accurate weight checks with any amount of air in your tank.

 

[ShakaZulu]

I'm diving in the same waters as you (7mm and all the other stuff), I'm 185lbs and I need 26lbs. 24 is enough, but for the peeps who like to do a safety at 10ft, it gets a little difficult with that AL80 sitting 500psi. Note - the better you are weighted, the less air you'd need to use to compensate for neutral bouyency. Good luck.

 

[Teg97RS]

Damn, so I'm not bad with 160 lbs, a 2 pieces 7mm and less than 18 lbs of weights

 

[O2BBubbleFree]

The above advice is excellent. One minor point, though, if you'll allow...

Archimedes’s Law more accurately deals with density, not weight. Density being weight per volume. That's why muscle weighs more than the same volume of fat, and, I believe, why an aluminum tank will be more buoyant than an equivalent-sized steel one.

Bob's (Grateful Diver) calculations above will give you a good starting point, but, as always, buoyancy checks are necessary to fine tune from there.

If you don’t want to do the calculations, here’s a chart that will also get you in the ballpark. It’s worked for me and several others that I’ve shared it with on this board:

In saltwater

Swimsuit or dive skin: 1-4#
1/16" (3mm) one-piece or shorty: 5% of bodyweight
3/16" (4.5mm) 2-piece (john and jacket): 10% of bodyweight
1/4" (6mm) 2-piece w/ hood, gloves, boots: 10% + 3-5#
Neoprene drysuit: 10% + 7-10#
Shell dry suit: 10% + 7-14#

For fresh water, subtract weight according the following:

If you weigh between 100 and 125#, subtract 4# of lead
If you weigh between 126 and 155#, subtract 5# of lead
If you weigh between 156 and 186#, subtract 6# of lead
If you weigh between 187 and 217#, subtract 7# of lead

I won’t mention here that the amount of lead required for high-altitude diving is different than sea-level, because that started a fire that’s still burning on the other thread. Oops, I guess I did mention it. If you disagree, please join the discussions elsewhere. For the rest of you who might be reading this and looking for weight guidlines, please accept this as fact.