The BG4 is a positive pressure type closed circuit SCBA. It has an overall mass of 12.3kg (without ice) and carries a 2l alloy steel cylinder that can be charged to 200 bar, giving a supply of 400l of oxygen. The cylinder valve is equipped with a safety cap that will vent at 300 bar. The cylinder couples to the pressure reducer which is fitted with a manual bypass valve in case of failure. These form the high pressure components of the set.
From the pressure reducer are the medium pressure line (yellow), the dosage line (blue) and the cable to the monitron (electronic system monitor and pressure gauge). The pressure reducer maintains a pressure of 4.5 bar in the medium pressure line to supply the lung demand (minimum) valve and feeds the constant dosage of between 1.5 and 2 litres of oxygen per minute into the dosage line. The constant dosage is normally sufficient for up to a medium work load and is fed into the cooler from where another blue line connects to the residual pressure sensor in the monitron control unit.
Should the wearer empty the breathing bag and create a sufficiently low pressure in the set (between 0.5 and 2.0 mBar) when inhaling, the pressure plate on the breathing bag will push the lung demand valve needle which causes the base of the needle to temporarily lose its seal and allow oxygen to flow into the breathing bag at approximately 80l per minute.
Should the wearer fill the breathing bag and create excess pressure in the set (between 5 and 7 mBar), the pressure plate will contact and open the pressure relief (maximum) valve and allow oxygen and carbon dioxide from the tube from the drägersorb canister to be vented to the outside air.
The facemask is connected to the set by the coupling on the breathing tubes. The inhalation tube feeds from the cooler over the right shoulder and the exhalation tube feeds to the drägersorb canister over the left shoulder. Either side of the coupling are two one way valves to maintain the correct flow direction through the set and minimise tidal rebreathing in the facemask.
The inhalation valve is in the inhalation tube connection and only allows oxygen to flow from the inhalation tube into the coupling and facemask.
The exhalation valve is in the exhalation tube connection and only allows oxygen (which at this point contains CO2) to flow from the coupling into the exhalation tube.
The drägersorb canister is filled with pelletised soda-lime (Drägersorb) which absorbs the carbon dioxide from the exhaled oxygen. The oxygen then flows into the breathing bag from where most of each breath is taken. A byproduct of the absorption process is heat.
To offset the rise in temperature in the oxygen from the heat generated in the drägersorb canister and the warmth of the exhaled breath, the set is fitted with a cooler which can be filled with a 1.2l block of ice. Oxygen from the breathing bag flows around the outside of the cooler (loosing heat into the cooler - even if there is no ice loaded) before flowing into the inhalation tube. The release of oxygen from the constant dosage and the lung demand valve also have a cooling effect on the set.
The positive pressure of approximately 3.5 mBar is created by the springs pushing against the pressure plate. Moisture which collects in the breathing bag is allowed to escape by means of the drain valve when sufficient head of pressure is created in the tube above it.
With a fully charged cylinder, the BG4 has a working duration of 4 hours, but should the rescue team become trapped, the duration can be extended to up to 16 hours by using the entrapped procedure.
SCHEMATIC DIAGRAM of the BG4
1. Oxygen Cylinder 10. Inhalation Tube 2. Pressure Reducer 11. Inhalation Valve 3. Monitron 12. Facemask Coupling 4. Constant Dosage Line 13. Exhalation Valve 5. Lung Demand Valve 14. Exhalation Tube 6. Pressure Relief Valve 15. Drägersorb Canister 7. Breathing Bag 16. Spring 8. Pressure Plate 17. Drain Valve 9. Cooler
Each time a set is reassembled, it must be properly tested before being stored, and due to the nature of closed circuit SCBA it is vitally important that an ordered startup procedure is followed each time the BG4 is worn.
The purposes of the startup procedure are to ensure that there are no faults in any sets after having been transported, no normal air is left in the sets when worn, each team member is wearing the set correctly and each team member is fit enough to take part in the operation.
