Setting up twin set for wife, Standard AL80 or Compact/neutral 80?

[Marktan]

As my wife and I progress in our diving together, I've started diving sidemount doubles. I was originally using a pair of neutral/compact aluminum 80's (Catalina 3300psi service pressure) and have recently switched to a pair of steel LP95s. My wife has a pair of (standard Luxfer shotblast) AL80s and is diving them as single back mount. She has no desire to do the sidemount setup (she'd rather not swap regulators or change her BC), however she has expressed interest in backmount doubles for intro to tech and so we can stay down longer together without having to go back up for her to swap tanks.

My question is, given we dive (7mm) wetsuits typically here in the midwest (neither dive dry yet), which set of aluminum 80s (standard or neutral) would be best suited for setting her up for a backmount twin set? I'm leaning towards the neutral 80s at they won't go so positive on her at the end of dive, but aren't so negative like steels. Her BC has 40lbs of lift, so I think that's sufficient. FWIW, she dives with about 12-14lbs weight in fresh water on her single AL80. We have all the regulators we need already, so it's just a matter of picking which tanks and attaching the bands & manifolds.

Thoughts and reasoning behind it would be appreciated. Thanks!

 

[Chilly_Diver]

What about double HP100’s? Better buoyancy characteristics than an AL80 and similiar in size.

 

[Marie13]

What about double HP100’s? Better buoyancy characteristics than an AL80 and similiar in size.

OP already has the cylinders he's asking about. Just trying to decide which ones to double up.

 

[Marktan]

Agree about the characteristics, but own the 80s now, and have several upcoming certs for the two of us that funds are being allocated to (not to mention a replacement for my recently flooded and now dead camera), so unless I find a great deal on a pair of 100s, no go for the time being.

 

[fish149]

If it's between the two then yeah, I'd go with the more neutral tanks . I agree that steel would be better but if you already have the AL's.....
I would seriously consider getting a drysuit before getting the doubles put together though as staying down longer for most Midwest diving means freezing your but off. Right now it seems like you can get used suits on eBay ridiculously cheap, especially med-small sizes. If you're somewhat handy gluing new seals on a fixer upper is a pretty simple endeavor.

 

[ofg-1]

How far does she have to walk with the doubles? What kind of boat does she have to crawl into. A set of Al 80's is a bit over 70 pounds, and depending on her size, that's a bunch of weight. A single lp 85 pumped to 3500 is about 112 cubic feet and about 1/2 the weight of the doubles.

 

[Marktan]

How far does she have to walk with the doubles? What kind of boat does she have to crawl into. A set of Al 80's is a bit over 70 pounds, and depending on her size, that's a bunch of weight. A single lp 85 pumped to 3500 is about 112 cubic feet and about 1/2 the weight of the doubles.

Appreciate the concern. At the quarry, walking anywhere from 50-300ft. Boat depends, but the one has a nice fin ladder to get on the deck. She's 5'9", athletic (open water marathon swimmer) and capable. In rescue class she hauled our 6'4" 300+lb instructor out of the water and onto the beach on her own multiple times throughout the day, so I'm not too concerned.
She also tried our friend's doubled 108's last year without much fuss, which is what got her thinking about it.

I'm trying to set up with what we currently have. Can always switch things around in the future.

 

[guruboy]

I have read that the neutral tanks can get bottom heavy. Also, unless you can reliably get HP fills, you're going to be missing gas.

 

[Marktan]

I have read that the neutral tanks can get bottom heavy. Also, unless you can reliably get HP fills, you're going to be missing gas.

Ironically, when I used the CF neutrals for side mount, the tails still did float/rotate a bit when empty, but not nearly as bad as the tails swinging really high with the AL80s. When full down to about 1000psi they trimmed out nicely. Not sure how that translates to back mount.
No worries on the fills at the shop.

 

[LiteHedded]

with a 7mil wetsuit I'd probably go for the lighter tanks