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Dräger BG174

The BG174 is a negative pressure type closed circuit SCBA. It has an overall mass of 14kg 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 frame and cover are manufactured from approved non-spark aluminium alloy.

The cylinder couples to the oxygen distributor which feeds the pressure reducer, the manual bypass valve and, through the pressure gauge shutoff valve, the pressure gauge. These form the high pressure components of the set.

The pre-flush unit is fed from the pressure reducer and activates when it is subjected to a surge of pressure, ie when the cylinder valve is opened normally. It rapidly injects approximately 7l of oxygen into the tube that connects the CO2 absorption canister to the breathing bag. The breathing bag has a volume of approximately 6l, so is fully inflated by the pre-flush with the effect that all normal air is removed from the breathing bag, valve group assembly and inhalation tube. If the cylinder valve is opened slowly, the set can be pressurised without the pre-flush activating.

The pressure reducer reduces the pressure of the oxygen supplied by the cylinder to maintain a pressure of 4 bar in the medium pressure line. The manual bypass valve by-passes the pressure reducer to supply oxygen at cylinder pressure to the pre-flush line at a rate of up to 120l per minute. The medium pressure line connects the pressure reducer to the metering orifice, lung demand valve and warning signal control bellows in the valve group assembly.

With 4 bar in the medium pressure line, the metering orifice allows oxygen to flow at approximately 1.5l per minute into the valve group assembly. This is the constant dosage supply and is normally sufficient for up to medium work load.

The release of oxygen by the metering orifice and the lung demand valve have a cooling effect on the set.

When the warning signal bellows are not pressurised, a flap with two slots covers the breathing bag connection in the valve group assembly. If oxygen is drawn from the breathing bag while the warning signal flap is covering the breathing bag connection, metal reeds in the slots of the warning signal flap vibrate to create a musical tone. When the warning signal bellows are pressurised, the warning signal flap is moved away from the breathing bag connection.

Past this point, all parts of the set operate at around atmospheric pressure.

The facemask is connected to the set by the breathing tubes. The saliva trap is on the inhalation tube. The valve group assembly also contains 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 valve group assembly into the inhalation tube.

The exhalation valve is in the short tube that connects the valve group assembly to the CO2 absorption canister (drägersorb canister) and only allows oxygen (which at this point contains CO2) to flow from the exhalation tube to the drägersorb canister.

Should the wearer empty the breathing bag and create sufficient negative pressure in the set (between -1 and -4 mBar) when inhaling, the diaphragm in the valve group assembly is drawn inwards which pushes the cotter pin against the lung demand valve needle. This causes the base of the needle to temporarily lose its seal and allow oxygen to by-pass the metering orifice at up to 30l per minute.

Should the wearer fill the breathing bag and create excess pressure in the set (between 1 and 4 mBar), the diaphragm in the valve group assembly is pushed outwards which moves an orifice in its centre off the sealing plate of the cotter pin. This allows excess oxygen to be released to the atmosphere through the vent hole. This is the pressure relief valve.

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 and for this reason the drägersorb canister is designed to give off heat.

With a fully charged cylinder, the BG174 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.

There is a total of 7 valves on the BG174, 3 are manually operated (cylinder valve, manual bypass valve and pressure gauge shutoff valve) and the other 4 operate automatically when affected by the wearer's breathing (inhalation and exhalation valves, lung demand valve and pressure relief valve).



Schematic of the BG174

1. Exhalation Tube		16. Cotter Pin
2. Exhalation Valve		17. Lung Demand Valve Needle
3. Drägersorb Canister		18. Lung Demand Valve
4. Breathing Bag		19. Warning Signal Control Line
5. Valve Group Assembly		20. Warning Signal Control
6. Inhalation Valve		21. Warning Signal Flap
7. Inhalation Tube		22. Pre-flush Unit
8. Facemask Connection		23. Pre-flush Diaphragm
9. Cylinder Valve		24. Pre-flush Line
10. Oxygen Cylinder		25. Pre-flush Chamber
11. Oxygen Distributor		26. Manual Bypass Valve
12. Pressure Reducer		27. Pressure Gauge Line
13. Medium Pressure Line	28. Pressure Gauge
14. Metering Orifice		29. Pressure Gauge Shutoff
15. Diaphragm			30. Saliva Trap



