How do I properly adjust my weighting with a steel tank?

[renegade17]

I have a steel 119. If I do my weight check at the surface, with a full tank, do I still need to add weight to offset the tanks buoyancy change later in the dive? Or, because the tank is actually negatively buoyant even when at 500psi, should I not add any additional weight? I'm looking for a good starting point before I check my buoyancy at the end of a dive at 15ft with a nearly empty tank. This thing is heavy and I don't want to dive overweighted, (whether I'm diving dry or wet).

[Gombessa]

I assume you've been diving an AL80 before? As a starting point, if you were properly weighted with the aluminum, you can subtract the empty buoyancy difference between the tanks: (4lb - (-2lb)) = 6lb. That's how much weight you can take off the belt to account for the difference between tanks when empty.

Then you can think about whether the decrease in ditchable weight (along with the weight of the additional gas when the 119 is full) overweights you at the beginning of the dive.

[spectrum]

I have a steel 119. If I do my weight check at the surface, with a full tank, do I still need to add weight to offset the tanks buoyancy change later in the dive? Or, because the tank is actually negatively buoyant even when at 500psi, should I not add any additional weight? I'm looking for a good starting point before I check my buoyancy at the end of a dive at 15ft with a nearly empty tank. This thing is heavy and I don't want to dive overweighted, (whether I'm diving dry or wet).

If you feel you have a lock on the right weight with a full cylinder consider that air weighs .08 pounds per cubic foot.

If you dive 101.8 cubic feet away you will be .08 X 101.80 = 8.1 pounds too light at the end of the dive. Add 8 to what you have.

Then retest it at the end of the dive your full weight may have been high if all of the gear was not saturated.

Pete

[renegade17]

I used to dive 28lbs ditchable with my drysuit/undergarment and an aluminum 80. I've since changed my entire rig (from a Seaquest Balance) to a BP/W with a 6lb SS backplate and the steel 119. I guess I'm more interested in the steel 119's fresh water buoyancy characteristics, because at the beginning of a dive currently, I can sink easily with 12lbs ditchable. I want to use less weight (and switch my backplate to an aluminum if neccessary) to keep enough ditchable weight. I feel better with more on my belt so I can ditch it if necessary.

[halocline]

I used to dive 28lbs ditchable with my drysuit/undergarment and an aluminum 80. I've since changed my entire rig (from a Seaquest Balance) to a BP/W with a 6lb SS backplate and the steel 119. I guess I'm more interested in the steel 119's fresh water buoyancy characteristics, because at the beginning of a dive currently, I can sink easily with 12lbs ditchable. I want to use less weight (and switch my backplate to an aluminum if neccessary) to keep enough ditchable weight. I feel better with more on my belt so I can ditch it if necessary.

Assuming your 28 lbs was also in fresh water, and nothing has changed except the tank and the backplate, somewhere around 14lbs would be a reasonable starting point. The tank is -2 empty as opposed to +4 for the AL80, the plate is -6 as opposed to +2 (I'm guessing) for the jacket BC, so that's 14lbs difference, 28-14=14.

You won't know for sure until you get in the water with the tank more or less empty, but that should be pretty close.

[H2Andy]

on your first dive with the steel tank, take the weight you usually dive with in easily divisible amounts (say 2 lb packets)

you'll be overweighted. then it's a simple matter of doing a safety stop and removing weight (your buddy can be your "storage unit") until you are more or less neutrally bouyant with a near-empty tank

you can then fine-tune in subsequent dives

[CalgaryDF]

You still need to account for the air that you have breathed and exhaled regardless of the type of tank.