SCBA - Self Contained Breathing Apparatus 5
SCBA - Self Contained Breathing Apparatus 5 : Positive Pressure (Preventing Inward Leaking): Open circuit SCBAs utilize either "positive pressure" or "negative pressure" operation. 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. Air is delivered to the wearer when he breathes in, or in other words, reduces the pressure in the mask to less than outside pressure, hence the name "negative pressure". The limitations of this are obvious, as any leaks in the device or the interface between the mask and the face of the wearer (caused for example by small face skin wrinkles) would reduce the protection offered". Positive pressure" SCBA addresses this limitation. By careful design, the device is set to maintain a small pressure in excess of the surrounding air pressure inside the facepiece. Although the pressure drops when the wearer breathes in, the device always maintains a higher pressure inside the mask than outside of the mask. Thus, even if the mask leaks slightly, there is a flow of clean air out of the device, automatically preventing inward leakage under most circumstances. Although the performance of both types of SCBA may be similar under optimum conditions, this "fail safe" behaviour makes a "Positive pressure" SCBA preferable for most applications. As there is usually no air usage penalty in providing positive pressure, the older "Negative pressure" type is, in most cases, an obsolete configuration and is only seen with older equipment. However some users refuse to use this technology as in case of a damage or loss of the facepiece the air will be released uncontrolled. The leakage rate can be so high that a fully charged SCBA will be drained in less than three minutes, a problem that does not happen with "negative pressure" SCBA systems
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