Schematic of the High Pressure Unit

1. Oxygen Distributor		9. Rod
2. Pressure Gauge Line		10. Pre-flush Line
3. Pressure Gauge Shutoff	11. Medium Pressure Line
4. Diaphragm			12. Sealing Plate
5. Spring (opening)		13. Metering Orifice
6. Spring (closing)		14. Pre-flush Spring
7. Piston			15. Pre-flush Chamber
8. Piston Cap			16. Manual Bypass Valve


How It Works

Before the unit is pressurised, the opening spring holds the pressure reducer diaphragm forward which holds the three rods (only two are shown in the diagram) forward which hold the piston cap back which allows the piston to unseat.

When the cylinder valve is opened, oxygen flows past the piston (which is a hexagon in a round cylinder), through holes in the piston cap and into the centre of the high pressure unit. As the pressure in the medium pressure line increases, the pressure reducer diaphragm is pushed back which allows the closing spring to push the piston forward and reduce the oxygen flow. If the pressure reaches 4 bar, the piston will be pushed forward until it has seated and closed off the oxygen flow.

Also, when the cylinder valve is opened normally, the pre-flush diaphragm is pushed back until the pressure equalises through the metering orifice. This causes the sealing plate to unseat and allows oxygen to flow into the pre-flush line.


BG174 Procedures

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 BG174 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:

  1. Attach their facemasks;
  2. Stand their sets up;
  3. Remove the covers;
  4. Check that the high and low beam on caplamp work;
  5. Ensure that the breathing bags are empty;
  6. Check that the drägersorb canister is full and the connections are tight;
  7. Check that the pre-flush connection is tight;
  8. Check that the pressure gauge shutoff valve is in the 'on' position;
  9. Check that the cylinder connection is tight;
  10. Check that the medium pressure line connection is tight;
  11. Check that the vent hole on valve group assembly is pointing down;
  12. Check that the breathing tube connections are tight;
  13. Put the covers on, lay the sets back and move the harness straps out to the sides;
  14. Check that the signalling device works;
  15. Check that the nuts on the top harness bracket are tight;
  16. Check that the 'A' frame is secure;
  17. Check that the bolts for bottom harness brackets are tight;
  18. Ensure that all of the straps on their sets are fully extended;
  19. Ensure that the visors have been prepared to prevent fogging;
  20. Check that the breathing tubes are straight and undamaged and the saliva trap nut is tight;
  21. Open the cylinder valve ensuring that the pre-flush activates;
  22. Check that the manual bypass valve operates;
  23. Check that the cylinder pressure is greater than 180 bar;
  24. Don their sets.

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:

  1. Remove their helmet;
  2. Exhale then don their facemask, keeping hair out;
  3. Hold their facemask against their face and inhale;
  4. Slightly lift the bottom of their facemask up and exhale downwards;
  5. Repeat steps 3 & 4 twice (the lung demand valve should operate on the last two breaths) (the set and the wearer's lungs are now purged of normal air);
  6. Tighten the facemask straps;
  7. Replace their helmets;
  8. Pinch off their inhalation tube and inhale to check that the exhalation valve works and that the facemask doesn't leak under negative pressure;
  9. Pinch off their exhalation tube and exhale to check that the inhalation valve works and that the facemask doesn't leak under positive pressure.

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.

  1. Check that the facemask straps are straight (the team member will hold his helmet with his right hand);
  2. Check that no hair is in the facemask;
  3. Check that the harness straps are not twisted;
  4. Turn the team member around and check that the set's cover is secure;
  5. Check the self-rescuer and safety rope;
  6. Turn the team member back and should hook the chin strap under the facemask connection.

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:






Turn Back



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 wearer’s back. The breathing bag should be topped up using the manual bypass before closing the cylinder valve and changing the cylinder. The cylinder valve should be opened without activating the pre-flush and the cylinder connection should be checked for leakage. No start-up is required.

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.

Route Marking


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 BG174, 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 and lift his arms through the shoulder straps. Lift him off his set, simultaneously moving him to the right of the set and rolling him so that he is placed on the ground facing his set. 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.