Before being briefed, the captain should choose a position for the startup so that the vice-captain can have the team members line their sets up in team order on the ground. The vice-captain will position the captain's set opposite the others and prepare it for him and then prepare his own. While doing this, he should have the team check the equipment that they will be carrying and position it behind their sets.
Following a list attached to the set, the team will:
The vice-captain will fill in the team members' names, set numbers and start cylinder pressures on the record sheet ready for the captain.
After the captain has briefed the team, he will tell the team to line up and go under oxygen, upon which they will:
Once team members are ready, they will pair up (No2 with No3, No4 with No5 and captain with vice-captain) and perform the following procedure with each other.
Just prior to leaving the FAB, the captain will record the start time of the operation and a time of return agreed to with the director.
At this stage, if a back-up team is not present, an estimated time of its arrival must be confirmed.
The captain instructs the team on how they will move by using the appropriate signals with his signalling device. All of these signals are returned by the vice-captain. Another signal, which is passed down the line after being initiated by the captain, is used to check that each man is OK.
Should a danger be observed by a team member that the captain hasn't seen, the team member will use the emergency signal to alert the captain.
If a team member sees another team member collapse, he should use the emergency signal then go to his aid. This is the only time that a team member can act without waiting for instructions from the captain.
The code used is:
1 pause 2
|I'm OK, are you OK|
The team should travel in order in single file, approximately 1.2m apart. The second-last team member has the job of regularly checking that the vice-captain is O.K.
Team members will often feel like they are following aimlessly behind the captain, but they must stay alert, looking at ground conditions, listening for signals from the captain and controlling their breathing (long, slow, deep breaths).
A rescue team entering an irrespirable atmosphere underground must be comprised of a minimum of three team members, with the ideal number being six and the maximum being eight. A team of three should only perform reconnaissance work.
A team can be split if necessary, as long as each group has a minimum of three members. No team member should ever be out of sight of the rest of the team, and must never be out of communication range.
Every 20 minutes, or if the team enters smoke, the captain should halt the team, check each member's condition and record cylinder pressures (comparing consumptions).
Should one set be found to have used considerably more oxygen than the others it should be checked for faults and a decision made on whether or not to continue.
A set with low cylinder pressure can have its cylinder replaced with a fresh one while the set remains on the wearers back. The breathing bag should be topped up using the manual bypass before closing the cylinder valve and replacing the cylinder. After the cylinder valve is opened, the cylinder connection should be checked for leakage but no other start-up is required.
If at any stage the monitron indicates a fault, remove the cover, remove the springs, top up the bag, close the cylinder valve, wait for the monitron to shut down then open the cylinder valve and replace the springs. If the monitron still indicates a fault, a decision will need to be made on whether or not to continue.
The route of travel underground must be marked either by placing reflective arrows on the ground or marking arrows on the walls. Either way, the arrows are placed at intersections pointing back to FAB, and must be placed so they will easily be seen and cause no confusion, either for the team returning or for a back-up team following. To achieve this, two arrows are placed at each and every intersection, on the outside of a corner turned.
When a team back-tracks, the arrows must be picked up or crossed out (as appropriate), and the captain of a back-up team following a first team must place arrows identifying his team next to the first team's arrows.
If the team encounters a closed ventilation door, unless instructed to leave the door open, the team captain will stop the team, hold the door open (checking that there are no hazards ahead), signal the team to advance, stop the team once the vice-captain has passed through, close the door and resume his position at the front of the team.
If the team enters smoke or unstable territory, the captain should instruct the team to link up. 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.
In thick smoke, the captain will need to use a rod (such as a tamping stick) to follow a wall and will need to pay close attention to the plans, counting his paces to measure distance.
If the team is searching for missing persons, they should first head to the last known position, briefly looking up other drives as they are crossed and watching for signs that the missing persons may have evacuated their workplace. If the missing persons are not found where they were expected to be, a systematic search of the other drives should be performed on the way out.
