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Backscatter
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When suspended particles in the water, such as sand, are illuminated by light from a flash, they reflect the light back to the lens. The particles appear as specks or snow in the photo.
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Backward Roll Entry
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means of entering the water in SCUBA gear from a sitting position such as from the gunnel of a boat whereby the diver, while securely holding his mask, leans backward and rolls into the water onto his tank and shoulders. Checking for an all clear is recommended.
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Bailout
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training technique used in some SCUBA classes wherein the student jumps into the pool while holding all equipment in hand and then dons the equipment on the bottom of the pool; or, pertaining to or consisting of a means for relieving an emergency situation.
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Barometric pressure
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Same as atmospheric pressure with the exception that it varies with the weather.
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Barotraumas
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Any disease or injury due to unequal pressures between a space inside the body and the ambient pressure, or between two spaces within the body; examples include arterial gas embolism and pneumothorax.
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BC or BCD
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See buoyancy compensator.
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Bends
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A form of decompression sickness caused by dissolved nitrogen leaving the tissues too quickly on ascent; is manifestation of decompression sickness.
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Bladder
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A pouch within a Buoyancy Compensator which holds the amount of air the diver desires to provide proper buoyancy.
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Body suit
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Garment that provides full length abrasion protection.
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Bootie
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A piece of foot protection, usually made of neoprene, worn inside an open-heeled fin; serves to protect the diver's feet while walking to and from the dive site and prevents blisters from the fins while swimming; also provides warmth, depending on thickness. May come in a varying sole thickness.
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Bottom time
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The time between descending below the surface to the beginning of ascent.
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Bow
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The front end of a boat.
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Boyle's law
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At a fixed temperature for a fixed mass of gas, pressure times volume is a constant value.
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Breath-hold diving
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Diving without life support apparatus, while holding one's breath.
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BTU
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Britsh Thermal Units or calories; measurment of heat.
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Bubble
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A collection of air or gas surrounded by a permeable membrane through which gases can enter or exit.
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Buddy
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Diving partner.
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Buddy Breathing
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Sharing of the same air supply by two or more divers; an emergency technique used when one person's air supply is exhausted or unavailable due to equipment malfunction.
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Buoyancy
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The upward force exerted on an object in liquid, whether the object sinks or floats. Objects that float are positively buoyant, those that sink are negatively buoyant and those that stay where placed are neutrally buoyant.
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Buoyancy compensator
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An inflatable vest worn by the diver that can be automatically or orally inflated to help control buoyancy; abbreviated BC or BCD (Buoyancy Control Device).
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Burst disk
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Thin copper disk held in place with a vented plug. Designed to rupture if tank pressure is greatly exceeded.
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