BSAC Sports Diver Training Breathing Gas Planning & Systems

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

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?

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.)

Cylinder Size for the Dive Cylinder capacity Size of cylinder in litres x bar pressure E.g. 10L cylinder at 230bar = 2300 litres air Dive to 20m for 20 mins. - 10L Cylinder OK? Breathing rate x absolute pressure x dive time (mins) 25 l/min x 3 bar (20m) x 20 mins = 1500 litres This represents two thirds (750 litres = 1/3) Reserve third (750 litres) needs to be added = 750 litres Total Air required = 2250 litres 10L Cylinder will be suitable for this dive

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 = 1500 litres This represents gas for dive (Two Thirds) Add reserve third - 1500 x 1.5 = 2250 litres Total Reserve judged against type of diving undertaken

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) = 875 litres Total required = 2625 litres OR Reserve = 1750 litres x 1.5 = 2625 litres 12L Cylinder needed for this dive

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

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!

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

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)

‘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

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

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

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

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

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

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

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!

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