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Collin O'Brien

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Quick Question: How many more pounds would you use with a Steel 120 vs with a Steel 100 to factor in the slightly more buoyant properties of the 120 toward the end of a dive?

Explained: A buddy is letting me hold onto his Steel 120’s as he is a CCR diver and never uses them, so I gave them a try today on two dives. From the mid-point of each dive and especially toward the end of the dive, I felt like my mid-section was floating up and staying down was getting tough and required more and more effort…I literally held a heavy rock at one point to stay neutral as I was sick of fighting myself to stay down.

I usually dive Steel 100’s with a Steel BP and Single Tank Adapter with an integrated 6lb weight from Halcyon so I usually use just 2 additional lbs or none at all. And I have been great with that set up for a while. What weight adjustments do you think would be best for this switch to a Steel 120 with the issues I was having from the midpoint of my dive on; possibly due to the slightly positively buoyant properties of an empty Steel 120. Thank you!
 

-JD-

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Which steel 100s and which steel 120s? The details matter.

Reserve pressure buoyancy on my PST 3500 HP120s vs. HP100s is close enough that I don't worry about it. FWIW, they are both slightly negative empty.
 

rx7diver

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Quick Question: How many more pounds would you use with a Steel 120 vs with a Steel 100 to factor in the slightly more buoyant properties of the 120 toward the end of a dive? ...


Using the Huron Scuba tank specification chart:

A Sherwood Genesis/PST HP 100 is -8.8# to -1.3#, full to empty.
A Sherwood Genesis/PST HP 120 is -10.3# to -1.3#, full to empty.

So, theoretically, you would not need to adjust your weighting when switching from this HP 100 to this HP 120.

But, more important, since you are able to actually dive the HP 120, can't you just do a buoyancy check and adjust?

Did I misunderstand what you are asking?

rx7diver
 
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Collin O'Brien

Collin O'Brien

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I am not sure which brand tank it is. I was using Faber as my reference which is -0.6 when empty for a S100 and +0.7 when empty for a S120. I keep track of gear used, weight and how it feels for each dive so Im sure I'll figure it out eventually. I was just seeing if anyone had any advice because it was very annoying today trying to stay down when I usually have zero issue in a S100. Maybe air was trapped in wing on both dives causing my mid section to float more.
 
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Collin O'Brien

Collin O'Brien

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If neither of you notice as big a difference between the two tanks then maybe it was some other factor today and not the tank. Because it was noticable...but also the first time I ever used the 120.
 

johndiver999

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If you think you need 2 lbs more lead to counteract the larger buoyancy swing associated with consumption of more gas from the larger tank, then do it.

I like to dive with a few lbs of excess lead, just to provide stability on the bottom in a current, so I generally do not adjust ballast when diving the different steel tanks I have: LP 85, LP 108, LP 121, HP 130 and HP 149 (and I think a 117? or something).. I often get 2 dives from the 149 and I can feel the difference between the first and second dive.
 
https://www.shearwater.com/products/peregrine/

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