Weight Question

[fierodiver]

Hi. I've been using HP steel 80's to dive with since I've been certified. I have my weight and trim dialed in for these tanks. I recently purchased a steel 120. Using the bathroom scale I see that the 120 weighs 54lbs,the 80 is 35lbs. These are tank full weights. I am assuming I will need to carry less lead on the belt because of the heavier tank. But the 19pound difference in tank weights is more than the amount of lead I already carry. With the 80's I use 13 lbs of lead. Is there any rule of thumb on this? I realize I'll probably need to play around with it,but I'm looking for a starting point. OR,is there a possibility that I wont need any lead? Thanks.

 

[cmburch]

It goes by buoyancy empty. You probably need to use the same amount of weight if the buoyancy empty are similar.

http://www.xsscuba.com/tank_steel_specs.html

If both were XSSCUBA then would need to add a pound.

 

[SteveAD]

The 19 pound dry weight difference will be mostly offset by the difference in displacement. If you want a better idea of the bouyancy difference put each tank into a trash can filled with water and weigh it (while submerged) with a fish scale.

 

[mark99]

You'll probably use the same weight, because the empty buoyancy is almost the same. You might find your trim off a little because the tank is longer, and the center of gravity is lower. I use a 1# ankle weight around the tank neck on my HP-120 to trim to horizontal better.

 

[Bubbletrubble]

If you are properly weighted with a known tank (model/size), then it should be very straightforward to calculate how much weight to wear with a different tank...so long as you know what the buoyancy characteristics are for each empty tank.

Step #1: Use your favorite tank specs chart. I like this one from TDL:
TECHDIVINGLIMITED.COM

Step #2: Find the "empty buoyancy" characteristics of each tank.

Step #3: Calculate the difference in empty buoyancy weight.
For instance, let's say that you use 13 lbs. lead with your HP80 steel tank = PST E7-80 tank (empty buoyancy, -2.5 lbs.). You want to know how much total lead to carry with a PST E7-120 tank (empty buoyancy, 0 lbs.). You would have to carry an extra 2.5 lbs. of lead with your "new" PST E7-120. Thus, you would need to carry a total of 15.5 lbs. of lead with the PST E7-120. Understand that, intuitively, you were probably thinking that changing to a larger capacity tank would allow you to take off some lead. That would not be the case with this example.

You say that you have your weight requirements "dialed in" for the HP80 tank. Please make sure that you have determined this using a near empty tank and wearing all your normal gear.

As others have pointed out, the "dry" weight of a tank means absolutely nothing with regard to its buoyancy characteristics. However, the "dry" weight will matter when you're carrying the tank to/from your car, LDS, boat, etc.

If you don't trust the calculations...or cannot properly identify the tank(s), then you should simply do a full-blown weight check.

Have fun and dive safe.