1. BSAC Sports Diver Training
Breathing Gas Planning & Systems

2. Breathing Gas Planning & Systems
Deeper, decompression stop or more challenging dives?
Failure of gas supply - life-threatening!!
Divers need sufficient gas to carry out planned dives
Have a sufficient reserve for emergencies
Carry an alternative gas supply
Depending on the type of diving being undertaken
Equipment configurations

3. Sufficient Gas for the Dive
Rule of Thirds
One Third Gas supply for descent, to half way or turn around point.
1/3
One Third Gas supply for second half of dive or return including surfacing
1/3
1/3
One Third Gas supply for reserve
How much gas is required for the Dive?
What size cylinder is needed?

4. Gas required for the Dive
Amount of gas required depends on
Personal breathing rate
Fitness
Energy output
Temperature
Nervousness
Safe Rule of Thumb - 25 litres/minute
Depth of Dive
Use more gas at depth
Duration of dive (minutes)
Amount of gas = 25 l/min x Abs. Pressure (bar) x Time (mins.)

5. Cylinder Size for the Dive
Cylinder capacity
Size of cylinder in litres x bar pressure
E.g. 10L cylinder at 230bar = litres air
Dive to 20m for 20 mins. - 10L Cylinder OK?
Breathing rate x absolute pressure x dive time (mins)
25 l/min x bar (20m) x mins = litres
This represents two thirds (750 litres = 1/3)
Reserve third (750 litres) needs to be added = litres
Total Air required = litres
10L Cylinder will be suitable for this dive

6. Calculating Reserve Gas
Dive to 20m for 20 mins
Breathing rate x absolute pressure x dive time
25 l/min x 3 bar (20m) x 20 mins = litres
This represents gas for dive (Two Thirds)
Add reserve third x 1.5 = litres Total
Reserve judged against type of diving undertaken

7. Gas Calculation Example
Dive Plan
25m for 20mins.
= 2200 litres
= 2640 litres
10L cylinder at 220bar
12L cylinder at 220bar
Breathing rate x absolute pressure x dive time
25 l/min x 3.5 (25m) bar x 20mins = 1750 litres
One third reserve (875 litres) = litres
Total required = 2625 litres OR
Reserve = 1750 litres x 1.5 = 2625 litres
12L Cylinder needed for this dive

8. Emergency Gas Planning
Gas Consumption - a Function of:
Depth
Time to ascend (and ‘settle’ time)
Any decompression stops required
Stress under water
End of dive to first stop/check depth
Out of gas - sharing single system
Accelerated breathing rates > 25l/min

9. Alternative Supply - AS
Why have an AS?
Gas supply loss - life threatening
Urgent need to re-establish air supply
Alternative supply carried by diver
Alternative supply carried by buddy
Sharing techniques stressful and have poor record of success
All divers should carry an AS!

10. AS Configurations
No one configuration of AS suitable for all circumstances
Configuration must be appropriate to diving conditions
Types of Configuration
Advantages
Disadvantages
Practicalities

11. BC Demand Valve Advantage Disadvantages Minimises number of hoses
No protection against first stage failure
Cannot be used by buddy
Restricts diver’s head movement
Divers must remain very close together
Both divers consume gas from the same cylinder (1st stage)

12. ‘Octopus’ demand valve
Should be visible & easily accessible
Advantages
Easy access for buddy
Diver retains own regulator
Long hose
Buddy use easier
Can also be used by wearer
Disadvantages
No protection against first stage failure
Both divers breathing from same cylinder
Both divers breathing from the same 1st stage
1st stage performance not assured

13. Two Independent Regulators
Pillar valve with two independent outlets
Advantages
All the advantages of an ‘octopus’
Can protect against some primary first stage failures
Disadvantages
No protection against massive leak or free-flow
Both divers breathing gas from the same cylinder

14. Pony Cylinder & Regulator
Comprises
Additional cylinder, usually of 3 litre capacity
Dedicated second regulator
Advantages
True emergency back-up to diver’s own gas supply
Additional gas capacity
Fairly light
Disadvantage
Capacity inadequate for deeper dives involving decompression stops

15. Twin Cylinders with Independent Regulators
Advantages
Same as pony cylinder
Greater gas capacity
Balanced trim underwater
Considerations
Monitoring two supplies
Regulator switching
Disadvantages
Regulator failure loses access to gas in that cylinder
Requires extra buoyancy

16. Twin Cylinders with Isolation Manifold
Isolation valve closes connection between cylinders
Advantages
No need for regulator switching
Monitoring one supply
Considerations
Isolator must be accessible to wearer
Disadvantages
Additional ‘O’ rings
Requires extra buoyancy

17. AS is a Back-up System
Gas consumption must be planned with any kit configuration
Gas back up/reserve should :
Always be kept for emergency use (Emergency Gas Consumption)
Not used to extend dive duration
Be adequate for dive depth or conditions
Be adequate for self and buddy at worst point in the dive

18. Maintenance Equipment is your lifeline Ensure
All equipment is washed in fresh water after use
To remove dirt
Prevent build up of salt crystals
Equipment is regularly serviced by competent technician
Equipment treated with respect

19. Updating or Buying New Equipment
Is the system right for you?
For the diving you are beginning to do?
For the diving you intend doing in the future?
Can you upgrade gradually?
Consider complete system
Overall weight of equipment
Check buoyancy changes
Shallow water check out dive
Skills
Practice - Practice - Practice!

20. Summary Gas Planning Alternative Supply New Equipment
Adequate size of cylinder for dive
Calculating reserves
Alternative Supply
All divers should carry an AS
Have suitable equipment configuration for planned dive
Maintenance and care
New Equipment
Updating or new - implications
Become familiar in safe, confined conditions
Practice - Practice - Practice