Aloha My fellow genius scuba divers! Bouyancy Question

[Kimo-Maui]

I just got a short 100 steel tank. Its full bouyance is -10. It looks like the average bouyancy of an aluminum 80 is about -2.

So, does this mean I should drop 8 pounds of weight?


Thanks for response. A little about me - fairly new diver, certified in Sept but since I live in the tropics, I get to dive about 2x a week this time of year. I'm interested in photography, and I'm just getting my arms around it. I have the sealive DC800 camera - very nice improvement over the last one I had (sealife 600?). The 600 was a severe challenge to take clear, unblurred pictures. The 800 has a quicker shutter, and I get nice clear pictures now.

Anyone who visits maui, I can give you tips on where to dive and what to look for, including a shallow shark cave dive.

Look forward to your feedback!

 

[cfelliot]

I made this change from an AL80 to a ST80 last year.

I don't think you want to base your weight change on the full tank buoyancy, but rather the ending tank PSI buoyancy.

Note that the volume of air remaining at 500 PSI in an AL80 may not be the same volume of air remaining at 500 PSI in a ST80 tank due to perhaps it being HP or a different total volume!

This info is generally in the tanks spec sheets too!

 

[ligersandtions]

In determining the correct weighting, you should see how much you need to keep you neutrally buoyant with little or no air in your wing/BC at your safety stop, with ~500 psi in your tank. Basically what that's saying is that you need to compare the bouyancy characteristics at empty....an Al 80 is +4.4 empty, while your steel 100 is probably somewhere between -0.5 and -7.2 empty, depending on which tank it is that you have. This would mean you'd be able to take off somewhere between 5 and 11 pounds, ASSUMING you are properly weighted right now.

I'd suggest making your dive, and then when you get to your safety stop, purge down to about 500 psi in your tank. Remove all air from your wing and see if your are positively buoyant (floaty), negatively buoyant (sinking and have to fin to keep yourself at 15 feet), or neutral (desired). If you are positive or negative, add or remove weight in small increments until you find the weighting that makes you neutral.

If at all possible, doing this over a sandy bottom where you can lay down and remove weights as needed is probably the easiest way to do this weight check. Of course you can do this at the surface, but I'm not trusting enough and prefer to do it this way. I'm sure someone else will chime in as to how to do a "proper" weight check at the surface.

 

[Scubakevdm]

In determining the correct weighting, you should see how much you need to keep you neutrally buoyant with little or no air in your wing/BC at your safety stop, with ~500 psi in your tank. Basically what that's saying is that you need to compare the bouyancy characteristics at empty....an Al 80 is +4.4 empty, while your steel 100 is probably somewhere between -0.5 and -7.2 empty, depending on which tank it is that you have. This would mean you'd be able to take off somewhere between 5 and 11 pounds, ASSUMING you are properly weighted right now.

I'd suggest making your dive, and then when you get to your safety stop, purge down to about 500 psi in your tank. Remove all air from your wing and see if your are positively buoyant (floaty), negatively buoyant (sinking and have to fin to keep yourself at 15 feet), or neutral (desired). If you are positive or negative, add or remove weight in small increments until you find the weighting that makes you neutral.

If at all possible, doing this over a sandy bottom where you can lay down and remove weights as needed is probably the easiest way to do this weight check. Of course you can do this at the surface, but I'm not trusting enough and prefer to do it this way. I'm sure someone else will chime in as to how to do a "proper" weight check at the surface.

Good advice. The only thing that I would add is have a buddy check your bcd for trapped air when you do your near empty tank buoyancy check at 10 or 15 feet, and remember to keep your and hands fins still.

 

[Kimo-Maui]

Sounds like solid advice. I can tweak the 15ft stop as I rarely go below 50 ft (not much reef on my side of the island below 50), and can just sit in the sand at 15 ft. If I'm too bouyant, I'll just stuff sand in my pocket if I need to remain at 15, or else more likely I'll just add weight the next time.

With about 500psi, my steel should be -2, and an alum is about 4.4 so It looks like I can drop 6-7lbs.

In determining the correct weighting, you should see how much you need to keep you neutrally buoyant with little or no air in your wing/BC at your safety stop, with ~500 psi in your tank. Basically what that's saying is that you need to compare the bouyancy characteristics at empty....an Al 80 is +4.4 empty, while your steel 100 is probably somewhere between -0.5 and -7.2 empty, depending on which tank it is that you have. This would mean you'd be able to take off somewhere between 5 and 11 pounds, ASSUMING you are properly weighted right now.

I'd suggest making your dive, and then when you get to your safety stop, purge down to about 500 psi in your tank. Remove all air from your wing and see if your are positively buoyant (floaty), negatively buoyant (sinking and have to fin to keep yourself at 15 feet), or neutral (desired). If you are positive or negative, add or remove weight in small increments until you find the weighting that makes you neutral.

If at all possible, doing this over a sandy bottom where you can lay down and remove weights as needed is probably the easiest way to do this weight check. Of course you can do this at the surface, but I'm not trusting enough and prefer to do it this way. I'm sure someone else will chime in as to how to do a "proper" weight check at the surface.

 

[ligersandtions]

I would probably suggest not putting sand in your pockets....unless you know exactly how much a handful weighs

When I did this weight check for myself, I went down with what I thought was more weight than I was actually going to need. I had a bunch of one pound soft weights....and when I got to my safety stop, dumped my air (I did have my buddy really squish my wing to make sure I had it all out), and purged my tank, I was definitely over weighted. I dropped weight one or two pounds at a time, inhaled and exhaled a few times.....if I was still overweighted, I dropped one or two more and tried it again. Don't rush through it and make sure that you are breathing how you will at your safety stop, otherwise you may seem artificially light or heavy.

 

[emttim]

To the OP: It sounds like you switched to a HP 100 steel judging by the buoyancy. I made the same swap from rental AL 80s to my own HP 100, and while you shouldn't exactly take off 8 pounds, I was able to remove about 6 pounds myself so pretty close to 8. I would start off with 4 pounds and then change in 1-2 pound increments until you get it dialed in the way you want it. Better to be a little overweighted than underweighted in the meantime since one involves you not having to swim back to shore and change out your weights in order to do the dive.

 

[spectrum]

Kimo-Maui

As mentioned the empty buoyancy specs are what you compare to adjust weighting between cylinders.

If you intend to dive underweighted and stuff your pockets with sand as needed you are on your own.

Pete

 

[don Francisco]

Kimo-
assuming you already established correct weighting with your AL80, then you're sort of right that you would add or subtract weight according to the difference in bouyancy of various tanks.

Where you go wrong is forgeting to account for the different capacities of the tanks. I guess that you plan to use the additional 20 cuft of air, so I'd allow about 2 pounds for that (.08#s/cuft) and shed only 6 pounds of ballast. As a rule, it's better to use the empty tank bouyancy in weight calculations, since ideal weighting is based on that condition.

Note also that this may also have some effect on trim, so you might also have to reposition your weights.