SS v. aluminum backplate - help me think this through

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I appreciate the input. I think my initial instinct to go with SS is correct. I may be able to get away with no weights for swimsuit diving, and less added for wetsuit. If I’m traveling with my dive gear that’s another checked bag anyway, so the weight shouldn’t be a big deal.
I assume you're diving with Al80 tanks, is that right? A typical steel plate pretty well compensates for the 4 lbs positive buoyancy of the empty tank, so it's almost certain that you'll need no weight if you're not wearing a wetsuit. You might even be a bit heavy depending on your body type and what else you're carrying. You asked the question about using weights on the cambands on an AL plate, and that works fine as well. To me it's not quite as good as having the ballast spread out evenly right against your back, but it's close.

I eventually switched to just an AL plate because I needed one for doubles and I was frequently diving doubles and singles on the same trip. It works fine, you'll be happy either way.
 
@Rayk,

The fact is, 80 cu ft of air/nitrox (for example) weighs approx 6.5 lbs. A diver just might need to overcome some or all of this weight at the beginning of a dive if he/she (or his/her buddy) is struggling at the surface.

rx7diver
You seem to be forgetting that the 6.5 lbs of air is contained in a tank that is 4.4 lbs positive, leaving a net negative buoyancy of a full AL tank at about 2 lbs. If you can't swim that up or maintain control at the surface, you have a problem that is not equipment-related.
 
You seem to be forgetting that the 6.5 lbs of air is contained in a tank that is 4.4 lbs positive, leaving a net negative buoyancy of a full AL tank at about 2 lbs. If you can't swim that up or maintain control at the surface, you have a problem that is not equipment-related.
@halocline,

This is how I'm thinking about this:

6.5 lbs of air weighs 6.5 lbs, correct?

Suppose a diver weights himself to be neutral with an "empty" cylinder and a completely deflated BC at 3 [sic] fsw (say). If his full cylinder contains 6.5 lbs of air at the start of his dive, then, if he is struggling at the surface, maybe by getting rid of ~6.5 lbs (of something) he or his rescuer will increase the likelihood of a positive outcome.

Does this make sense?

NOTE: I chose 80 cu ft for no special reason--except that it's in the ballpark of what many recreational divers will be using (namely, 77.4 cu ft in a Luxfer Al 80).

rx7diver
 
@halocline,

This is how I'm thinking about this:

6.5 lbs of air weighs 6.5 lbs, correct?

Suppose a diver weights himself to be neutral with an "empty" cylinder and a completely deflated BC at 3 [sic] fsw (say). If his full cylinder contains 6.5 lbs of air at the start of his dive, then, if he is struggling at the surface, maybe by getting rid of ~6.5 lbs (of something) he or his rescuer will increase the likelihood of a positive outcome.

Does this make sense?

NOTE: I chose 80 cu ft for no special reason--except that it's in the ballpark of what many recreational divers will be using (namely, 77.4 cu ft in a Luxfer Al 80).

rx7diver

I think you are looking at weight instead of buoyancy. I was under the impression AL80 full were negative 2ish.
 
I think you are looking at weight instead of buoyancy. I was under the impression AL80 full were negative 2ish.

The typical aluminum 80 is approximately (with valve) 1.4 pounds negative when full and 4.4 pounds positive when empty. Since we are generally supposed to be back on deck/shore at 500 psi then you are about 3 pounds positive (somebody else can be exact I am sure) at the end of your dive as your crawl ashore or climb the ladder. A steel 100 HP cylinder might be around negative 1 pound empty and perhaps 8 or 9 pounds negative when full. A LP steel 85 is about 0 pounds empty and around 6 or 7 pounds negative when full.

Oh heck, here is a good link to pdf:


James
 
@halocline,

This is how I'm thinking about this:

6.5 lbs of air weighs 6.5 lbs, correct?

Suppose a diver weights himself to be neutral with an "empty" cylinder and a completely deflated BC at 3 [sic] fsw (say). If his full cylinder contains 6.5 lbs of air at the start of his dive, then, if he is struggling at the surface, maybe by getting rid of ~6.5 lbs (of something) he or his rescuer will increase the likelihood of a positive outcome.

Does this make sense?

NOTE: I chose 80 cu ft for no special reason--except that it's in the ballpark of what many recreational divers will be using (namely, 77.4 cu ft in a Luxfer Al 80).

rx7diver
Yes, I understand that, but nobody uses 6.5 lbs of air diving with an AL80. First, an AL80 holds 77 cft, which weighs roughly 6.2 lbs, not 6.5. Second, people do weight checks to be slightly positive on the surface (water surface at eye level) with 500 PSI. (If they are following the generally accepted recreational training guidelines as I remember them) This brings the weight of the air compensated for to around 5.2 lbs, and keep in mind that divers are slightly positive, maybe about 3 lbs, I suspect that's low, but just for arguments sake. The amount positive is equal to the weight of the portion of the diver's head that remains above the surface during the weight check.

So that means that a diver properly weighted using that standard check, with a full AL80, will actually be around 2 lbs negative on the surface, not anywhere near 6.5 lbs.

I've been diving a long time, most of it in warm water with AL tanks; single tanks, manifolded doubles, and sidemount. I have a lot of experience dialing in my own buoyancy and helping others do so in those scenarios. Most recreational single tank divers are carrying more weight than they need. There are a few reasons for that which we don't need to go into in this post. But my point is that overweighted divers might need ditchable weight on the surface, and technical divers using much larger volumes of air and/or heavy gear certainly benefit from it, but single tank warm water recreational divers don't, as long as they are properly weighted and reasonably competent. Don't forget, we're talking about divers that are using BCs with 25-40 lbs of lift. BC failure at the beginning of a dive? Whose fault is that....part of a pre dive check is making sure your BC works.

Actually, I have been guilty, more than once, of jumping into the water with my tank valve off. I used be a DM and from time to time I would embarrass myself by carefully checking everyone else's tank and forgetting about my own. In that case, I did have a full tank on the surface with no functioning BC. But, since I can swim a few lbs to the surface and maintain positive buoyancy, and I can reach my valve, those scenarios were a little embarrassing but not dangerous.

Sorry for this long post, but it is an interesting topic.
 
The whole point is to be able to deal with a failed BC, a tank that is off, a buddy who might be wearing way too much non-ditchable lead (who you have to rescue), very rough conditions with waves breaking over your head and adverse currents, waves crashing onto big rocks that presents as the only feasible exit, a tank that has been run down to zero, jumping in with no fins and the tank off, etc. etc.

Being able to ditch ballast is a good idea for anyone who can't breathe water - We shouldn't have to run through every possible (stupid) scenario to remind ourselves of this fact. I don't think I have ever had to drop my own lead, however I prefer to have that option as a plan c or d and when I am diving with no ditchable lead (say a heavy steel tank and little exposure protection), it always makes me a little nervous and I have to remind myself that one of "my usual options" isn't gonna be there if I need it.
 
https://www.shearwater.com/products/perdix-ai/

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