Weighting for Neutral Buoyancy

[Rollin]

No, you want to be neutrally buoyant at your safety stop with about 500 psi in your cylinder and no gas in your BC.

How do you access that that last 500 psi in case you need it, without ascending?

 

[Still Kicking]

This works well in my experience IF evaluated at the end of a normal exhale and carrying 80ish cuft of air.

But if you do it at the beginning and then add 5-6lbs it's the same as doing it at the end. Get it?

If people don't understand the concept of neutral buoyancy, all the other suggestions are just noise.

 

[inquis]

But if you do it at the beginning and then add 5-6lbs it's the same as doing it at the end. Get it?

Yes, I get it. The additional weight compensates for what you'll lose during the dive. My point was that your post did not specify how much air to have in the lungs, which is actually quite important. I was trying to agree with you while adding that missing bit of information.

I don't think it's the concept of neutral buoyancy that trips people up; I'd say it's the details.

 

[inquis]

How do you access that that last 500 psi in case you need it, without ascending?

Exhale a little bit. That 500 PSI weighs 1 lb in an AL80 tank.

 

[scubadada]

How do you access that that last 500 psi in case you need it, without ascending?

Exhale a little bit. That 500 PSI weighs 1 lb in an AL80 tank.

Good answwer by @inquisit.

I'm never light at the safety stop or last deco stop as I also want to be able to do a controlled, slow last ascent to the surface. If anything, I'm just a touch heavy.

 

[rsingler]

What is a good quick practical way to make sure I am perfectly weighted? I've struggled a bit with this particularly because we sometimes rent BCs or wetsuits on vacation. I've done the bob in the ocean level with eyes/top of head, and found it lacking. Calculators don't work because I don't have access to inherent buoyancy specs for rental gear. I try to be within 1 lbs over weighted because it's easier to mitigate than being slightly under weighted in an emergency.

In theory I aim to be neutrally buoyant with 0.00 psi in the tank at 15-20 ft, BC empty. In practice it's kind of difficult to achieve because, up to this point, I always end dives with at least 500 psi, usually closer to 1,000 psi. What is the typical inherent buoyancy of an AL80 at 0.00 psi and 1,000 psi?

While all of you have been arguing about theoretical buoyancy, and buoyant aluminum vs empty steel, no one's paying attention to what the OP has really asked! He already knows the "top of the head or eye level in the ocean" trick, so he's either tried that with an empty tank or tried it with a full tank and added 5#. But he "finds it lacking". Why? Because there's a factor that has been ignored amidst all the usual ScubaBoard squabbling.

@smackboy1 , you'll continue to be frustrated at consistently getting it right with rental gear and varying locations.
If the objective is to be neutrally buoyant at your last stop with either 500psi (normal dive) or 100psi (problem dive where you're trying to hold a stop with an empty tank), the requirements are still inconsistent across a range of exposure suits.

Why? Because surface estimates (accounting for the weight of tank air not yet breathed), do NOT account for the variance in exposure suit compression. Between the surface and 10-20 feet, your rash guard will lose little if any buoyancy. So your weighting at the surface will be little different from what you actually need at your last stop. Doing the same "eye-level" or "top of the head" estimate in a 7-8mm cold water suit, or even worse, a Farmer John, may or may not account for the compression of the thick suit and loss of buoyancy at 15ft. At 15ft, a new, soft neoprene "Aqualung-style" comfort fit wetsuit, plus neoprene hood and boots may lose nine pounds of buoyancy between the surface and 15 feet. That's why the various videos of the "correct technique" differ between cold and warm water settings. That's why some say "head just awash" and others say "eye level", since you have another four pounds of head weight being supported above the surface at eye level.

There is NO universal answer.
But the best suggestion started with the one above, where it was recommended to "jump in the water with a full tank", and add 5-6 lb.
So with this suggestion, if your head is just awash, we have correct neutral buoyancy accounting for air consumption only, and it has the advantage of not requiring an empty tank to accomplish. Just add the weight of the air you'll use during the dive (all, some).
Next, you need to guess at your suit compression at the stop, and remove weight accounting for the loss of suit buoyancy at 15', that you had during your test at the surface:
Trilam Dry Suit - 0 removed (you'll keep the suit inflated the same at both depths)
Dry Rash Guard with a little trapped air during the test: 1 pound removed
3-4mm wetsuit without hood: 2-3 pounds removed
7-8mm wetsuit with gloves and hood, used rental: 5-6 lb removed
New, thick, soft neoprene wetsuit with gloves and hood: 8-10lb removed, or more!

And in this, we see why some folks recommend "eye-level" for the test! Because floating at eye-level is 4 pounds lighter than awash, which equates to the buoyancy loss of an average 5mm wetsuit at 15 feet.

What else does this mean? It means that if you carried a big tank (100cf?) for a long dive in your 5mm wetsuit, did a surface test and added 8lb for all that gas you were going to use, then removed 4lb for your suit, you'll be perfect at your stop. But you'll also be increasingly buoyant in the last 15 feet, when you should be ascending the slowest. Time for lots of exhalation and small lungs in the last 5-10 feet! And when the captain asks you to pop down and grab the shallow, but stuck anchor (assuming you'd even want to after a long dive, but that's another discussion), you'll maybe find you can't exhale enough to even get back under the surface with only 400 psi left in your tank.

 

[Mike Boswell]

I carry about 5 pounds more lead than I need, because:

1. I don't ever want to be underweighted, which is a PITA.
2. When another diver needs more weight, I've got them covered.
3. I might want to lie or kneel on the sand while I watch critters swim around, or take pictures.
4. I can easily add air in the BC to compensate for the extra weight.

YMMV

 

[boulderjohn]

It's not about weight at the surface; it's about weight in the water. In other words, buoyancy. Buoyancy is all about displacement. A full 80 cf aluminum tank weighs about 35 lbs on land and about 3.6 lbs in the water. A full 80 cf steel tank weighs about 32.5 lbs on land and about 13 lbs in the water. So it is lighter on land but more negative in the water because steel is denser than aluminum, and because a steel tank of the same volume is smaller than an aluminum one (and thus displaces less water, thereby making it less buoyant). Subtract the amount of air breathed during a dive and the aluminum tank will be positively buoyant at the end of the dive while the steel tank will still be negative.

So yes, weight is weight. But we are really interested in buoyancy, not just weight. Concrete weighs a lot, and a concrete block will immediately sink. But turn that concrete into a boat hull and it will float.

Thank you for the explanation of buoyancy.

Now, before I bow out of this altogether, take a look back and see how many damn times I explained what buoyancy is before I got this lecture.

 

[Outbound]

Thank you for the explanation of buoyancy.

Now, before I bow out of this altogether, take a look back and see how many damn times I explained what buoyancy is before I got this lecture.

My sincerest apologies. I didn’t intend to anger or offend.

 

[The Chairman]

I can complete most dives with no air, or very little air, in my bladder. That's with 3mil or less and using the head awash at the beginning of the dive method. I don't have any issues at my safety stop,