Wetsuit buoyancy: Loss in depth - how big should be BCD

[АлександрД]

Once we discussed about minimal BCD volume for neoprene wetsuit. Some guys told that just 2 L is enough (to compensate air weight in tank).
Some use just marketing and experience, and told like for most BCD - 10-14 L. Some told about 30 L ...

So, I offer you my estimation for minimal volume of BCD. Any remarks or corrections be welcome! Also mathematics and phisics: Welcome for discussion!

Lets go...
(I will use many assumptions, but our target - just to receive estimation of measured weight)
I will use Decimetre, as standard length in my counts, because easy to convert - 1dm3 = 1 L of water = 1 kg

Spherical cow in vacuum ... Ups, sorry, .... intermediate diver body surface is around ~1.9m2 = 190 dm
Lets count buoyancy loss for worst conditions, and we will use 9mm neoprene thikness. (0.09dm)
Total neoprene volume 190*0.09=17.1 dm3
When it consists vacuum only - it will have buoyancy 17 kg, BUT! it has also Neoprene inside.

Neoprene density is around 150-250 kg/m3 (same as g/dm3)

ok, will use 0.2 kg/dm3 as medium

It mean that suit weight is around 17*0.2=3.4 kg (it is just weight of Neoprene itself, like a rubber)
And buoyancy of that suit is 17-3.4= 13.5 kg (approx)

It means that all our suit contain 13.5 L of air.

Once I counted formula of piped depthmeter and got this
V(current)=V(total)*10/(10+depth)

Will use maximal allowed recreational depth - 40m
V(at 40) = 13.5 * 10 / (10 + 40) = 13.5 * 0.2 = 2.7 l

Sure, J-Y Cousteau used rubber suits with glass spheres inside rubber, to prevent compression, but we have just Neoprene... so, for worst condition we will allow, that Neoprene not elastic, but has not crushing resistance.

It mean that we loss from surface till 40m around 13.5-2.7=10.8 kg of buoyancy.

BTW: Neoprene itself (as rubber, without air foam) has dencity around 1.5 g/cm, (1.5 kg/dm)
It mean that "sinkancy" (neoprene itself, without air) is around 3.4*(1.5-1) = 1.7 kg underwater weight.
Theoretically at 70m we will have just 1.68 L of air inside neoprene foam, and our suit start to be sink!

All this calculation means - you should not have BCD less than 11 L of air.
Global conclusion: Use drysuit!

 

[USMC CPL.]

I can't afford a drysuit. Lol. But my BP/w with 25 pound wing works great. Had a lp inflator malfunction at 40 feet and was able to comfortably swim my rig up. With a HP100 tank. Good read.

 

[Gradester]

If I wore a drysuit how much lift would my bc need?

 

[EireDiver606]

Once we discussed about minimal BCD volume for neoprene wetsuit. Some guys told that just 2 L is enough (to compensate air weight in tank).
Some use just marketing and experience, and told like for most BCD - 10-14 L. Some told about 30 L ...

So, I offer you my estimation for minimal volume of BCD. Any remarks or corrections be welcome! Also mathematics and phisics: Welcome for discussion!

Lets go...
(I will use many assumptions, but our target - just to receive estimation of measured weight)
I will use Decimetre, as standard length in my counts, because easy to convert - 1dm3 = 1 L of water = 1 kg

Spherical cow in vacuum ... Ups, sorry, .... intermediate diver body surface is around ~1.9m2 = 190 dm
Lets count buoyancy loss for worst conditions, and we will use 9mm neoprene thikness. (0.09dm)
Total neoprene volume 190*0.09=17.1 dm3
When it consists vacuum only - it will have buoyancy 17 kg, BUT! it has also Neoprene inside.

Neoprene density is around 150-250 kg/m3 (same as g/dm3)
View attachment 453679
ok, will use 0.2 kg/dm3 as medium

It mean that suit weight is around 17*0.2=3.4 kg (it is just weight of Neoprene itself, like a rubber)
And buoyancy of that suit is 17-3.4= 13.5 kg (approx)

It means that all our suit contain 13.5 L of air.

Once I counted formula of piped depthmeter and got this
V(current)=V(total)*10/(10+depth)

Will use maximal allowed recreational depth - 40m
V(at 40) = 13.5 * 10 / (10 + 40) = 13.5 * 0.2 = 2.7 l

Sure, J-Y Cousteau used rubber suits with glass spheres inside rubber, to prevent compression, but we have just Neoprene... so, for worst condition we will allow, that Neoprene not elastic, but has not crushing resistance.

It mean that we loss from surface till 40m around 13.5-2.7=10.8 kg of buoyancy.

BTW: Neoprene itself (as rubber, without air foam) has dencity around 1.5 g/cm, (1.5 kg/dm)
It mean that "sinkancy" (neoprene itself, without air) is around 3.4*(1.5-1) = 1.7 kg underwater weight.
Theoretically at 70m we will have just 1.68 L of air inside neoprene foam, and our suit start to be sink!

All this calculation means - you should not have BCD less than 11 L of air.
Global conclusion: Use drysuit!

I don’t think it needs to be as complicated as that.
Just weight yourself properly

 

[АлександрД]

If I wore a drysuit how much lift would my bc need?

With dry suit is much more simple calculations.
In case of full flood of the dry suit you loss ALL your buoyancy.
It mean that you have to have same air volume of your wing or jacket, as total weight, that you have (including underwater weight of tank and other rigs).
E.g. for me with thick undergarment I use 12 kg of lead, and my composite tanks has near neutral buoyancy (with 3kg extra leads).
But my SM wing have near 8-9 kg. I know, it is wrong way, but I have dives not far from land, and I can take up by my fins over 4 kg.
and... I have separated dropable weights

I don’t think it needs to be as complicated as that.
Just weight yourself properly

Yes, but this complicated calculation is allow to have estimation base for proper weight.

 

[Gradester]

With dry suit is much more simple calculations.
In case of full flood of the dry suit you loss ALL your buoyancy.
It mean that you have to have same air volume of your wing or jacket, as total weight, that you have (including underwater weight of tank and other rigs).
E.g. for me with thick undergarment I use 12 kg of lead, and my composite tanks has near neutral buoyancy (with 3kg extra leads).
But my SM wing have near 8-9 kg. I know, it is wrong way, but I have dives not far from land, and I can take up by my fins over 4 kg.
and... I have separated dropable weights

Yes, but this complicated calculation is allow to have estimation base for proper weight.

So if the lead is droppable like on a weight belt than your wing doesn’t have to have the same amount of lift as the weight?

 

[-JD-]

@rsingler has developed a very comprehensive tool for bouyancy calculation that includes wetsuit bouyancy swing at depth and change due to air consumption. It also looks at (partial) weight ditching to deal with BC failure.

Check out the thread here: Buoyancy, Balanced Rigs, Failures and Ditching – a comprehensive tool

 

[АлександрД]

So if the lead is droppable like on a weight belt than your wing doesn’t have to have the same amount of lift as the weight?

yes, but I`m a little bit greedy, and is pity to lose any equipment