Quick steel tank buoyancy question

[amajamar]

Just to confirm, if the full buoyancy of my HP 80 is -8.5 lbs and the full buoyancy of an HP 100 is -10 lbs than the difference of 1.5 lbs is what I could remove from my weight belt. I don't need to consider the weight of the tank, correct?

Thanks in advance!

 

[eponym]

Correct--if you know your weighting with one tank you can switch to the other by adding or removing the difference between them. However, you might want to check that the difference between those two tanks when "empty" (or below 500 psi) is also 1.5 pounds. That could be a more useful metric.

-Bryan

 

[amajamar]

Thanks, Bryan!

Interestingly, the empty buoyancy is the same for both tanks, at -2.5 lbs, so I suppose that another way to look at it is that if I keep the weighting the same, I may be slightly overweighted at the start of the dive, and properly weighted at the end of the dive. Otherwise, I may be a little light with a low tank... hmmm.

Thanks a lot!

 

[Scared Silly]

Just to confirm, if the full buoyancy of my HP 80 is -8.5 lbs and the full buoyancy of an HP 100 is -10 lbs than the difference of 1.5 lbs is what I could remove from my weight belt. I don't need to consider the weight of the tank, correct?

No, the 1.5 lbs is the different for the weight of the extra 20cuft in the cylinder that you will breath. What you want to look at is when the cylinder is "empty". So given both cylinders have the same empty buoyancy you do not want to remove any weight.

 

[eponym]

Thanks, Bryan!

Interestingly, the empty buoyancy is the same for both tanks, at -2.5 lbs, so I suppose that another way to look at it is that if I keep the weighting the same, I may be slightly overweighted at the start of the dive, and properly weighted at the end of the dive. Otherwise, I may be a little light with a low tank... hmmm.

Thanks a lot!

I think you've got it now.

When I have to switch from my own HP steel 100s to Aluminum 80s I have to add the difference between them when almost empty so I can still control my ascent in the top fifteen feet at the end of a long dive. That's about six pounds more I have to carry.

-Bryan

 

[TSandM]

Yes, it's the empty buoyancy you consider, not the full. The extra 1.5 pounds is gas you intend to consume, so if you take that weight off your belt, you will be too light at the end of the dive.

 

[amajamar]

Thank you all for you responses. I can confirm that you were all correct! I dove my new 100 cf steel tank this weekend without making any changes to my weighting and found myself a little heavier at the start of the dive and just right at the end!

Thanks again!