SCBA - Self Contained Breathing Apparatus 4
SCBA - Self Contained Breathing Apparatus 4 : Fullface Masks: The fullface masks of breathing apparatus designed for use out of water are sometimes designed in a way that makes them unsuitable for scuba diving, although some may allow emergency very shallow submersion: (a) The seal at the edge of the mask is a wide tube with thin, flexible walls running around the edge of the mask, full of air at atmospheric pressure. On the surface it pushes against the edges of the wearer's face, causing a tight seal despite small variations in head shape. At more than a few feet depth pressure (underwater or in a caisson) this tube collapses, destroying the seal and making the mask leak. (b) Curved window which underwater would severely distort the image by refraction. The mask can have a big fullface window, or small eye windows. The mask might have a small orinasal breathing mask inside, reducing breathing dead space. (c) Link to image and description of a negative-pressure fullface mask. The mask can also incorporate a two-way radio communicator. Some old industrial rebreathers (e. g. , the Siebe Gorman Proto) had a mouthpiece and attached noseclip instead. (1) Positive pressure (preventing inward leaking): Open circuit SCBAs utilize either "positive pressure" or "negative pressure" operation. (a) A "negative pressure" SCBA may be used with a type of fullface mask which could be used as a gasmask (with a filter canister on the facepiece's air inlet) or with an open-circuit breathing set connected to the air inlet
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