Once you have your weighting dialed in for a given equipment configuration, it's REALLY simple to go on vacation and be presented with a different tank, or different bcd, or bring new fins, or add a pony bottle, and still pick exactly the right weight for your dive...on your very first try!
The front page in my logbook is a blank, on which I've written a column of Magic Numbers. One for every wetsuit or drysuit liner I use. On another page is a little summary of various tank buoyancies.
That's all I need.
Briefly, the Magic Number is the amount of buoyancy you need to add to your given wetsuit, to be neutrally buoyant. Since wetsuits float, the Magic Number is always negative, to neutralize that. NOTE: The Magic Number is NOT the amount of lead you carry, as you'll see below.
Here's how it works.
What you don't know:
Wetsuit: Unknown positive buoyancy
You: Slight positive or negative buoyancy, which you haven't measured
What you DO know:
EMPTY Tank plus valve: Known buoyancy (positive or negative)
Reserve Air Left in tank: Call it -1 lb (see end of post for exact calcs)
BCD/Wing/Backplate/Fixed Weight buoyancy: Many jackets are close to zero once all the air is squeezed out. Those who dive BP/W will know how much their plate weighs.
First stage on tank: -1 lb, unless you've got one of those fly-weight titanium beauties
Fins: between -2 lb and +1 lb, based upon mfr and composition. Note: this is buoyancy, not weight of the item.
Extra Ditchable Lead carried: Whatever you determined upon doing your neutral buoyancy check with a near-empty tank.
The TOTAL is your magic number for that wetsuit.
Let's do an example:
I float at eye level with 10 lb in my 5mm wetsuit, a normal lung, and a near empty tank. Let's run the numbers:
Empty Aluminum 80 with valve: +4.4 lb
Reserve Air: -1 lb
Scubapro GlidePro BCD: 0 lb
First stage: -1 lb
Scubapro SeaWing Nova Fins: +0.5 lb
Lead carried: -10 lb
Total: -7.1 lb
My Magic Number for my 5mm wetsuit is -7
So now I go on a vacation dive with steel tanks. I've never dived with a steel tank before.
I look up a generic steel 80 on my chart and I find: -2 lb
So if my Magic Number is -7...
Empty Steel 80: -2 lb buoyancy with empty (with valve)
Reserve Air: -1 lb
BCD: 0 lb
First stage: -1 lb
Fins: +0.5 lb
Sub-total: -3.5 lb
How much lead do I need to carry to hit -7?
3.5 more pounds.
For this dive I'll carry 4 lb and be perfect right out of the gate.
What if I change to old heavy Jet-fins and try out my buddy's steel backplate with my wetsuit?
Empty Steel 80: -2 lb
Reserve Air: -1 lb
BCD: -4 lb
First stage: -1 lb
Fins: -1 lb
Sub-total: -9 lb
I'm 2# heavier than my Magic Number already. I don't need ANY weight. I'm going to be diving just a shade heavy, and will need air in my bcd at all times, including about 2 lb worth (1 liter) at my safety stop. If my bcd fails, I may have to swim up as much as 10-12# if my tank is still full and my wetsuit is compressed at depth. At my safety stop, I'll be about 2# heavy with a near empty tank.
If you have added junk (dive light canister, pony bottle, seventeen steel D-rings on your BP/W, just add that to your BCD/Wing/Backplate/Fixed Weight buoyancy entry. The rest is all the same.
Dive Safe!
Reserve Air Buoyancy calculation:
PSI reserve/PSI tank capacity x tank volume = residual gas volume
Residual gas volume x 0.08 lb/cu ft = residual gas weight
Example: AL80 (77.4 cu ft at 3000 psi)
500psi/3000psi = 0.167
0.167 x 77.4 = 12.9 cu ft residual gas
12.9 x 0.08 = 1.032 lb residual gas weight
Thus, diving a LP steel down to 2500 may leave you with a bit more negative buoyancy, since you're leaving more air behind. Either account for it, or use a lower residual gas pressure for this type of tank:
LP Steel 80 (80 cu ft at 2640 psi)
500/2640 x 80 x 0.08 = 1.21 lb residual gas weight. (Do you really need a reserve of 15 cu ft?)
