How to Calculate a Scuba Weight Belt

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Unless you want to bob around at the water’s surface like a cork, you're going to need a weight belt to get down to that reef teeming with tropical fish. Neutral buoyancy lets you get up close with coral, skates, turtles and clownfish, but it takes some experimenting to get it right when you're first starting to scuba dive. Buoyancy calculations should be part of prep for each dive as you trade off gear, move from a pool to open water, suit up for the cold or dive in your bathing suit.

Buoyancy and Ballast

  • You are naturally buoyant -- the more fat cells you have in relation to your denser muscle and bone, the easier it is to float. In addition, your scuba gear has its own degree of buoyancy. A wet suit traps air in its foam; a buoyancy compensation vest usually retains a residue of air even when deflated; and your tank starts out heavier when full of compressed purified air, but is more buoyant as you breathe it toward empty. Just holding your breath traps air in your lungs like a balloon. You need some ballast to slip below the surface and cruise over a reef or a wreck. But not too much or you'll bounce along the bottom, damaging the marine environment you meant to admire, expending unnecessary energy and using up precious air to hover and swim smoothly.

Find Your Neutral Weight

  • Your compensatory ballast changes with the changing circumstances of each dive. but you can calculate a basic weight load and estimate to near accuracy for the variables. First, get into a pool wearing just a bathing suit, fins and snorkel, and test drive some weights. Grasp a weight belt with some weights on it, take a half breath, hold it, cross your ankles and keep your hands still to avoid creating upward propulsion. If you hang in the water without rising or sinking, you are at neutral buoyancy. Exhale. If you start to sink, you've got it right. Adjust the weights to find your ideal body-weight ballast. Now add your wet suit and reconfigure the weights. The difference between body weight and wet-suit weight is what you need to add when you suit up. PADI, the Professional Association of Diving Instructors, estimates that a 150-pound diver in a one-piece full wet suit needs around 7.5 pounds of lead. But each person is different, so start with an estimate and refine.

Don't Pop to the Top

  • Add the rest of your gear to the calculation. Let all the air out of your buoyancy compensator, submerge it and place individual lead weights on it, one at a time, until it doesn't bob to the surface. When you dive with a BC vest, you'll need to add that much extra weight to your weight belt or the pockets of the vest. Tanks differ from dive boat to dive boat, and you may use your own or the charter's tanks. All tanks are heavier when they are full, but steel tanks stay heavy enough to provide negative compensation -- they add weight -- and aluminum tanks become positively buoyant as they empty. "SCUBA" magazine estimates that a standard aluminum tank can be almost 4 pounds lighter towards the end of a dive. If you're doing a controlled ascent from a deep dive with an aluminum tank, you'll want that extra 4 pounds or so on your weight belt so you don't rise too quickly.

Putting on the Pounds

  • There are several ways to add weights, configured to your body weight, gear and dive conditions. The common solution to balance buoyancy is a wide, nylon webbing belt with a secure, stainless steel quick-release buckle. Lead weights -- plastic-coated or bare -- are threaded onto and removed from the belt to control buoyancy. Some belts have pockets that flip open to make weight adjustment simple, even underwater. Weight belts can be unbuckled with one move and dropped in an emergency. Buoyancy compensators and body suits may come with accessible pockets for removable weights. Buoyancy is an imprecise science -- you should aim to get it right within a few pounds, knowing that you'll be a bit lighter toward the end of the dive. And you should definitely recalculate for fresh or saltwater dives. If you estimated neutral buoyancy in a fresh-water pool, "SCUBA" magazine recommends you multiply your ballast weight by .025 to account for greater buoyancy in salt water.

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