Deco on the
Fly and Gas Planning
Following on
from the short article we made on recreational gas planning it seemed a god
idea to follow up with some guidelines on gas planning at the next level –
entry level technical diving.
For most
technical divers, dives down to 50m are often done on the fly. We still normally start with a plan, but
often something comes up or we can often end up staying longer than the plan or
going a little deeper.
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| Modern technical computers make deco on the fly viable |
Carrying out
deco on the fly is often seen as a risky approach to technical diving, although
this isn’t always the case with the correct planning and knowledge. Most technical divers have spent $1,000 or
more on a technical dive computer (often more than one!) so already have the
decompression information at hand for a revised profile – the risky part is
ensuring we have enough gas to complete the dive, and that's what we’ll cover
here.
Basically
what we are going to do is work out how long a deco bottle will last, we can
then use this to estimate how much decompression (or time to surface on most
computers) we can accumulate without running the risk of being out of gas
during the deco.
EAN50 Gas Planning
 |
| EAN50 is generally the most versatile deco gas |
The most
common decompression gas is the trusty 12l EAN50 bottle, this is normally
breathed at 21m and shallower and is often combined with O2 for longer or
multiple dives. Like most decompression
profiles the stops are generally longer as the stops become shallower, almost
resembling a Fibonacci
series
pattern (i.e. 1-1-2-3-5-8-13-21 etc…) in many cases.
If we use a
conservative average depth of 15m for EAN50 (it would normally be shallower)
this would give an adjusted breathing rate of 2.5ata x 20l/min = 50 litres per
minute. A typical 12l tank at 200 bar
will hold 2,200+ bar of gas – 2,200 litres / 50 l/min gives 44 minutes of gas –
we’ll say 40 minutes to be conservative.
We can now
use the 40 minute bottle as a benchmark – basically for every 10 bar we have 2
minutes of decompression gas in a 12l tank, or 1 minute per 10 bar in a 6l
tank. Using this logic a tank with 130
bar would have 26 minutes of decompression gas, or a 6 litre tank at 160 bar
would have 16 minutes of decompression gas.
Factoring in
Oxygen to Gas Planning
When using a
second decompression gas as oxygen we can also factor this in. In most case, the decompression time between
21m to the surface is split evenly between EAN50 depths and O2 depths; that is
we’ll spend 50% of the time at 21m to 9m and the other 50% between 6m and the
surface. This has one obvious advantage,
it effectively doubles the amount EAN50 deco gas we need, IF (and this is an
important if) the O2 gas volumes are also planned!
Assuming the
average O2 depth is 5m or 1.5 ata, our adjusted breathing rate would be 1.5 ata
x 20l/min = 30 litres per minute. Using
a 12l bottle at 200 bar as the benchmark we can say this bottle has 2,200
litres / 30 l/min = 73 min, say 70 minutes of deco gas use.
Using the
same numbers as above this corresponds to every 10 bar in a 12l cylinder
representing 3.5 minutes of deco gas, we like to make sure we have some O2 left
over for any post dive emergencies or just for in water contingencies so lets
use 3 minutes per 10 bar as our benchmark, of 1.5 in a 6l bottle. So, a 12l at 80 bar would be 24 minutes of O2
deco gas while a 6l bottle at 120 bar would be 18 minutes of O2 deco gas.
Back Gas Planning
and Reserves
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| Maintaining sufficient back gas is critical |
The key
concept in this scenario is that in a worst-case(gas sharing at minimum
reserve) situation we are prepared to skip any deco below 21m and ascend from
depth directly to the gas switch depth.
Unless the dive is excessively long, or part of a multiple long dive
series, it is unlikely we’d have any significant decompression obligation below
21m for dives up to 50m in depth. This
principle also requires EAN50 to be available at 21m, dives using only O2 as a
decompression gas aren’t suitable for this method of gas planning and are not
suggested as a result – make a plan and stick to it for these dives!