Entrapped Procedure

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 unsling their set, bringing them around their left hand sides 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 position their set in front of them with the breathing bag facing them. Using the manual bypass valve, they will gently fill the breathing bag 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.

After the oxygen flow from the pressure reducer runs out, the warning signal will be heard when the team member inhales (which will be slightly harder) and the breathing bag will partly deflate. Upon exhalation, the breathing bag will re-inflate, but be slightly less full than before. Team members must relax themselves and breathe slowly and deeply.

When the breathing bag is nearly empty (after approximately 20 minutes), it is gently re-inflated by cracking the cylinder valve, pressing the manual bypass (if necessary) and closing the cylinder valve. While the cylinder valve is open the cylinder pressure can be checked if desired.

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 wearer’s 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 breathing bag, valve group assembly and hoses should be hung by the nut on the connection. After they have been hanging for a few minutes, the bags should have the bottom corners held up and the hoses should be stretched to remove water from corners and corrugations.

Drägersorb canisters should be vibrated as they are filled so that after the filler plug has been put in the canister will not rattle if it is shaken.

The double plug must be put on the valve group assembly prior to washing and must be checked at each trough - it is important not to get moisture into the valve as this may cause a blockage of the metering orifice or the dosage screw in the warning signal bellows line.




The results of the tests on the BG174 should be recorded on every occasion to allow for future comparison. Should any adjustments to the BG174 be required to rectify faults, the positive and negative pressure leak tests 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 the stopwatch. After one minute, the pressure must be below -6 mBar.

Remove the plug and fit the adaptor.

2. Preparing the BG174

Position the BG174 to the right of the RZ25 with the back facing and the cover off. Inspect the frame and harness of the BG174, 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 connection on the breathing tubes and attach the connection to the adaptor on the RZ25.

3. Warning Signal

With the tester set to negative pressure pumping and the breathing bag partially inflated, operate the pump a few times. The warning signal should be heard with each pump.

If the pump seems to operate unusually slowly, check that the warning signal reeds are free.

4. Positive Pressure Leak Test

Fit the plug in the vent hole on the valve group assembly and set the tester to positive pressure pumping. Operate the pump to fill the breathing bag and create a pressure above 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 above 6 mBar.

5. Exhalation Valve

Set the tester to negative pressure pumping, pinch off the inhalation tube with the right hand and with the left hand push the pump handle approximately halfway down and release it. The breathing bag should not deflate (also, the gauge should remain approximately constant and the pump handle should only slowly rise).

6. Negative Pressure Leak Test

Remove the plug from the vent hole and operate the pump to empty the breathing bag and create a pressure below -7 mBar (again, the warning signal should be heard with each pump). 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.

7. Inhalation Valve

Set the tester to positive pressure pumping, 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 the gauge should remain approximately constant.

8. Pre-flush

Set the tester to negative pressure pumping and open the cylinder valve (open fully then back half a turn), the breathing bag should inflate within a few seconds.

If the cylinder valve was hard to turn, replace the cylinder and have the valve serviced.

9. Pressure Relief Valve

Set the tester to leak test. The gauge should stabilise between 1 & 4 mBar.

10. Constant Dosage & Manual Bypass Valve

Plug the vent hole and set the tester to dosage (red dot). Gently press the manual bypass until the gauge reads about 1.7 l/min. The gauge should stabilise between 1.4 & 1.7 l/min.

11. Warning Signal, Breathing Bag Volume, Lung Demand Valve

Remove the plug and fit the cover. Set the tester to negative pressure pumping. Counting the number of pumps, operate the pump (the warning signal should not sound) until the breathing bag is empty and the lung demand valve is heard to operate. This should not occur until after the tenth pump. Press the pump handle down and slowly release it, the lung demand valve should operate at between -1 & -4 mBar.

12. Manual Bypass, High Pressure Leak Test and Gauge Shutoff

Place the BG174's pressure gauge where it can be read and close the cylinder valve. When the pressure gauge reads 180 bar, start the stopwatch. When the pressure has dropped to 35 bar, simultaneously stop the stopwatch and open the cylinder valve 2 turns (the pre-flush should not activate).

A high pressure leak will be indicated if the time on the stopwatch is less than 25 seconds and will usually occur at the cylinder connection. Any other fault with any high pressure component will need to be repaired by a technician.

