Weighting for Neutral Buoyancy

[Julius SCHMIDT]

Here's an English guy that will set you straight

Scuba Weighting | 4of9 | Scuba Buoyancy Masterclass

I dive with more weight and a bigger bladder than is required and with excellent buoyancy because it is SAFER
that's capitals for safer

 

[scubadada]

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?

No, you want to be neutrally buoyant at your safety stop with about 500 psi in your cylinder and no gas in your BC. You want to be able to do a nicely controlled final ascent to the surface. Yes, there will be about a 5 lb shift in weight from a full AL80 to one with around 500 psi.

Make you best guess based on previous dives, you should be able to get it near exact in just a few dives, closer every time.

 

[EFX]

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.

Here is what I did to get my weight and trim dialed in: link

Once you know your weight and where that weight should go, it should be OK for rental equipment.

 

[jadairiii]

Your comment specified a buoyancy swing for aluminum tanks. I was just pointing out that steel tanks have the same buoyancy swing, and it does not matter if they are positive of negative at the end of the dive.

We are staying the same thing. My bias is, that even at 60, I'm personally just a tad positive, below 25' with full lungs (free diving) I'm neutral. So I am less concerned with "swing weight" verses "what's going to float". When I dive steel tanks, I'm always negative, beginning, middle, end of dive, which translates to more gas in the wing. You should see me diving 104's in caves!

 

[Still Kicking]

Adjust weighting to float at eye level with a full tank. Add 5 or 6lbs.

 

[The Chairman]

In theory I aim to be neutrally buoyant with 0.00 psi in the tank at 15-20 ft, BC empty.

With a full tank, hands and legs crossed, empty bladder and on a normal full breath, your head should be just awash in the water. Not eye level... just at the top of your head.

However, this won't work if you're not horizontal in the water. That is far more important than being within a pound or two of optimum weight. Your thrust angle affects your weighting significantly.

 

[inquis]

Adjust weighting to float at eye level with a full tank. Add 5 or 6lbs.

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

 

[boulderjohn]

We are staying the same thing. My bias is, that even at 60, I'm personally just a tad positive, below 25' with full lungs (free diving) I'm neutral. So I am less concerned with "swing weight" verses "what's going to float". When I dive steel tanks, I'm always negative, beginning, middle, end of dive, which translates to more gas in the wing. You should see me diving 104's in caves!

You are still not understanding.

You need less weight with steel tanks because they weigh more from the start, and are this much less buoyant from the start. If an aluminum tank weighs 8 pounds less than a steel tank of the same size, you will need to add 8 more pounds of lead to make up the difference. During the dive, either tank will become more buoyant by the weight of the air exhaled. At the end of the dive, it does not matter whether the tank itself floats or sinks. It is the weight of the entire package, including all the gear and all the weights, that matters.

 

[jadairiii]

You are still not understanding.

You need less weight with steel tanks because they weigh more from the start, and are this much less buoyant from the start. If an aluminum tank weighs 8 pounds less than a steel tank of the same size, you will need to add 8 more pounds of lead to make up the difference. During the dive, either tank will become more buoyant by the weight of the air exhaled. At the end of the dive, it does not matter whether the tank itself floats or sinks. It is the weight of the entire package, including all the gear and all the weights, that matters.

I do understand. And I understand the "entire package". I've done that numerous times when I am dealing with double 104's, stages, lights, scooter(s), undergarments, and such.

with a recreational set up, my goal is to limit the amount of gas in my wing. I dont float nor do I sink, with some steel tanks, even with an alum back plate, I cannot achieve "neutral" buoyancy at the end of the dive, I'll always be negative. Yet, with an Alum 80, I can, during a portion of the dive, have an empty wing.

So, for me, it does matter if the tank sinks, even at the end of the dive, because I cant make my regulator any lighter, I cant make my alum back plate any lighter, my jets are slightly negative. The "entire package" before I add that tank is negative. Maybe I'm unique?

 

[Outbound]

You are still not understanding.

You need less weight with steel tanks because they weigh more from the start, and are this much less buoyant from the start. If an aluminum tank weighs 8 pounds less than a steel tank of the same size, you will need to add 8 more pounds of lead to make up the difference. During the dive, either tank will become more buoyant by the weight of the air exhaled. At the end of the dive, it does not matter whether the tank itself floats or sinks. It is the weight of the entire package, including all the gear and all the weights, that matters.

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.