BSAC Ocean Diver Training
Planning to go Diving
OT4.1
08/02
Planning to go
Diving
Planning to go Diving
Aims
Every dive requires a plan for
safety and enjoyment
Two of the most important
considerations
 Nitrogen management
 Ensuring sufficient breathing gas
for the dive
OT4 2
08/02
Planning to go
Diving
Diving & Effects of Nitrogen
Atmosphere
O2
 79% Nitrogen, 21% Oxygen
Nitrox
 A breathing gas usually having more than 21%O2
Gases
 Dissolve in the blood - a liquid
 Oxygen utilised, carbon dioxide produced
 Oxygen, carbon dioxide come out of
blood as gases in alveoli
Diving - ambient pressure increases
OT4 3
08/02
 Higher concentration of gas delivered to diver
 Higher concentration of gases dissolved in
the blood including nitrogen
N2
Planning to go
Diving
Nitrogen Absorption
On the surface
 Nitrogen in body in equilibrium with air
On a dive
 Increase in ambient pressure results in
more nitrogen absorbed into body
tissues - “on-gassing”
 Going deeper or longer increases
body’s uptake of nitrogen
 Different tissues absorb nitrogen at
different rates
 ‘Fast tissues - good blood supply
 ‘Slow’ tissues - poorer blood supply, fatty
tissues
OT4 4
08/02
Planning to go
Diving
Nitrogen Release
On ascent
 Nitrogen released from tissues - “off-gassing”
 Tissues release nitrogen at different rates
 Body can tolerate a certain excess of nitrogen
 Nitrogen still being released when back on the
surface
If a diver ascends too quickly
 Nitrogen will be released as bubbles in the
tissues or bloodstream
 Can cause Decompression Illness (DCI)
Nitrogen release needs to be managed
OT4 5
08/02
Planning to go
Diving
Nitrogen Management
BSAC Decompression
Tables
Dive Computers
Understanding the principles of tables will
help to understand computers
OT4 6
08/02
Planning to go
Diving
BSAC Tables - Definitions 1
Depth
 Deepest depth reached during the
dive
6m
Descent rate
 Maximum is 30m/minute
Ascent rate
Deepest Depth
 Maximum is 15m/minute up to 6m
 1 minute from 6m to the surface
6m
Ascent/Dive time check depth
 6m pause to check dive time
Dive time
OT4 7
08/02
 From leaving surface, to arriving at 6m
Dive time
Planning to go
Diving
BSAC Tables - Definitions 2
Current Tissue Code (CTC)
 Represents body’s present level
of nitrogen saturation
 Starting point for dive planning
CTC
Surfacing Code (SC)
 Represents change to CTC at
end of dive
Tissue Codes :
Nitrogen Load
OT4 8
08/02
A B C D E F G
SC
Planning to go
Diving
Using BSAC Tables
Ocean Divers
 Maximum depth 20 metres
 “No-Stop” dives for
minimum risk - the white
zone
Ocean Divers dive
within this area
Table ‘A’
 For divers with a CTC
of ‘A’
OT4 9
08/02
Planning to go
Diving
BSAC Tables - Planning a
No-Stop Dive
Dive to 9m for 17 min.
 Surface Code is B
If depth or time between
values
 Use next longer time
 Use next greater depth
Dive to 20m for 30 min.
 Surface Code is E
OT4 10
08/02
Planning to go
Diving
Planning a Second Dive
On the surface after a dive
 “Off-gassing” reduces tissue Nitrogen loading
 Surface Interval reduces Surfacing Code
 Gives new Current Tissue Code for the second dive
Plan second dive shallower than first
CTC
SC
CTC
Surface Interval
OT4 11
08/02
Planning to go
Diving
BSAC Tables - Using the
Surface Interval Table
To find your CTC for a second dive
 Enter at your SC
 Move across to Surface
Interval
 Read new CTC from
table
Example
 SC = ‘E’
Surface interval + 5 hours
 New CTC = ‘B’
OT4 12
08/02
Planning to go
Diving
Table Levels
Table ‘Level’ depends on atmospheric pressure
 Pressure range marked on each
Table
 Use Altitude/Atmospheric Chart
with Transfer Table
When to use:




OT4 13
08/02
Travelling - hills and mountains
Living or diving at altitude
Inland sites and bad weather
Flying
Planning to go
Diving
Flying & Diving
Flying before diving
 Tissues off-gas
 Stresses of flying
 Recover before diving - 10
hours minimum
Flying after Diving
OT4 14
08/02
 Increased level of off-gassing DCI
 Stresses of flying
 Ensure long Surface Interval
between last dive and flying
 Need CTC of ‘A’ or ‘B’
Planning to go
Diving
Diving using BSAC Tables
To Dive the Plan using Tables you need
 A depth gauge to monitor depth
 A dive watch to monitor time
 A dive slate as a reminder of the plan
PLAN THE DIVE AND DIVE THE PLAN
OT4 15
08/02
Planning to go
Diving
Dive Computers
The basic functions:
 Theoretical body tissue
‘models’ to calculate nitrogen
on and off-gassing
 A timer & pressure sensor
 Planning information depth/time
 Audible or screen warnings
 Stored log of previous dives
 Integrated temperature
sensors
OT4 16
08/02
Planning to go
Diving
Difference between Tables
and Computers
Dive Profile
 Profile of depth and time
Dive Table Profile
 ‘Square’ profile
 ‘Calculated for maximum depth
Dive Computer Profile
 Continuous tracking depth and time
 Continuous tracking of nitrogen
load
 No-stop times vary from square
profile of tables
OT4 17
08/02
Planning to go
Diving
Plan the Dive and Dive the Plan
Whether using Tables or Computers
 Two dives - plan the deepest first
 Never push the limits of tables or
computers
 Table and computer mix, dive the tables
 Plan a dive that avoids “saw-tooth”
profiles
 BSAC ’88 Tables allow for 3 dives in 24
hours
 Computers may have similar allowance
 Ensure a good surface interval after
diving and before flying
OT4 18
08/02
Planning to go
Diving
Using Tables or Computers
Tables or Computers are not
infallible
 They are planning tools to minimize
the risk of DCI
DCI can happen . . .