Injured Team Member
If a team member becomes injured or unconscious while wearing the BG4, it is usually best to swap him from his set to a resuscitator. To do this, lie him on his back supporting his head, undo his harness straps, lift his arms through the shoulder straps and roll him onto his side (rolling the set with him). With the resuscitation/demand unit of the resuscitator ready, quickly remove his facemask, check that his mouth is clear, tilt his head back, check for breathing and place the mask of the resuscitator on his face. Then continue the process of DRABC normally. The inhalation hose should be pinched off after the facemask has been removed until the cylinder valve is closed.
If all means of escape open to the team become blocked, the team captain should choose an appropriate site, remembering to mark the route, and instruct the team to adopt the entrapped procedure to conserve oxygen until the back-up team can rescue them. Obviously team members will be anxious, and the captain must reassure them to maximise the duration of the sets.
The team will prepare a place to sit so that half of the team is on one wall of the drive opposite the other half. They will undo their harness straps and lift their set over their heads so they have them in front of themselves. They will then make themselves comfortable, leaning against something so that they are in a half sitting position, with their set on their lap. They will remove the cover from their set and remove the springs. Using the manual bypass valve, they will gently fill the breathing bag three quarters full then turn the cylinder valve off. It is for this situation that effective purging is most important. Each team member must watch his breathing bag and keep his hand on the cylinder valve in case he has difficulty taking a breath. To conserve battery life, caplamps should be turned onto low beam.
When the team member inhales (which will be slightly harder) the breathing bag will partly deflate. When he exhales, the breathing bag will re-inflate, but be slightly less full than before. Team members must relax themselves and breathe slowly and deeply to minimise oxygen usage.
When the breathing bag is nearly empty (after approximately 20 minutes), it is re-inflated by opening the cylinder valve a couple of turns, pressing the manual bypass and closing the cylinder valve. While the cylinder valve is open the cylinder pressure can be read if desired. If the bag has been overfilled, the pressure relief valve can be guarded with the left hand to prevent oxygen wastage.
Team members should regularly check those beside them, and should someone pass-out, the nearest team member should reach over, turn their oxygen on and perform DRABC.
Each time after the sets have been used, they must be stripped down and all parts that have been in contact with the wearers breath washed and disinfected. The ideal procedure is rinse the parts, wash them in warm water with a mild detergent, put them through disinfectant in warm water then rinse again.
There should be a drying rack above the troughs so that the parts can be arranged to properly dry before the sets are reassembled. The cooler should be placed with the outlet downwards for drying. The breathing bag should be hung with the lung demand valve port down. After they have been hanging for a few minutes, the bags and hoses should be stretched to remove water from corners and corrugations.
If refillable drägersorb canisters have been used, they should be emptied and washed before being refilled. The canister should be vibrated as it is filled so that after the bottom plate has been put on there will be minimal rattle if the canister is shaken.
The lung demand valve should be removed with the medium pressure line attached to it. To wash the demand valve, hold the medium pressure line and dip the valve in the solution for a few seconds - it is important not to get moisture into the back of the valve.
After changing the battery in the monitron control unit, it is necessary to calibrate the unit. To do this, attach the probe to the middle socket on the monitron control unit and hold the button on the probe until the monitron reads zero.
TESTING THE BG4 WITH THE RZ25 TESTER
The results of the tests on the BG4 should be recorded on every occasion to allow for future comparison. Should any connections on the BG4 be disconnected to rectify faults after the positive pressure leak test has been performed, the leak test should be repeated. A set that has not been tested should not have its cover fitted.
1. Preparing and Testing the Tester
Check that the stopwatch is wound, extend the handle on the bellows, remove the cover from the socket on the RZ25 and zero the scale.
Positive Pressure Leak Test
Fit the rubber plug in the socket, set the tester to positive pressure pumping and gently press on the bellows handle to create a pressure above 7 millibar (70 mm WS). Set the tester to leak test, press the relief button until the gauge reads 7 mBar and start the stopwatch. After one minute, the pressure must be above 6 mBar.
Negative Pressure Leak Test
Set the tester to negative pressure pumping, press the bellows handle down a small distance and release it to create a pressure below -7 mBar. Set the tester to leak test, press the relief button until the gauge reads -7 mBar and start stopwatch. After one minute, the pressure must be below -6 mBar.