400/2640 x 80 x 0.08 = 0.97 lb
The front page in my logbook is a blank, on which I've written a column of Magic Numbers. One for every wetsuit or drysuit liner I use. On another page is a little summary of various tank buoyancies.
That's all I need.
Briefly, the Magic Number is the amount of buoyancy you need to add to your given wetsuit, to be neutrally buoyant. Since wetsuits float, the Magic Number is always negative, to neutralize that. NOTE: The Magic Number is NOT the amount of lead you carry, as you'll see below.
Here's how it works.
What you don't know:
Wetsuit: Unknown positive buoyancy
You: Slight positive or negative buoyancy, which you haven't measured
What you DO know:
EMPTY Tank plus valve: Known buoyancy (positive or negative)
Reserve Air Left in tank: Call it -1 lb (see end of post for exact calcs)
BCD/Wing/Backplate/Fixed Weight buoyancy: Many jackets are close to zero once all the air is squeezed out. Those who dive BP/W will know how much their plate weighs.
First stage on tank: -1 lb, unless you've got one of those fly-weight titanium beauties
Fins: between -2 lb and +1 lb, based upon mfr and composition. Note: this is buoyancy, not weight of the item.
Extra Ditchable Lead carried: Whatever you determined upon doing your neutral buoyancy check with a near-empty tank.
The TOTAL is your magic number for that wetsuit.
Let's do an example:
I float at eye level with 10 lb in my 5mm wetsuit, a normal lung, and a near empty tank. Let's run the numbers:
Empty Aluminum 80 with valve: +4.4 lb
Reserve Air: -1 lb
Scubapro GlidePro BCD: 0 lb
First stage: -1 lb
Scubapro SeaWing Nova Fins: +0.5 lb
Lead carried: -10 lb
Total: -7.1 lb
My Magic Number for my 5mm wetsuit is -7
So now I go on a vacation dive with steel tanks. I've never dived with a steel tank before.
I look up a generic steel 80 on my chart and I find: -2 lb
So if my Magic Number is -7...
Empty Steel 80: -2 lb buoyancy with empty (with valve)
Reserve Air: -1 lb
BCD: 0 lb
First stage: -1 lb
Fins: +0.5 lb
Sub-total: -3.5 lb
How much lead do I need to carry to hit -7?
3.5 more pounds.
For this dive I'll carry 4 lb and be perfect right out of the gate.
What if I change to old heavy Jet-fins and try out my buddy's steel backplate with my wetsuit?
Empty Steel 80: -2 lb
Reserve Air: -1 lb
BCD: -4 lb
First stage: -1 lb
Fins: -1 lb
Sub-total: -9 lb
I'm 2# heavier than my Magic Number already. I don't need ANY weight. I'm going to be diving just a shade heavy, and will need air in my bcd at all times, including about 2 lb worth (1 liter) at my safety stop. If my bcd fails, I may have to swim up as much as 10-12# if my tank is still full and my wetsuit is compressed at depth. At my safety stop, I'll be about 2# heavy with a near empty tank.
If you have added junk (dive light canister, pony bottle, seventeen steel D-rings on your BP/W, just add that to your BCD/Wing/Backplate/Fixed Weight buoyancy entry. The rest is all the same.
Dive Safe!
Reserve Air Buoyancy calculation:
PSI reserve/PSI tank capacity x tank volume = residual gas volume
Residual gas volume x 0.08 lb/cu ft = residual gas weight
Example: AL80 (77.4 cu ft at 3000 psi)
500psi/3000psi = 0.167
0.167 x 77.4 = 12.9 cu ft residual gas
12.9 x 0.08 = 1.032 lb residual gas weight
Thus, diving a LP steel down to 2500 may leave you with a bit more negative buoyancy, since you're leaving more air behind. Either account for it, or use a lower residual gas pressure for this type of tank:
LP Steel 80 (80 cu ft at 2640 psi)
500/2640 x 80 x 0.08 = 1.21 lb residual gas weight. (Do you really need a reserve of 15 cu ft?)
400/2640 x 80 x 0.08 = 0.97 lb
