A Self-Contained Breathing Apparatus is a completely portable unit that supplies the wearer with oxygen (either pure or in air) independent of the surrounding atmosphere. This allows personnel to survive in atmospheres that, due to the presence of poisonous gases or a reduced level of oxygen, are unsafe to breathe, ie irrespirable.
The basic functions of SCBA are to:
SCBA must be strong but light-weight, comfortable to wear and very reliable. They must have easily operated valves that wont catch on projections when worn in confined spaces. There must be minimal resistance to breathing, and no undue heat build-up.
SCBA can be divided into two main categories:
Both of these systems can have either Positive Pressure or Negative Pressure units.
In positive pressure units, the demand valve operates when the pressure inside the facemask is reduced to a point that is above atmospheric pressure.
In negative pressure units, the demand valve operates when the pressure is reduced to a point that is below atmospheric pressure.
In the event of a leak in the apparatus, positive pressure units are less likely to have the supply contaminated by the outside atmosphere than negative pressure units and there is less resistance to breathing, however they don't utilise the supply as economically and they are more likely to leak oxygen to a fire or other chemical reaction.
OPEN CIRCUIT SCBA
Air is supplied to the wearer at a breathable pressure by the demand valve when he inhales. When he exhales, the flow of air stops, and the exhalation valve in the facemask opens, allowing the exhaled air to be discharged to the atmosphere.
As air contains 78% nitrogen and 1% other gasses, only 21% of the volume of the cylinder is the oxygen that is needed for respiration, and of the oxygen that is inhaled, about three quarters is exhaled. In other words, in an open circuit SCBA, only 5% of the volume of the cylinder is actually utilised.
As a result, even though they have a large cylinder, open circuit SCBA have a fairly short working duration. Also, as the rate of air consumption is dependent on factors such as fitness, nervousness and level of exertion, the duration varies substantially between wearers, and there is no procedure whereby the duration can be greatly extended should the person wearing the unit become trapped.
Open circuit SCBA have certain features that make them ideal in some situations - they are simple to operate, quick to put on and the air is supplied at a constant temperature.
Schematic Diagram of a Two Stage Open Circuit SCBA
CLOSED CIRCUIT SCBA
The closed circuit principle is only suitable with oxygen. The oxygen is breathed in from a flexible reservoir, and the exhaled air, which is approximately 96% oxygen and 4% carbon dioxide, is passed through a substance that absorbs carbon dioxide before returning to the reservoir. In compressed oxygen sets, a constant dosage of oxygen is fed into the system to supplement the oxygen that is used by the body, and should this be insufficient, an extra supply is injected by means of a pressure sensitive valve. These sets also have a relief valve to prevent pressure building up in the set if the constant dosage is not fully used, but generally, very little of the oxygen supplied is wasted. Therefore, these sets have a specific duration. Also, because the cylinder only contains the oxygen that is needed, a small cylinder gives a long duration, and if necessary, there is a procedure which can extend the duration to up to sixteen hours should the rescuers become trapped.
Closed circuit SCBA do however have disadvantages when compared to open circuit sets - they are complicated to use, the absorption of carbon dioxide produces heat and they require a lengthy start-up procedure to check all of the functions of the set, and to eliminate the normal air from the system.
If air was left in the set, and all of the oxygen in the reservoir was used, only pure nitrogen would remain, which if breathed causes an instant blackout.
Closed circuit SCBA can also use liquid oxygen or chemically generated oxygen. Each of these systems has their own merits, however compressed oxygen sets are the most suitable for Mines Rescue.
Schematic Diagram of a Closed Circuit SCBA
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