Reasons for aluminum b/p with single aluminum tank?

[chris_b]

I wasn't sure if I should post this in the "tek" area since the topic is backplates but with so many related questions in this forum I figured this is a good spot.

Thanks to all of you who continue to put up with all my newbie questions but now I have another one.

As I understand it, the difference in buoyancy of a full vs. empty 80cf Aluminum tank is ~6 lbs., also the weight of most stainless steel backplates. This would just offset the air used up, leaving the stuff on your back just a little negative.

Even if you were diving in your birthday suit (apparently people actually do this!) and were minimally buoyant (i.e. no wetsuit) would you ever want to use an aluminum backplate? Wouldn't using one just mean more weight on your belt in order to stay a little negative at your end-of-dive safety stop?

I'm still trying to think through the ins and outs of weighting and buoyancy; thanks again for indulging (and educating!) me.

Chris

[Scubaroo]

What would be REALLY cool is a table of tank configurations (63 alum, 77 alum, 72 LP steel, 95 LP steel, 120 HP steel, PST 104 LP doubles etc etc) versus exposure suit styles (shorty, 3mm, 5mm, 7mm, dry, etc), with a recommended backplate type/weight for each combination. Obviously there would be some overlap! but it would be a handy reference for us backplate newbies. I guess a near-neutral BP/tank bouyancy could form the basis of the table, with room to "upgrade" your backplate from aluminum to steel, or add an STA, to increase integrated weight...

It would be a huge table though... and probably a good source for heated debate!

Or is there a simple rule of thumb somewhere that says aluminum BPs are only for tropical or drysuit diving with steel doubles???

[ericfine50]

AL BP is used for diving with a wetsuit and AL80s. This will allow you to swim up to the surface if you have a wing failuar. Yes, you would still need a weight belt.

SS BP = Drysuit, Steel Tanks.

Eric

[jetblast00]

Originally posted by ericfine50
AL BP is used for diving with a wetsuit and AL80s. This will allow you to swim up to the surface if you have a wing failuar. Yes, you would still need a weight belt.

SS BP = Drysuit, Steel Tanks.

Eric

I know plenty of people who dive steel doubles in drysuits with Al backplates... you're wearing steel doubles, what do you need more wight for if you know you're not going to breathe down the all of the tanks? that's just extra gas in your wing to manage. It's simply a matter of putting some non-ditchable weight somewhere if you need it... Personaly, I'd like to have the SS backplate for a single Al80 to take the weight off my waist, but I know that I can swim that up without didching it.

[Lost Yooper]

The only thing that matters is that you're weighted such that you can swim up with a full tank by ditching some amount of weight (with a wetsuit). I don't personally think it's a good idea to have all of your weight on a weight belt. IMO, (idealy) the weight belt should have just enough weight on it so that if it's ditched, you will be able to swim up with a full tank and a fully collapsed suit. I don't think ditching all of your weight at once is a particulary good idea.

Mike

PS. If I wear a belt, I wear it under the crotch strap. If the belt accidentally comes off, the crotch strap will catch it. To ditch the belt, I would have to unbuckle the waist strap and then unbuckle the belt. I also use two weight belt buckles too. You don't want your belt to accidentally coming off.

[100days-a-year]

I use a S/S b/p with everything,same harness only thing that changes is weight worn.With a skin I am nuetral empty and can swim up the 6 lbs when full.With 3 mil 6-8 lbs in weight pockets.With 7 mil(winter spearfishing only)I need 16 lbs.I found weight pockets at LDS coupla years ago to replace belt.They fit behind the waist rings ,are accessible even with stages and hold 10lbs apiece.

[devjr]

Chris, one argument for the light aluminum setup centers on the wing itself. You could probably get by with the smallest possible air bag. That's not a bad thing; these harness, plate, wing, keel, bungee, weight setups look rather heavy and busy to begin with.

One issue might relate to surface riding. You don't want your face forced into the water. However, I'm not sure with which plate the advantage lies. The aluminum tank might jerk the butt up and the plate may force the chest down. Who knows? We probably need a "Dr. van Halcyon" on this board for all the controversy these gadgets cause.

[scubabunny]

I'm guessing that all the questions about aluminum 80 tanks deal with standard 80s that get 5lbs positive when empty. What about Neutral 80s or Compact 80s that are both neutral at 500psi? Using no wetsuit or .5mm?

Will someone still be able to get to the surface if they had a bladder failure? Or...do they just "swim like h*ll"?

[VTWarrenG]

I never dive without at least a minimal exposure suit, even in warm water. I always have to wear at least some weight on my weightbelt. I always use a stainless-steel plate to let me use a little less weight on the belt. I have no problems with keeping my head up on the surface -- and no one I know who dives a plate regularly has a problem, either. It's a myth.

Go for the SS.

- Warren

[devjr]

Halcyon apparently doesn't regard the surface problem as a myth. On their website they discuss options to deal with it.

http://www.halcyon.net/mc/weights.shtml