Airflow System

At this point, you should know and understand the use of each component of the

OBA. Figure 6-10 shows an OBA with a canister installed. The arrows indicate the airflow through the OBA.

As you exhale, moist breath passes through the exhalation tube (7), through the valve housing to the bottom of the canister (6), and upward through the chemical. The carbon dioxide is absorbed, and the moisture present reacts with the chemical to give off oxygen. This oxygen passes into a breathing bag (4) (part of the breastplate group) from which the inhalation tube (3) allows the breathable mixture to be drawn into the facepiece (1) by your normal intake of breath.

Check valves (2 and 8) are used in the inhalation and exhalation passages. An automatically operated pressure-relief valve (5) in the breathing bag relieves excess pressure in the breathing bag. The speaking diaphragm, as described earlier, is built into the facepiece.

Figure 6-10.-Airflow system.

OBA Operating Procedures

Damage Controlmen use the OBA on a regular basis for fire-fighting and training purposes. It will protect you; however, improper use of the OBA could make you a personnel casualty. The following topics discuss the type A-4 OBA operating procedures. It is very important that you learn to use them properly. You should practice the operating procedures under the supervision of a leading petty officer who is qualified in the use of the OBA. Do not use the green quick-starting canister for training; check with the DCA about using a red training canister.

DONNING THE OBA.- When donning the OBA, do so in the following order:

1. With one hand, grasp the facepiece at the combination valve housing and the apparatus at the operating handle. With the other hand, grasp the straps of the body harness and the body pad D ring. Bring the pad and harness over your head while positioning the OBA on your chest. See figure 6-11.

2. Find the two straps hanging free in back (view A, fig. 6-12). Attach the end of

Figure 6-11.-Step 1 of donning the OBA.

Figure 6-12.-Step 2 of donning the OBA.

each strap to the ring on each side of the breastplate (view B, fig. 6-12).

3. Position the breastplate on your chest so that the breathing tube connections are slightly below your shoulders. Your head movement should not be restricted when you don the facepiece. While holding the apparatus in this position, adjust the two underarm straps and then adjust the two shoulder straps (fig. 13)

Figure 6-13.-Step 3 of donning the OBA.

until the apparatus fits comfortably. The harness pad should be located in the center of your back down from the neck for a comfortable fit.

4. Place the facepiece over your head in the standby position (fig. 6-14) until you are ready to activate the canister.

5. Snap the waist strap to the bracket on the lower side corner of the breastplate

Figure 6-14.-Step 4 of donning the OBA. 6-7

Figure 6-15.-Step 5 of donning the OBA.

(fig. 6-15) and adjust to hold the apparatus snugly to your body. To secure the excess loop of the waist strap, wrap it under the secured part of the strap. Secure the ends of the lower body harness straps under the waist strap if they extend down to your waist after being adjusted.

6. Install the canister in the following manner:

a. Remove the tear-off cap of the canister by pulling the tab backward and downward to expose the copper-foil seal (fig. 6-16). Discard the cap.

b. Remove the canister candle cover by rotating the swivel plate 1800. Push it down toward the center of the canister, as shown in view A of figure 6-17. Leave the cover dangling by the lanyard, as shown in view B of figure 617. When you remove the candle cover, do NOT

pull the lanyard so that the cotter pin is removed. The removal of the cotter pin fires the candle, and the canister starts generating oxygen. If this happens while the copper-foil seal is intact, internal pressure in the canister will build up. This pressure will cause the copper foil seal or the canister seam to rupture.

c. Hold the canister with the neck up and the concave, or ribbed side, toward your body. Insert the canister upward into the guard and breastplate assembly (fig. 6-18) until the canister is firmly in place. The canister is now locked in a standby position with the copper-foil seal still intact. If the copper-foil seal is pierced when the canister is placed in the standby position, the standby stop will need to be adjusted. An OBA that pierces the copper-foil seal in the standby position is NOT to be used until the adjustments are made.

7. Don and adjust the facepiece as follows:

a. Place the head harness straps over the front of the facepiece.

b. Insert your chin into the chin stop of the facepiece (view A of fig. 6-19).

c. Pull the head straps from the front of the facepiece over your head (view B of fig. 619). Be sure your hair is not under the facepiece shield.

d. Make sure the straps lie flat against your head.

e. Tighten the lower straps (neck straps) first.

f. Tighten the side straps.

g. Place both hands on the head harness pad (on the back of your head) and push it down toward your neck.

h. Repeat steps e and f.

Figure 6-16.-Step 6 removing tear-off cap.

Figure 6-17.-Removing the candle cover, views A and B.

i. Tighten the forehead or front strap, if needed.

8. Test the facepiece for a good seal by squeezing the corrugated breathing tubes tightly to prevent the passage of air (fig.

Figure ^{6-18.-Inserting the canister.}

Figure 6-19.-Donning the facepiece, views A and B.

6-20). Inhale gently so the facepiece collapses slightly, and hold your breath for 10 seconds. The facepiece will remain collapsed while your breath is held if the assembly is gas tight. If you detect any leakage around the face seal, readjust your head harness straps. If you detect other than face-seal leakage, investigate the condition and correct it. You MUST test the facepiece for a seal before each use.

9. Make final adjustments on all four body harness straps. You should be able to look up or down without having the facepiece shift or catch on the timer or the main valve housing.

10. If you are going into a standby or ready condition, loosen only the lower facepiece straps and then remove the facepiece. Place the facepiece over your head and out of the way until ready to start the canister and put the OBA into operation. In an emergency, eliminate this step.