Weighting differences between AL80 Doubles and HP100 Doubles

[ahereng]

I have found this tank-buoyancy calculator to be pretty helpful for this:

Scuba tank size and buoyancy calculator

 

[Still Kicking]

When I dove doubles regularly, I found 80s with a 10lb V weight to be a pretty good set for dives in the 150' range. Beyond that, I used steels or a stage.

 

[grantmac]

Which 100s do you have? They sound like 3AA tanks, IE: very negative.

FX100s are just barely neutral when empty and could have you closer to neutral.

 

[PfcAJ]

I sink like a stone. As mentioned before, double hp100s with an aluminum backplate and drysuit w/ thick undergarments leaves me heavy by about 6lbs at the end of the dive. When I jump in the water (fresh water) without any gear on except my drysuit, fins, mask, it takes about 12lbs of lead to sink me with my thickest undergarments. My rig mentioned previously is 17lbs negative with 500psi in the doubles (measured this by suspending the rig in the water with a luggage scale). Ended up needing 4lbs of weight on the crotch strap in al80s to be correctly weighted when I tried them, so the estimate of being 10lbs lighter was pretty close. I'm diving in low 50s water so my "thick" undergarments may not be as thick as some other people. I do wish I could wear just a tshirt though

17lbs negative seems like a lot.

So options to make it lighter:

2.5” bands vs 3”
Lighter 1st stages
Painted vs dipped galvanized tanks