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A diver's buoyancy is determined by the mass of the water they (and their attached equipment) displace. Water density varies with temperature and salinity, but a good rule of thumb is that 1 cubic meter of water "weighs" about 1029 kilograms. That pencils out to about 64 pounds per cubic foot.

A typical diver displaces about 2.5 to 3 cubic feet of water. That's equivalent to about 160 to 192 pounds of sea water. Let's assume the diver displaces 2.5 cuft of water. That means that the water they displace will "support" 160 pounds. If the diver weights less than 160 pounds, she'll be positively buoyant and will float. If she weights more, she'll be negatively buoyant ad will sink. The ideal circumstance that a diver strives for is to be neutrally buoyant, so no energy is expended to keep from sinking or floating.

If our diver and her gear weigh 160 pounds, she'll be neutrally buoyant. In practice, our diver will carry enough weight to make her approximately neutrally buoyant. If she inhales air from her Scuba cylinder, her chest cavity will expand, displacing a higher equivalent weight of water, and she'll ascend. If she exhales, she'll displace less water, and will descend.

This ideal neutrally buoyant state allows a diver to ascend and descend with minimal effort. In practice, however, things are constantly changing during a dive. Wetsuits compress with depth, meaning the deeper our diver goes, the less water she'll displace. So as she descends, she'll have a tendency to descend more quickly.

Her scuba cylinder displaces a constant amount of water, but the more air is used, the less it will weigh. This means that our diver will have a tendency to become more positively buoyant... she'll "float" more... as the dive progresses.

In order to adjust for all of these variables, divers wear a "Buoyancy Compensation Device" (BCD). This is generally a vest shaped bladder that can be inflated using air from the scuba cylinder. It is fitted with a dump valve that allows fine tuning of the amount of inflation, and thus the amount of water it displaces.

During a dive, our diver will adjust the amount of air in her BCD to maintain neutral buoyancy. Then she'll use her breathing to make fine adjustments in her vertical position.

Divers diving in colder environments will often use a dry suit in place of the wet suit worn by most recreational divers. This dry suit is filled with variable amounts of air from the scuba cylinder, much the same way a BCD is. A dry suit can be used alone, or together with a BCD, to control buoyancy.

In all cases, a diver will use a depth gauge, or a dive computer which measures depth, to monitor their depth throughout a dive.

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9y ago
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13y ago

In order to ascend you need to become positively buoyant. This can be done by inflating your BCD (stab jacket - the jacejt divers wear), in to your drysuit if you are using one or simply by inhaling and storing air in to your lungs.

More dramatic ways to ascend (only to be used in the right situtions) include dropping weight (i.e. removing your weight belt) or dropping your entire kit.


-Inflate BCD jacket

-Inflate dry suit

-Breathe in

-Drop weight

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13y ago

Scuba diver descends or ascends using proper buoyancy control by inflating/deflating his/her scuba BCD (Buoyancy Compensator device)

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Q: How does a scuba diver control their depth?
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