st5 - scuba diving, snorkeling & land tours in sabah · divers need sufficient gas to carry out...

ST5.1 08/02

Breathing Gas Planning & Systems

BSAC Sports Diver Training

Breathing Gas Planning &

Systems

ST5.2 08/02

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


Systems

ST5.3 08/02

Sufficient Gas for the Dive

Rule of Thirds

1/3

1/3

1/3 !  One Third Gas supply for

second half of dive or return including surfacing

!  One Third Gas supply for reserve

!  One Third Gas supply for descent, to half way or turn around point.

How much gas is required for the Dive? What size cylinder is needed?


Systems

ST5.4 08/02

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.)


Systems

ST5.5 08/02

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


Systems

ST5.6 08/02

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


Systems

ST5.7 08/02

Gas Calculation Example

Dive Plan 25m for 20mins.

!  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

10L cylinder at 220bar 12L cylinder at 220bar

= 2200 litres = 2640 litres


Systems

ST5.8 08/02

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


Systems

ST5.9 08/02

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!


Systems

ST5.10 08/02

AS Configurations

No one configuration of AS suitable for all circumstances

Configuration must be appropriate to diving conditions Types of Configuration !  Advantages !  Disadvantages !  Practicalities


Systems

ST5.11 08/02

BC Demand Valve

Advantage !  Minimises number of hoses

Disadvantages !  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)


Systems

ST5.12 08/02

‘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


Systems

ST5.13 08/02

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


Systems

ST5.14 08/02

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


Systems

ST5.15 08/02

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


Systems

ST5.16 08/02

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


Systems

ST5.17 08/02

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


Systems

ST5.18 08/02

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


Systems

ST5.19 08/02

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!


Systems

ST5.20 08/02

Summary

Gas Planning !  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


Systems

ST5.21 08/02

st5 - scuba diving, snorkeling & land tours in sabah · divers need sufficient gas to carry out...

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