If we assume
our ascent is at a maximum rate of 10m/min and we allow an additional 1 minute
at the bottom to signal the team and start the ascent, plus 2 minutes at 21m to
change gas as a team we get the following gas consumptions:
|
30m Depth
|
35m Depth
|
40m Depth
|
45m Depth
|
50m Depth
|
Max Depth Pressure
|
4.0 ATA
|
4.5 ATA
|
5.0 ATA
|
5.5 ATA
|
6.0 ATA
|
Average Ascent Depth
|
25.5m
|
28.0m
|
30.5m
|
33.0m
|
35.5m
|
Average Ascent Pressure
|
3.6 ATA
|
3.8 ATA
|
4.1 ATA
|
4.3 ATA
|
4.6 ATA
|
Ascent Time
|
1.0 min
|
1.5 min
|
2.0 min
|
2.5 min
|
3.0 min
|
Gas Use at Bottom
|
80 litres
|
90 litres
|
100 litres
|
110 litres
|
120 litres
|
Ascent Gas Use
|
71 litres
|
114 litres
|
162 litres
|
215 litres
|
273 litres
|
Switch Gas Use
|
124 litres
|
124 litres
|
124 litres
|
124 litres
|
124 litres
|
Total Gas Use
|
275 litres
|
328 litres
|
386 litres
|
449 litres
|
517 litres
|
|
|
|
|
|
|
Assuming 11.3l twin tanks
|
12.2 Bar
|
14.5 Bar
|
17.1 Bar
|
19.9 Bar
|
22.9 Bar
|
You’ll
notice the gas pressure required in twin tanks is just under half the depth in
metres of the pressures above; if we allow for two divers both breathing at an
increased rate of 30 l/min our gas consumption rates increase by a factor of 3
(20 l/min to 60 l/min) – so the half the depth becomes 0.5 x 3 = 1.5 times the
depth in m.
Our
benchmark then becomes 1.5 times the depth in metres assuming we are going to
switch to EAN50 at 21m, we then complete the deco on EAN50 alone or a
combination of EAN50 and O2.
Putting it
all together
So, how do
we use this? Lets say we’ll do a dive to
50m and will be using EAN50 (in a 12l tank at 130 bar) and O2 (in a 6l tank at
110 bar) – how do we plan our gasses to suit the deco?
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| Bring a little too much gas for the dive is not a bad thing |
The first
factor is the back gas, we need 1.5 times the depth as a reserve to bring two
of us up to the 21m gas switch – 50m x 1.5 = 75 Bar in this case.
Now the
deco, we have 130 bar of EAN50 at 2 minutes per 10 bar, so 26 minutes of EAN50
gas. The O2 tank is a smaller 6l bottle
at 110 bar, at 1.5 minutes per bar that's just over 16 minutes of O2 deco.
So, if we
keep 75 Bar of back gas at 50m we can run up a total deco time of 26 minutes if
we use EAN50 alone, or 32 minutes if we use both EAN50 and O2 (16 minutes on
EAN50 and 16 minutes on O2).
Things to
consider
These
numbers are conservative but give some flexibility in deco on the fly as having
a little too much gas for a dive is rarely a problem; these figures give ball
park estimates and should be used in place of proper gas planning. We’d normally suggest factoring in maximum
deco times of gas into the equipment check at the start of the dive; for
example “I am carrying two decompression gasses,
EAN50 and O2, my EAN50 is in a 12l bottle with 160 bar of pressure giving me a maximum
of 32 minutes of decompression gas, my O2 is also in a 12l bottle at 90 bar of
pressure giving me 27 minutes of decompression gas. Using both gases I have enough decompression
gas to complete 54 minutes of total decompression”.
A key
consideration if using a computer to provide total deco time of time to surface
(TTS) values is what this TTS time is based on, many computers use all gasses
in memory to estimate this time – it’s important the gasses are switched on or
off correctly when using this type of gas planning – for example if I am diving
with 40 minutes of EAN50 and 10 minutes of O2 I need to consider I may not have
enough O2 to complete the required deco if my deco is longer than 20 minutes –
it’s best to switch off the O2 in the computer and use the TTS based on EAN50
alone – the O2 can always be switched back on the ascent portion of the dive or
after the switch to EAN50 occurs.
Ratio
decompression is also another method of providing decompression information on
the fly, however we don’t suggest this should be used as a primary method of
decompression as the availability of more accurate technical computers at a
relatively low price and excessive times normally given for ratio decompression
for dives in the 40-50m range make this technique less than ideal. Ratio decompression does however make an excellent
back-up tool.
Key Rules
There are basically
three numbers to remember for this method of gas planning, as follows:
- EAN50 gives 2 minutes of gas per
10 bar in a 12l bottle
- O2 gives 3 minutes of gas per 10
bar in a 12l bottle
- Back Gas reserves in bar should
be 1.5 times the depth in metres when using 12l twins
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| A predive review of all gases carried is recommended | |
|
Try using
these rules at the start of each dive as a double check on how much deco gas is
available to each diver, it’s a good practice to get into regardless of whether
deco on the fly will be performed or not.