Close the pressure gauge shutoff valve and record the cylinder pressure. Close the cylinder valve and operate the manual bypass. When the sound of oxygen being released ceases, start the stopwatch. After one minute the pressure gauge reading should be unchanged. Open the pressure gauge shutoff valve, the pressure should drop to zero.

13. Storage

Replace the pressure gauge in its holder, disconnect the breathing tubes from the tester, and fit the cap to the facemask connection.

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.



RZ25 Fails Leak Tests

Instructions not followed correctly.

Rubber plug not properly fitted.

RZ25 requires servicing.

Warning Signal Fails To Operate

Reeds bent or stuck - straighten, free-up or replace.

Cylinder valve open.

Warning signal flap stuck open.

Breathing Bag Will Not Inflate

RZ25 not switched to positive pressure pumping.

Saliva trap nut not fitted.

BG174 not fully assembled.

All connections not properly seated.

O-ring missing.

BG174 Fails Positive Pressure Leak Test

RZ25 not switched to leak test.

Rubber plug not properly fitted.

All connections (eg valve group assembly lock ring) not properly seated and tight.

O-ring missing.

Breathing bag or breathing tube punctured.

RZ25 faulty - test RZ25, test another BG174.

Exhalation or Inhalation Valves Fail Test

Incorrect breathing tube pinched off.

Breathing tube not properly pinched off.

Valve fouled.

Valve requires moistening.

Valve needs to be replaced.

BG174 Fails Negative Pressure Leak Test

RZ25 not switched to leak test.

Cylinder connection not properly seated and tight.

Diaphragm damaged or not seating correctly on cotter pin.

RZ25 faulty - test RZ25, test another BG174.

Pre-flush Fails

Cylinder valve opened too slowly.

Oxygen cylinder empty.

Pre-flush unit faulty.

Pressure Relief Valve Outside Tolerance

RZ25 incorrectly zeroed.

RZ25 not set to leak test.

Cylinder valve closed.

Oxygen cylinder empty.

Diaphragm not seating correctly on cotter pin.

Pressure relief valve spring needs to be replaced.

Zero constant dosage.

Constant Dosage Insufficient

RZ25 incorrectly zeroed.

RZ25 set to wrong scale.

Cylinder valve not fully open.

Oxygen cylinder empty.

Metering orifice fouled.

Medium pressure line blocked.

RZ25 faulty - test RZ25, test another BG174.

Pressure reducer faulty.

Constant Dosage Excessive

RZ25 incorrectly zeroed.

RZ25 set to wrong scale.

Demand valve needle incorrectly fitted.

Demand valve needle bent and touching cotter pin - replace.

Metering orifice enlarged - replace demand valve needle.

Pre-flush or manual bypass leaking - undo pre-flush connection to check.

Warning signal bellows connection leaking - tighten, replace washers.

Warning signal bellows leaking - replace.

RZ25 faulty - test RZ25, test another BG174.

Pressure reducer faulty.

Warning Signal Operates During Breathing Bag Volume Test

Cylinder valve not fully open.

Oxygen cylinder empty.

Warning signal bellows stuck - free up.

Feeder line for bellows blocked - check metering screw.

Warning signal bellows faulty.

Breathing Bag Volume Low

Breathing bag wasn't full to start with.

Breathing bag constricted.

Zero constant dosage.

Breathing Bag Volume Over 14

Pump operated slowly.

Constant dosage excessive.

Lung Demand Valve Will Not Operate

Breathing bag is not empty.

Pump handle was released too slowly.

Cylinder valve closed.

Oxygen cylinder empty.

Demand valve needle bent and not touching cotter pin.

Cotter pin jammed.

Lung Demand Valve Outside Tolerance

RZ25 incorrectly zeroed.

Demand valve needle bent.

Cotter pin doesn't move freely.

Cotter pin spring needs to be replaced.

Pump handle was released too quickly.


Emergency Response Considerations

Self-Contained Breathing Apparatus

Drager PA93

Hazardous Chemicals

Drager BG4



Fire Fighting

First Aid

Rope Rescue

Case Study - Pasminco Fire

Major Disaster Case Studies


Summary of the Principles of Rescue Work

Guidelines for the Frequency of Practice Sessions

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