Newb question on added weight calculations

[Youngandfree]

So I got my open cert last summer using borrowed gear. I bought some used gear last fall, and went to the pool once to check it out. Since then, I'm down about 35 pounds. So I just got my BC and regs back from overhaul in prep for a trip end of the month to do advance open water in Florida. So I hit the pool today to work in my weight calculations with my lower body weight. I started with 10pounds added. 2 in each tank band pocket, and 3 in each removable pouch. BC is an older aqualung Pro LT with the huge weight pockets. Website says it should have 4lbs of lift, and is at the verge of being too big, but that's a different discussion. So with 10lbs on board, and I'm wearing a shorty wetsuit, and scubapro cargo dive shorts over them, I felt like I was struggling with my trim. I didn't sweet it too much since I just wanted to reacclimate and review my underwater skills. Main goal was to do a buoyancy test with 500 psi left in my aluminium 80 tank. 10 lbs was too much, so I removed a weight pocket. Still sank. Removed other pocket, down to 4lbs in trim pockets and still went to the bottom. Removed 2 lbs, and got to float with water line between eyes and forehead. No air in BC, and full lungs. I felt good that should be where it needs to be. So I'll get my tank filled and hit the pool again in a few days.

Am I on track to think I should start with the 2 pounds added, and a full tank, and see how my trim feels? I'm still a greenhorn newbie, with 6 dives last summer in one week. I know I'll have to add a bit more to account for salt water also. I just want to know if my thought process is wrong or not on calculating a starting weight.

 

[rsingler]

Good for you on going to all this effort! And your concern about your trim at this stage speaks volumes about how well you paid attention during your training. Bravo!
So let's talk about details.
Your process was excellent.
Your comment about needing more for salt water was also correct (perhaps 5#).

However, when I teach neutral buoyancy, I have my students hold, not FULL lungs, but a normal breath. The objective after being neutral is to rise when you inhale, and sink when you exhale. You shouldn't have to hold full lungs at your safety stop to keep from sinking. So if you were neutral with 2# with your technique, I'd guess you needed zero added weight with only a half breath at eye level. Now add 5-6# for the extra density of ocean water and I'll bet you'll be close...WITH THAT TANK (and that wetsuit). But not for diving...just for the buoyancy check in salt water. See below.

Keep in mind, by the way, that you don't need to have a tank at 500psi. If you have a 3000 psi 80CF tank (whose true capacity is 77.4CF), then at 500/3000 full, you weren't carrying 64.5 CF of air. Air weighs 0.08lb/CF, so 64.5CF was 5.1lb. So if you try your buoyancy check in the ocean with a full tank, since you need ~5# more for salt water, on top of your 0-2# f.w. neutral buoyancy check, I'll bet you're close, with a normal breath hold, because you're now carrying 5.1 lb of air.

BUT, since you're going diving with that full tank, you also need to carry lead for the air your going to breathe on the way down to 500psi. When the tank is at 500, and you're carrying 5# more lead, voila! You're at the same neutral buoyancy you started with.

Of course that means you'll start 5# heavy, but all divers start a dive overweighted by the weight of the air they expect to consume. You'll need air in your bcd to compensate. But by the end of the dive you should be able to float at eye level with an empty bcd, now that that 5# of air is gone.

Short answer: you can do a buoyancy check with any amount of air in your tank, as long as you include the weight of the "extra" air as though it were lead on your belt. Then, to dive, add lead in the amount of air you expect to consume to end up neutral.

But all of this is based upon the buoyancy of that type of tank when empty. An aluminum 80 needs a little over 3 lb lead to be neutral when at 500 psi. A steel task of the same size is commonly 1-2lb negative at 500 psi. So you'll need 4-5 lb more lead to do that buoyancy check with an aluminum tank than a steel one, unless it's one of the new "neutral" aluminum tanks.

Make sense?

BTW, I'm assuming you meant 40# of lift for that big BCD, yes?

Once you're diving neutral, trim is the next challenge. You'll have less drastic changes with smaller amounts of air in your bcd. It's all about balancing the center of gravity with the center of flotation. You might need floaty fins, or you might need to use a trim pocket more than a bcd pocket to move weight up or down on your body. Don't underestimate the effect of shifting your tank in its band up or down 2-3", as long as you're not banging your head on the first stage, or risking having it fall out. Most of all, get neutral, and then HOLD STILL! 30 seconds or more! Accept whatever position you get until you find out whether you're tail heavy or head heavy. Move a little bit of weight at a time until you dial it in. And don't forget that at the beginning of a dive you'll be 5# heavy and balancing on 5# of flotation in your bcd. Where is the center of flotation? Where is your center of gravity? That'll be useful info too. Things should get easier toward the end of the dive, as the amount of air you need to be neutral decreases.

Cheers! Have fun!

 

[Youngandfree]

Thanks for the details, that makes sense. And yes, 40 or 45 lbs of lift on the BCD. One thing with the BCD I have being bigger than I need, it rides up a decent amount, and I have the cumberbun as tight as it will go, which isn't snug. I can't change it at this point, and may use my brother in law's extra one in florida. That being said, I have the tank fairly low in the band, and it still rides up closer to my head than I would like. After July, trying to sell this one and get a smaller BC is on my list. It worked better last year when I was 35lbs heavier.

 

[rsingler]

A great interim solution might be to rig a crotch strap like those of us with backplate and wing setups. You can order from any number of places, like Dive Great Express, or make one yourself. It's not uncomfortable and will solve the rising up issue for now.

 

[Youngandfree]

Thanks for the suggestion on the crotch strap. I grabbed some basic strapping and buckles from the camping store. I connected it to the bottom of the back plate and put the other side of the buckle on the waist strap. I went to the pool today to try it out. It worked great. I also used 4 pounds and my trim felt much better. I was able to swim laps in the lap lanes even into the 3 ft end fairly easily without going up and down.