Module 7 - IAFF

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IAFF Training for Hazardous Materials: Technician© Student Text properly, positive pressure inside the mask unit forces air to be expelled, preventing contaminants from entering. However, the positive pressure effect is not 100% failsafe, and contaminants do occasionally enter the mask. Over the past years a number of improvements and modifications have been made to positive pressure SCBA: • Cylinders of air are now available in 30 minute, 45 minute and 60 minute durations. • Weight (with 30 minute cylinders), has been reduced to about 20 pounds. • Backplates, harnesses, facepieces and controls are more ergonomic (user friendly). • Flow rates are more consistent with user demands. Advantages to Positive Pressure SCBA • Most responders are proficient in its use • Readily available • Provides highest level of protection against airborne contaminants and oxygen deficiency Limitations to Positive Pressure SCBA • Bulky, heavy • Limited air supply limits work duration (this may actually be an advantage) • May impair movement in confined spaces • Unknown resistance to chemicals Closed Circuit Self Contained Breathing Apparatus (Rebreathers) Closed Circuit Respirators, sometimes known as “rebreathers” operate on the principle of rebreathing the air inside the breathing circuit of the SCBA. By rebreathing the air, the duration of the SCBA can be significantly extended. This differs from the common “open circuit” SCBA in the fire service, which expels exhaled air through the exhalation valve in the facepiece. A closed circuit respirator works as follows. When a cylinder valve is opened, 100% oxygen flows from the cylinder) through a pressure reducer to fill the breathing circuit. As the wearer inhales, the oxygen is drawn through 7-14 Module 7: Personal Protective Equipment
Student Text IAFF Training for Hazardous Materials: Technician© the inhalation hose and valve and hose. The exhaled air, containing carbon dioxide, flows through a chemical “scrubber”. The scrubber converts the carbon dioxide into water. This chemical reaction also generates heat in the process. The “scrubber” breathing air flows to the breathing bag and past a cooler. A constant dosage outlet continually adds oxygen to breathing air to replenish the oxygen which is consumed by the wearer. New oxygen is usually supplied via a cylinder of pure, compressed O 2 , although experiments with liquid oxygen (LOX) have also been done. The cooler and the constant dosage help to cool the warm breathing air from the scrubber. For peak demands of breathing air from strenuous work, a valve opens and fills the breathing circuit with additional oxygen. Any excess breathing air in the circuit is expelled through a relief valve. As can be seen, this method of operation is quite different from that of most fire service SCBA. “Rebreather” technology has progressed a long way from its beginnings (in the 1930s-40s), using state of the art manufacturing, metering and computer chip technology. To comply with NIOSH and) OSHA laws, these units are now positive pressure, just like “normal” SCBA. Some fire departments have started to use closed circuit SCBAs in specialized operations, such as tunnel rescue and fires, where travel to the actual emergency site may well exceed even 60 minute duration cylinders. Use of these systems in chemical protective clothing (CPC) is largely unknown. Extended duration of working time in CPC is generally avoided because of the physical and psychological stresses of working in CPC. Also, the fact that these systems do not expel air and therefore would not “inflate” a “Level A” suit would seem a disadvantage (The positive pressure inside the suit acts as a safety factor.) Any department considering the use of closed circuit units in conjunction with CPC should contact and discuss the procedures with the manufacturers of both the CPC and closed circuit respirators. Module 7: Personal Protective Equipment 7-15
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Module 7 - IAFF

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