Mouthpiece (scuba)
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In breathing sets, a mouthpiece is a part that the user grips in his mouth, to make a watertight seal between the breathing set and his mouth. It is composed of a short flattened-oval tube that goes in between the lips, with on its free end a flange that fits between the lips and the tooth and gums. On the flange there are two projections with enlarged ends, which are gripped between the teeth. Most sport diving scuba sets use a mouthpiece rather than a fullface mask.
In many industrial breathing sets and a few scuba sets (e.g. the Siebe Gorman Salvus, and a 1980's model of twin-hose diving regulator made by Nemrod), the mouthpiece also has an outer rubber flange that fits outside the lips and extends into two straps that fasten together behind the neck. That makes the mouthpiece more airtight and watertight, and helps to keep the mouthpiece in if the user lets his lips or jaws get slack through unconsciousness or absent-mindedness or being preoccupied with work.
A mouthpiece in the mouth prevents clear speech; as a result, many work divers, and many people who use a breathing set on land (e.g. firefighters), use a fullface mask.
[edit] In breathing sets with two wide hoses
In a loop of wide breathing tubes like in a twin-hose aqualung or some rebreathers, the word "mouthpiece" is sometimes taken to mean the whole of the rigid assembly between the two flexible tubes.
In the 1960's a mouthpiece assembly of this sort (for a twin-hose aqualung), called a "Godel mouthpiece" was made, which had a snorkel attached, and a valve so the diver could breathe from the aqualung or from the snorkel.
