Proper Lift?

[LJ_OREILY]

Is there a calculation or formula to figuring out how much lift one would need, with say a 120 HP tank, 7mil suit, for a 210lb person?

 

[cool_hardware52]

Is there a calculation or formula to figuring out how much lift one would need, with say a 120 HP tank, 7mil suit, for a 210lb person?

To calculate minimum required lift you need to know two things;

1. The initial buoyancy of your wetsuit, roll it up, throw it in the pool and add lead until it's neutral.

2. The "in water weight" or negative buoyancy of your rig. You can estimate this by adding up the pieces, i.e. the negative bouyancy of the tank, full + ~2 lbs for the regs, + what ever else is attached to the rig, integrated weights, lights etc.

Any BC needs to do two things, float your rig at the surface without you in it, and compensate for the compression of your wetsuit.

Example: Full Steel 120 ~ -10.5
SS Back Plate and harness -6.0
5 lbs bolt on weights -5.0
Regs -2.0
Can light -4.0

Rig total -27.5 lbs


7 mil wetsuit for a 210 lbs diver Guestimated initial buoyancy 30 lbs.

Minimum lift required is the larger of the two, 30 > 27.5, you need a 30lb lift at a minimum.

This is ONLY AN EXAMPLE. You need to determine your own values. Neoprene varies quite a bit. There is no magic equation. For cold water your exposure usually dicates the required lift.


Regards,


Tobin

 

[*Floater*]

Is there a calculation or formula to figuring out how much lift one would need, with say a 120 HP tank, 7mil suit, for a 210lb person?

The 120 HP tank has a buoyancy swing of about 9# and to be conservative assume your wetsuit loses 75% of it's buoyancy at depth, or maybe all it's buoyancy to be even more conservative. Now how much do you need to sink it at surface waters?

As an example, let say you need 24# to sink it at surface, so it loses maybe about 18-24# at depth, and your hood and boots lose a few # too, then you could probably get away with a 30# wing if the 75% figure is correct, but 35# would do for sure. This analysis assumes you don't dive otherwise over-weighted. i.e. you are neutral close to surface with an empty tank and no gas in your wing. With a drysuit you'd need even less lift.

Some people want their wings to be able to float their heavy rigs on the surface (without them strapped in i.e. no wetsuit buoyancy), so that's another thing to consider if you are so inclined.

I'd actually like to know how much buoyancy wetsuits do lose in %-terms. The figure I've heard on the internets is 67% at 30m, but it's just what someone said. If anyone knows then please let me know.

 

[cool_hardware52]

The 120 HP tank has a buoyancy swing of about 9# and to be conservative assume your wetsuit loses 75% of it's buoyancy at depth, or maybe all it's buoyancy to be even more conservative. Now how much do you need to sink it at surface waters?

As an example, let say you need 24# to sink it at surface, so it loses maybe about 18-24# at depth, and your hood and boots lose a few # too, then you could probably get away with a 30# wing if the 75% figure is correct, but 35# would do for sure. This analysis assumes you don't dive otherwise over-weighted. i.e. you are neutral close to surface with an empty tank and no gas in your wing. With a drysuit you'd need even less lift.

Some people want their wings to be able to float their heavy rigs on the surface (without them strapped in i.e. no wetsuit buoyancy), so that's another thing to consider if you are so inclined.

I'd actually like to know how much buoyancy wetsuits do lose in %-terms. The figure I've heard on the internets is 67% at 30m, but it's just what someone said. If anyone knows then please let me know.

I don't have the reference handy, but neoprene has been tested, and it looses all it's initial buoyancy at ~165. Testing was done by a Florida University IIRC. I doubt the effects are linear, as the pressure change is not linear with respect to depth. Remember the pressure doubles in the first 33 ft.......

For most single tank, no stop diving with a 5 mil or thicker wetsuit, if one is neutral at the surface with no gas in their wing and full cylinder holding shallow stops with 500 psi is no problem. Why? because the wetsuit compresses quite a bit in the first 15 ft, usually more than enough to offset the weight of the gas consumed from a single cylinder.

Not being able to float your rig at the surface could be expensive, not being able to over come the loss of exposure suit buoyancy could be tragic.

Tobin