 Be fit to dive
 Avoid getting cold
 Do not dive in a dehydrated state
 Avoid excessive exercise
 Do not dive with a hangover
 Abort a dive if anxious or worried
OT4 19
08/02
Remember
Planning to go
Diving
Use of Nitrox to manage Nitrogen
Ocean Divers may use 32% and 36% Nitrox
mixes using air tables or air computers
Advantages of Nitrox – less nitrogen means
 Safety factor
 Reduced risk of DCI
Disadvantages of Nitrox
 Nitrox divers can still get DCI
 Certain equipment must be dedicated
to the use of Nitrox
 Oxygen Toxicity (explained in OT6)
OT4 20
08/02
Planning to go
Diving
Introducing
BSAC NITROX Tables
Ocean Divers
 “No-Stop” dives using 32%
Nitrox, planned on 21% air table
 Maximum depth 20 metres
Ocean Divers dive
within this area
Table ‘A’
 For divers with a CTC of ‘A’
OT4 21
08/02
Planning to go
Diving
Nitrox – Procedures and
Practices
Higher percentage of oxygen
 Oxygen supports combustion
 Materials and greases must be safe
with higher oxygen percentages
 Equipment must be cleaned periodically
removing combustible contaminants
Percentages must be checked
with an O2 analyser before use
 Follow analyser manufacturer’s guidance
 Must not use gas more than 1% different
from desired mix
 All cylinders should be marked with mix %
and MOD
OT4 22
08/02
Planning to go
Diving
Nitrox – Using an analyser
1. Switch on the
gas analyser
3. Slowly open
the valve
- gas gently
hissing out
OT4 23
08/02
2. Calibration
- essential
before use
4. Present the
analyser to
the cylinder
valve
Planning to go
Diving
Nitrox – Analysing a mix
5. Take a reading
6. Record the analysis
OT4 24
08/02
Planning to go
Diving
Analysing essentials
 Calibrate prior to use
 Keep flow rate even and low as possible
 Erratic readings point towards a analyser failure
 Avoid windy conditions
 Avoid moisture
 Your analysis must be within +/- 1% of your
target mix
 Store analyser away from elevated oxygen levels
OT4 25
08/02
Planning to go
Diving
Dive Planning – Breathing Gas
The rule for planning and monitoring breathing gas
on a dive
Rule of Thirds
1/
 One third gas supply for
descent, to half way or
turn around point
 One third gas supply
for second half of
dive or return journey
including ascent
 One third gas supply
as a reserve
OT4 26
08/02
1/
3
1/
3
3
Planning to go
Diving
Breathing Gas Planning
The Rule of Thirds
10 Litre cylinder filled to 210 bar
Divide 210 bar by 3 = 70 bar for each 1/3 of dive
 First third
210 - 70 = 140 bar - at turn around/half way
point
 Second third
140 - 70 = 70 bar - at surface
 Third third
70 leaves approximately 70 bar as reserve
Always aim to surface with 1/3 cylinder as reserve
OT4 27
08/02
Planning to go
Diving
Gas Monitoring
Always monitor contents
gauge throughout a dive –
it’s your lifeline!
Running low or out of gas
should not arise under normal
diving conditions
Gas consumption increases when:
 Physical effort increases
 Divers are cold or anxious
Shorten or terminate the dive
OT4 28
08/02
Planning to go
Diving
Summary
Plan the Dive and Dive the Plan
Management of Nitrogen
 Tables or computer
 Plan with common sense
Management of Breathing Gas
 Rule of Thirds and constant
monitoring
OT4 29
08/02
Planning to go
Diving
OT4 30
08/02