Doubling AL80's, bad idea?

[AndrewJD]

To answer the first question, I think that double Aluminum 80's are a good set of tanks given the right environment.

As for double steels and wetsuits, I think that this is generally a bad idea.

To illustrate why double steels may be a poor choice when diving a wet suit, I've put some boyancy calculations together for different double tank and suit combinations that I am familiar with (my equipment) which are below. The two critical bouyancies are the Tanks empty at the Surface and the Tanks full at Depth.

A balanaced rig should be neutral when empty at the surface to allow a diver to maintain stops and at the same time be able to be swum up from depth given no bouyancy in the wing. Rule of thumb is that a diver could swim up -10lbs.

Double 100 (PST)
Wet Suit
Surface and Empty -3
At depth and Full -31
*Ditchable Weight 0

Dry Suit
Surface and Empty -0
At depth and Full -15
*Ditchable Weight -4

Double 80 (Luxfer)
Wet Suit
Surface and Empty 0
At depth and Full -25
Ditchable Weight -7

Dry Suit
Surface and Empty -0
At depth and Full -12
*Ditchable Weight -16

Double 72 (Faber 3300)
Wet Suit
Surface and Empty -13
At depth and Full -37
Ditchable Weight 0

Dry Suit
Surface and Empty 0
At depth and Full -11
*Ditchable Weight 0

D72 (OMS 2640)
Wet Suit
Surface and Empty -4
At depth and Full -28
*Ditchable Weight 0

Dry Suit
Surface and Empty -0
At depth and Full -11
*Ditchable Weight -5

In all instances of steel tanks with the wet suit, the diver is overwieghted with empty tanks at the surface and at depth is extremely negative. Where as the dry suit a diver is only overwieghted by the amount of gas that he/she is carrying since dry suit's bouyancy will not change throughout the dive (assuming the diver adds gas) while the wet suit does. This is why I believe that it not the redundant lift that a drysuit provides that makes it better suited for steel doubles but that it doesn't loose bouyancy at depth.

Can double steels be used with a wet suit? Yes, given the conditions. Say a thin wetsuit that doesn't have a large change in bouyancy with depth and tanks that allow you to be neutral when empty at the surface.

Also, why dive double aluminum 80s with a drysuit and heavy undergarments if you need to add 16+ pounds of lead? Might as well get the higher capacity steel tanks that are more negative.

Just my thoughts.

Jonathan


Notes on Buoyancy Numbers Used
Divers kit includes Backplate, Cannister Light, and Dry suit inflation bottle (drysuit)

Buoyancy for Dry suit w/ Steel plate (Surface and Depth) 18
Buoyancy for Wet Suit w/ Alum plate at Surface 9
Buoyancy for Wet Suit w/ Alum plate at Depth -4

Breakdown of buoyancy by components
Wet Suit at Surface 15
Wet Suit at Depth 2
Dry suit (Surface and Depth) 28
Steel Backplate (dry suit) -6
Aluminum Backplate (wet suit) -2
Canister Light -3
Dry suit Inflation Bottle -1
D100 Empty -12
D100 Full -27
D72 Empty -22
D72 Full -33
D80 Empty -2
D80 Full -14
D72 (OMS) Empty -13
D72 (OMS) Full -24


*Tanks include Bands and Manifold assumed to be -10.

 

[PerroneFord]

Jonathan,

Good stuff. However, you should not that LP cylinders tend to be the order of the day here in cave country. So check the buoyancy on those worthington LP95s and LP108s. And even the LP121s that the WKPP guys use for their rebreathers.

Steph, You mentioned that lift bags tend to be the popular here in cave country??? THink about that. Lift bag... cave... No.

On the double bladders, it was my understanding that the bag was NOT seperated right to left, but front to back, so you didn't get unbalanced inflation should you need to go to your backup bladder. Unfortunately, the kind of stuff that can pierce the outer fabric AND an innner bladder, will mostly likely puncture both inner bladders. Best bet it to dive a balanced rig.

Do Twin80s work? Yessir they do. And the GUE recommends this config for several reasons for ocean diving. Dead easy to get to the surface with. Tanks are cheap, and easy to get, don't have the rusting issues in salt water like steels, etc. I have a set of twin 80s and dive them all the time. Except in caves, where I prefer 108s.

Some steels are fine to wear with a wetsuit depending on the wetsuit and the depth. If you have an emergency and have to get up, there is nothing that says you can't vent some of the gas from your tanks to make you lighter. If you empty the AL tanks, they actually act like two floats.

For openwater, I'd prefer to set my buoyancy so that I need about 3-6 pounds at 10ft to stay down at the end of a dive. That will get me nicely positive if need be at the surface, and a 3-6 pound belt is negligible.

 

[meesier42]

I have used double AL80s for at least 150 of my work dives ( this accounts for about 90%). It was a rare day that we wouldn't dive doubles. But we were always shallow and needed lots of time (2-3 hours, at 20-30ft was normal) more than anything else.

J-valves or isolator manifolds would be recommended.