Remove the plug and fit the adaptor.
2. Preparing the BG4
Position the BG4 in front of the RZ25 with the back facing and the cover off. Inspect the frame and harness of the BG4, check that the drägersorb canister is full and that the signalling device works properly. Check that the harness and facemask straps are extended and untwisted. Remove the cap from the coupling on the breathing tubes and attach the coupling to the adaptor on the RZ25.
3. Response Threshold of Low Pressure Warning
Set the tester to positive pressure pumping and gently operate the pump. The acoustic alarm should sound before a pressure of 1 mBar is reached. The red indicator on the monitron should flash and the low pressure symbol should be displayed four times.
4. Inhalation Valve
Pinch off the exhalation tube and gently press the pump handle down. Resistance against the pump should be felt, the breathing bag should not inflate and a pressure of 10 mBar should be reached.
5. Static Pressure
Operate the pump until the breathing bag is approximately half full. Set the tester to leak test and the gauge should stabilise at approximately 3.5 mBar.
6. Exhalation Valve
Pinch off the inhalation tube with the left hand, set the tester to negative pressure pumping, push the pump handle fully down and release it. The breathing bag should not deflate and a pressure below -10 mBar should be reached.
7. Drain Valve
Hold the open side of the cap from the breathing tubes coupling over the pressure relief valve and set the tester to positive pressure pumping. Operate the pump until a pressure of 10 mBar is reached. The drain valve should not be heard to vent (ignore the low pressure warning from the monitron).
8. Positive Pressure Leak Test
Set the tester to leak test, press the relief button until the gauge reads 7 mBar and start the stopwatch. After one minute, the pressure must be above 6 mBar.
9. Pressure Relief Valve
Set the tester to negative pressure pumping until the coupling cap can be removed. Set the tester to positive pressure pumping and operate the pump until the relief valve can be heard to operate. The gauge should read between 5 and 7 mBar.
10. High Pressure Leak Test
Set the tester to leak test and open the cylinder valve a couple of turns - the acoustic alarm should sound once. Check that the segments on the monitron show more than 180 bar and that Ccr is indicated. Close the cylinder valve (must be done within 30 seconds). After approximately 20 seconds the acoustic alarm should sound once and Ocr should be displayed. Open the cylinder valve.
11. Constant Dosage
Set the tester to positive pressure pumping and hold the coupling cap over the pressure relief valve. Operate the pump until the breathing bag is full then set the tester to dosage (red dot). The gauge should stabilise between 1.5 & 2.0 l/min.
12. Lung Demand Valve
Set the tester to negative pressure pumping and once the breathing bag is empty operate the pump. The lung demand valve should be heard to operate at between 0.5 and 2.5 mBar.
13. Manual Bypass
Set the tester to leak test and operate the manual bypass valve. The breathing bag should be seen to inflate.
14. Low Pressure Warning and Battery Charge
Record the cylinder pressure, close the cylinder valve and watch the monitron. When the pressure has dropped to approximately 50 bar the acoustic alarm should start to sound intermittently and the red indicator should flash. When the pressure drops to approximately 5 bar the acoustic alarm should sound five times. The display should indicate bat and the red indicator should flash for approximately 30 seconds before the monitron switches off. If the red indicator lights up continuously and the display does not go blank, the battery needs to be changed. Replace the monitron in its holder.
15. Negative Pressure Leak Test
Set the tester to negative pressure pumping and operate the pump to create a pressure below -7 mBar. Set the tester to leak test, press the relief button until the gauge reads -7 mBar and start the stopwatch. After one minute, the pressure must be below -6 mBar. It may be necessary to hold pressure on the coupling during this test.
Pinch off the exhalation tube and disconnect the coupling from the tester. Fit the cap to the coupling, release the exhalation tube and fit the cover to the set.
As with all equipment, tested sets that are ready to be used should have a storage place separate from sets that are not ready to use.
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