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DRÄGER PA93

 

The PA93 is a two stage open circuit SCBA. Older models were switchable between positive pressure and negative pressure operation, whereas the current model automatically operates on positive pressure mode once the first breath is taken.

The PA93 has an overall mass of approximately 16 kilograms and carries an alloy steel cylinder with a water capacity of 6 litres that can be pressurised to 300 bar.

Cylinder Capacity = 6l x 300 bar = 1800l

Using 40 litres per minute as the average person's working air consumption:

Full Duration = 1800l ÷ 40l/min = 45 minutes

Applying the 10 minute safety margin:

Working Duration = 45 min - 10 min = 35 minutes

The cylinder is secured to the carrying plate by a strap and the cylinder connection. The cylinder connection forms part of the unit that contains the pressure reducer (which reduces the pressure from the cylinder to 6 bar), the pressure gauge take-off and the low pressure warning whistle (which operates when the cylinder pressure drops to 50 bar). The warning whistle can be adjusted to sound at 68 bar to indicate 10 minutes supply left.

The medium pressure line feeds the demand valve from the pressure reducer, at which point there is a relief valve that will open at 10 bar if there is a malfunction with the pressure reducer.

The demand valve on switchable models had the control for selecting either positive or negative pressure mode. First-breath models have a button for selecting negative pressure mode. It is onto the demand valve that the facemask attaches.

The facemask contains the exhalation valve and speech diaphragm, and an inner mask that covers the mouth and nose (ori nasal), which directs air across the visor to eliminate misting and dead space. The exhalation valve vents at 5 millibar and, in positive pressure mode, the demand valve opens when the pressure inside the facemask drops to 3.5 millibar.


High Pressure Leak Test

With the facemask disconnected and negative pressure mode selected, open the cylinder valve a few turns. Listen for air leaks and note the cylinder pressure - if it is less than 250 bar (5/6ths full) replace the cylinder. Close the cylinder valve and watch the pressure gauge, the pressure should not fall by more than 10 bar in one minute.

Next, hold the palm of one hand against the outlet of the demand valve and set the control to positive pressure mode (on first-breath sets, press the centre of the demand valve so that it clicks into positive pressure mode). Ease your palm off the outlet to slowly release the pressure (this procedure should also be followed when closing down the set) - note the pressure at which the low pressure warning sounds.

This test should be performed each time a fresh cylinder is attached to the set, and before the set is stored ready for use.


Donning The Set

Attach the facemask to the demand valve and check that the set is in negative pressure mode. Open the cylinder valve a few turns and check that the cylinder pressure is adequate. Stand the set up and slip the right arm through the right hand harness strap then pick the set up, allowing the harness strap to slide down to the shoulder. Slip the left arm through its strap then adjust the straps so that the set sits high on the back. Do up and adjust the waist strap and slip the neck strap of the facemask over the head. Slip the facemask straps over the head, ensuring that no hair is inside the mask, and inhale to check that the demand valve operates. Tighten the facemask straps evenly, starting from the bottom.


Low Pressure Leak Test

Close the cylinder valve - do not remove your hand from the cylinder valve while it is closed. Slowly breathe to release the pressure, noting the pressure at which the warning whistle operates. Once the pressure is released, inhale to check the facemask seal, then open the cylinder valve fully (and back half a turn) and (with switchable sets) select positive pressure mode.

This test should be performed each time the set is donned.


Team Procedures

Before responding to an emergency requiring the use of PA93s, an entry point must be chosen and made identifiable. The entry point must be as close as possible to the scene of the emergency, without being at risk of being reached by air contaminated by the cause of the emergency. The area at risk should be cordoned off.

When entering an irrespirable atmosphere with PA93s, there must be a minimum of two rescuers together (the Entry Party), with an Entry Control Officer (ECO) and a Back-up Party of two rescuers at the Entry Point, ie the minimum size team is five.

After the team has been briefed, the entry party will don their sets (assisted by the back-up party), and perform the low pressure leak test. If there is any chance that the rescuers will be out of sight of the ECO, or be more than a minute’s walk to fresh air, the full entry procedure must be used. For this, rescuers will unhook the ‘in tags’ from their sets and hand them to the ECO. As he receives them, the ECO will attach each rescuer's tag to the entry control board and record on it the rescuer's name and the name of the party that the rescuer is in (eg ‘Entry’).

Once all members of the party are ready and have handed in their ‘in tags’, the ECO will record the cylinder pressures of each rescuer on their ‘in tags’ and write the current time in the ‘start time’ box for that party. The ECO will calculate the ‘return time’ for the party by adding the working duration of the set with the lowest cylinder pressure to the start time. He will write the return time in the return time box as well as on one ‘out tag’ for each rescuer. Once all members of the party have attached an ‘out tag’ to their sets, the party can depart from the entry point.

After having helped the entry party don their sets, the back-up party will don their sets and perform the low pressure leak test (without selecting positive pressure mode on switchable sets). Then, they will disconnect the demand valves from their facemasks (with first-breath sets it will be necessary to hold your breath and select negative pressure mode before this is done), hand their ‘in tags’ to the ECO, and wait. The ECO will record their names, party name and cylinder pressures in readiness.

Should the back-up party be needed, they only need to attach the demand valve to their facemasks, select positive pressure mode (if switchable) and receive their ‘out tags’ from the ECO (the ECO will record the time that the demand valves were attached for calculating the return time).

If a party is advancing into an area of poor visibility, the means of marking the route must be either a guide line or a fire hose. When a guide line is used, it is payed out from the entry point and each member of the party should be attached to it by a personal line. Once the furthest point of travel has been reached, the guide line should be secured.

To avoid injury from obstacles or openings, the rescuers should crab walk, leading with the same foot each step then bringing the back foot up to the front one. Also, one hand should be raised in front of the face, with the back of the hand forward to avoid locking onto live wiring and to minimise injury.

If the emergency involves fire and persons unaccounted for, the fire should be brought under control then a search should start, commencing at the area of the fire. Areas that have been searched should be marked to avoid re-searching, or if a wide area is to be searched, a drawing divided into grid sections should be marked to show which sections have been searched.


Closing Down

Once the party has returned to the entry point and has finished working, negative pressure mode should be selected and the facemasks removed (with first-breath sets hold your breath while this is done) with the closing down time being recorded. The ‘out tags’ should be returned to the entry control board as soon as possible to confirm that rescuers have returned.

Final pressure readings should be taken and the cylinder valves should be closed and the sets bled once removed. Once it has been calculated how individuals have compared to the 40l per minute figure, the ‘in tags’ should be returned to the sets.

Entry Control Board

Emergency Response Considerations

Self-Contained Breathing Apparatus

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Drager BG174

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