weighting & buoyancy

[inquis]

2.4% seems like it would yield more realistic numbers. I'd like to believe it. I'd like even more to understand it.

It falls out of the density of fresh and salt water. Although the later varies, an average value is 1.025 kg/liter (vs. 1.0 kg/liter for fresh). (I believe Wikipedia's number is rounded). TBH, I got the 2.4% from a different source that I can't find now, which used imperial values: 63.9 lb/cuft (seawater) and 62.4 lb/cuft (fresh), resulting in a ratio of 1.024. If your "seawater" comes from the Red Sea, 3% would be a better number to use. This map of salinity gives you some insight into the variability:

At any rate, you probably know that to be neutral, the buoyant force must equal your mass. The buoyant force is just the water density times your volume. You don't know your volume, but you know it's the same regardless of whether your eyes are stinging or not, so it winds up cancelling out of the equations.

 

[inquis]

It would be nice to actually have the buoyancy specs for each piece of neoprene gear.

Put it in a mesh bag (or pillowcase or whatever) along with something heavy enough to sink it. Measure with a travel scale both with and without the article of interest. The difference is the article's buoyancy. A hot tub, pool, or even a trashcan (that doesn't leak) works. I didn't have enough lead to sink my 3mm, so I just threw one of those metalworking vices in the bag. Rocks can work in a pinch, but the ones I've tried were surprisingly light in the water.(You do have to be careful no air is trapped: sink the suit from the middle so air can escape then put it in the bag.)

 

[Doby45]

LMAO @ the micro percentages. Do what @NW Dive Dawg said and get to 15-20' with a 500psi tank and see what you need. All the other "science of micro percentages" is crap. You are dealing with a NEW 7mm too, so this will change as that suit compresses over time.

 

[DogDiver]

With a full 7mm suit in fresh water I would start with 10% of your body weight plus 4 pounds. This is just a starting point.

 

[Green Frog]

I'm wondering about the weight difference between a full aluminum 80 (3000 psi) and one with 500 psi. I live 300 miles from the nearest salt water (Chesapeake Bay) and am making a switch from a vest type BCD to a BC/W. My calculations are complicated by the fact that I've lost 50 lb since I was doing a lot of diving and had my weight needs firmly established. My exposure suit is an old tri-lam skin with only minimal positive buoyancy, and will be used in both warm lake setting and the warm ocean.

I plan to assemble the rig and go to a local lake, but would like some sort of guidance on how to calculate weight I'll need to use in the Bahamas. The problem may become moot if I get to go Ocean diving off SC sometime in the meantime and can take weights and extra trim weight bags, but sitting here in the foothills of the Appalachians it's quite the conundrum for me.

Froggie

 

[L13]

The usable air in an AL80 is ~5 pounds.

 

[inquis]

If you dial in your freshwater weighting, the additional weight needed for salt water is 2.4% of you+gear dry weight. (This based on average salinity/density, and you can ballpark a single AL80 rig at 40 lbs.) If you weigh 210 lbs, figure 6 lbs more: (210+40)*0.024

 

[johndiver999]

If you dial in your freshwater weighting, the additional weight needed for salt water is 2.4% of you+gear dry weight. (This based on average salinity/density, and you can ballpark a single AL80 rig at 40 lbs.) If you weigh 210 lbs, figure 6 lbs more: (210+40)*0.024

No, not really, you need to include the weight of your weight belt, BC, wetsuit, fins, (everything) when you apply the conversion of 2.4% from fresh to salt. Or just pick up the gear bag, throw the belt in it and estimate that weight.

 

[inquis]

True enough, it is total dry weight. (I only allowed for 8 lbs over the tank weight alone, which is a bit light in hindsight. Good catch.) That example probably should add 7 lbs instead of 6 lbs.