LITTON LITPAC II

The LITPAC II (figures 12-13) is a positive-pressure,compressed-gas (39% O2), closed-circuit apparatus with a refillable, CO2-absorbent canister. It was certified by NIOSH in 1994 as a 2-hr, positive-pressure apparatus. Its targeted market is mainly the fire service and other users who requirepositive pressure even at high work rates. These users desire durations longer than open-circuit apparatus can provide, but do not want high O2 levels in the breathing circuit. This apparatus was designed to pass the anticipated National Fire Protection Association 1984 standard for closed-circuit apparatus. The two gas cylinders contain 1,036 L ATPD of gas when filled to 4,500 psi. The LITPAC II is a demand-only apparatus with approximately 20% of each breath coming from the cylinders through a positive-pressure-biased demand valve regulator. The same portion of exhaled air is vented out of the circuit on every exhalation through a pressure-biased exhalation/relief valve. This strategy spares the CO2-absorbent to some extent and provides cooler breathing air, but requires a large quantity of supply gas. If one considers that open-circuit apparatus draw 100% of each breath from the gas supply versus approximately 4% for typical closed-circuit apparatus, this apparatus can be seen as a cross between open- and closedcircuit. The user exhales into the face mask, through the exhalation breathing hose, into an exhalation/relief valve assembly containing a check valve and a relief valve, both of which are spring-loaded. The exhaled air then either exits the circuit via the relief valve or enters the scrubber inlet bag. The air that enters the scrubber inlet bag then passes through the CO2- absorbent canister into a scrubber outlet bag, then into the pressure vessel bag, which encases the two compressed-gas cylinders. A small fourth bag made of thicker material, functioning more as a duct, connects the pressure vessel bag with the regulator. Inhalation draws air from the breathing bags and the gas cylinders. The gas from the cylinders enters the regulator through an injector nozzle, creating negative pressure in the regulator, which draws air in from the breathing bags, functioning as a sort of power-assist. This combination  gascylinder/ breathing-bag air is then drawn through the inhalation breathing hose and back into the face mask.