forced supplied air

01:23 Dec 7, 2010 

Roughly speaking, "forced" means here "compressed" (not to a high value, but just above the atmospheric pressure). However, what may be meant here is positive-pressure breathing apparatus.

"Positive pressure" SCBA addresses this limitation. By careful design, the device is set to maintain a small 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.
http://en.wikipedia.org/wiki/Self-contained_breathing_appara...