backplates shift center of gravity NOT reduce weighting

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Posted by Karl S. on October 31, 2001 at 14:41:08:

In Reply to: Re: Don't forget the tank posted by Kendall Raine on October 31, 2001 at 11:36:24:

a backplate will help you lie horizontally prone underwater. it does this by shifting your center of gravity closer to your spine and more towards your upper chest. its the same principle as integrated B/C weights, only better. [by the way, you can accomplish the same thing by mounting two small weight pouches on the upper extremes of your shoulder straps of your B/C and putting bag weights into them. works like a charm, and you dont have to go out and buy a backplate!]

a backplate has nothing to do with dropping weight from your weight belt or weight pouches, other than the shift that occurs due to the negative buoyancy of the backplate and its positioning. the backplate weighs X amount of lbs, normally 6, and has Y amount of negative buoyancy, normally 5.5 lbs or so.

If you are dropping more weight than 5.5 lbs because of a steel backplate and a new backpack, it is because you were overweighted before, or you had trapped air in your B/C, or else it is because you are now "cheating" and using your tank weight plus your full air weight in your tanks plus the weight of your other gear to give you only slightly negative buoyancy at the surface at the start of your dive.

if you ever want to know how much weight you need to make yourself neutrally buoyant without your gear, put on your wetsuit or drysuit and fins, mask, snorkel and go into the Ocean with your weights in a dive float and with a pouch-style weight belt. then slowly add more weight to the pouches until you float at eye level with a full chest of air, and sink when you exhale [this is where the fins come in, to get you back to the surface so you can breathe in again.]

you can do this at a pool too, only you wont be right on for the Ocean.

steel tanks are better than aluminum for your primary air supply because they stay negative or neutrally buoyant as they drain. A plastic backplate or lite aluminum one works best with steel tanks from a safety point of view, if you absolutely must have a backplate, either because your friends all have them, or you belong to a group of divers that requires them of their participants [read: D.I.R.]

a steel backplate works best with aluminum tanks because the weight of the plate offsets the positive buoyancy of the aluminum tank as it empties, if you are warm water diving. the whole rig in warm water can sink you easily at the beginning of the dive, and as your air gets consumed, then you become almost exactly neutrally buoyant from the combination backplate and aluminum tank. then you dont need any lead weight. the same thing is true of a steel tank and plastic or aluminum backplate.

a steel backplate with steel tanks is a dangerous concoction that spells negative buoyancy for cold water diving which you cannot ditch if you needed to, unless you have a bail-out pony bottle slung and you are prepared to ditch all your gear, backplate, tanks, backpack, all of it.

a slung aluminum pony bottle is always a good idea and insurance against anything that could go wrong. just remember when you are breathing-in on your pony for whatever reason, you need to be breathing-out into your B/C oral inflator. if youre using a pony, that means your tanks are out of air, and that your power inflator doesnt work anymore.

there are lots of crazy East Coast diving ideas proliferating on the West Coast these days [read: D.I.R.]

the DIRs are adjusting their own buoyancy at 20 fsw to be able to hold a 20 foot stop, where they can breathe pure O2 for decompression. thats why they can shed so much weight. they are not weighting themselves for neutral buoyancy at the end of a dive at the surface with an almost empty tank. Im not saying there is anything wrong with that, its just different.

its got nothing to do with the backplate.

/s/ Karl S.

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