The Science of Weight + Neutral Buoyancy

[lowwall]

The shops are wrong. 4lbs in saltwater with an AL tank and a 5mm shorty would be far more unlikely than 13.

What is your BCD?

As to the science...

Body fat has a density of around .9 (g/cc), while everything else averages out to about 1.06.

So if we take a 100kg (220lb) person with 25% body fat, we get 75x.06=4.5kg negative and 25x.1=2.5kg positive. For a net of 1kg positive. But then we have to add in 2kg positive for the typical amount of air that is left in the lungs after a normal exhale, so 3kg (6.6 pounds) positive before taking into account your gear.

This is for fresh water. Seawater has a density of around 1.027 g/cc, so every pound/kilo of lean mass contributes .033 pounds/kilos of negative bouyancy and every pound/kilo of fat contributes .127 positive.

Thus for a 190 pound person with 20% body fat. That's 38x.127 - 152x.033 + 4.4 = 4.826 - 5.016 + 4.4 = 4.21 pounds positive buoyancy. Note that none of this is as precise as the digits indicate. I'd call it "around 4 pounds".

 

[rongoodman]

That's the motto I need! I never put air into my jacket (unless going below 30m) and I feel like I am flying gracefully through the water with my weight setup.

You're starting the dive heavy by 5+ pounds due to the gas in your tank. If you never put air in your BC to compensate for it you're either too light or you're compensating for it entirely with your breathing. If you always finish your dives with a substantial amount of air you just might not have realized it yet but it could come back to bite you at your safety stop if you ever do finish with a nearly empty tank.

 

[cfberry]

yes the less muscle, more fat and decreased bone density a diver has, the more weight they will need. Lung volume might affect things a little too. However, I have seen at least one young man who was not fat, was reasonably fit and simply required considerably more lead than you would expect.

13 lbs is not a whole lot of lead for saltwater and an aluminum tank and some wetsuit.

this is totally true

 

[Doc Harry]

You can tell your weight is correct if you have water in your BC back on the boat.

Let me explain.

Towards the end of the dive, I am dumping air from my BC. My BC is empty the last 15 minutes of a typical reef dive.

As I ascend to the safety stop, I find myself trying to dump just a little more air to be neutral at the safety stop. If your BC is already empty, then all this does is add water to the BC.

If there is no water in my BC after a dive, then I know that I am still overweighted.

 

[lowwall]

Thus for a 190 pound person with 20% body fat. That's 38x.127 - 152x.033 + 4.4 = 4.826 - 5.016 + 4.4 = 4.21 pounds positive buoyancy. Note that none of this is as precise as the digits indicate. I'd call it "around 4 pounds".

I should finish this calculation. Next is the tank. You want to be neutral with 500psi left in your tank, assuming an AL80, that's another 3 pounds positive that has to be offset. Now the wetsuit, in this case a 5mm shorty probably in XL. You'd have to measure yours with some weights, a luggage scale and enough water to sink it to be sure, but the normal approximation would be 2 pounds per mm. So that's 10 pounds for a fullsuit, maybe 7 for a shorty?

And then you have all the rest of your equipment. We don't have your info, but assuming near neutral fins and a standard BCD (approx 1 pound positive) and regs (approx 1 pound negative), this will net out within a pound or two of neutral.

Let's total it up.

4 pounds positive - body
3 pounds positive - tank
7 pounds positive - wetsuit
0 - rest of gear
------
14 pounds positive

Given that all of these numbers are approximations, your 13 pounds of lead is well within the range we would expect. And your shops guess of 4 pounds is silly, even if they think your shorty is a 3mm instead of a 5mm.

 

[Glewellyn]

@rsingler has created a buoyancy calculator in the form of an excel sheet if you really want to analyse the amount of lead you'd theoretically need.

Info - Optimal Buoyancy Computer

The Optimal Buoyancy Computer A tool to help nail buoyancy and improve safety, before you splash 1) How much lead should I carry with my new wetsuit? 2) How big a wing should I buy? 3) Will my BCD support my lead, both at the surface and when my wetsuit is compressed at depth? 4) Will my BCD...

scubaboard.com

If you decide to take the time to complete it I'd be interested to see what the model predicts and how far off of 14lbs. it will be.

 

[We are Osea]

The title of this post makes it sound a lot more exciting than it's going to be, but I'm curious about body composition and how it impacts the weights you dive with to achieve neutral buoyancy.

Diver Profile:
100+ Dives
36 years old
190lbs
20% Body Fat (Acceptable)
58% Body Water (Standard)

I'm a calm diver, always relaxed in the water. I usually end Dives with plenty of air left (the same as the dive masters who are leading the tours). I share this because one theory is poor breathing technique and/or muscle stiffness, which can cause the need to carry more weight.

But with a 5mm shorty, and 5mm boots, I need 13lbs of weight. (5.89kgs)

And every dive shop I dive with is always adamant that I am wrong, that I should only need 4lbs - that I'm too stressed or breathing incorrectly etc.

At the end of the day, I don't really care what others think or say about the weights that I use.

But I am curious if there is something to do with body science that impacts all of this.

Appreciate any thoughts or theories from the community.

Cheers,

Joe

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