15 How much air do you need to be safe?
Description
This article is from the Scuba Diving FAQ, by njs@scifi.maid.com (Nick Simicich) with numerous contributions by others.
15 How much air do you need to be safe?
The following chart was produced by Dave Waller, and presents a
picture that I feel is conservative. You should probably assume that,
in an emergency, you will be breathing at one of the higher breathing
rates. It also assumes a 60 fpm ascent rate, which is considered too
fast by many computer models and some training agencies. Therefore,
these numbers should be considered minimums, and any deviation from
these conditions would be likely to cause these numbers to increase.
Total consumption (ft^3) # Total consumption (ft^3)
without 15_ft Safety Stop # with 15_ft Safety Stop [1,2]
#
Consumption rate (ft^3/min) # Consumption rate (ft^3/min)
Depth | 0.5 | 1.0 | 1.5 | 2.0 # 0.5 | 1.0 | 1.5 | 2.0
-----+------+-------+-------+-------#-------+-------+-------+------
60 | 1.66 | 3.32 | 4.98 | 6.64 # 2.75 | 5.50 | 8.25 | 11.00
80 | 2.33 | 4.66 | 6.99 | 9.32 # 3.42 | 6.84 | 10.27 | 13.69
100 | 3.10 | 6.21 | 9.31 | 12.41 # 4.19 | 8.39 | 12.58 | 16.78
130 | 4.45 | 8.90 | 13.36 | 17.81 # 5.54 | 11.08 | 16.63 | 22.17
150 | 5.48 | 10.95 | 16.43 | 21.91 # 6.57 | 13.13 | 19.70 | 26.27
200 | 8.48 | 16.96 | 25.45 | 33.93 # 9.57 | 19.14 | 28.72 | 38.29
Notes:
Total consumption includes 30 seconds at indicated depth, and
a 60_ft/min ascent rate.
Assuming a 1/2 consumption rate during a 15_ft safety stop
for 3 minutes.
The numbers beyond sport diving depths are here only for reference,
and not to encourage you to dive those depths. Redundant air only
reduces one of the dangers you would face in diving to those depths.
The largest Spare Air holds just under 3 cubic feet. The smallest
available pony bottle holds 13 cubic feet. You can look at the chart,
estimate your surface consumption rate, try to estimate what it would
be in an emergency, and see where you fit in.
It is almost certain that if you were diving deep, you'd want more air
than the chart shows, as you might need to make a longer decompression
stop.
While some people have tested bailout bottle ascents from as deep as
100 fsw, it should be emphasized that these tests were not performed
under stressful conditions. Typically, they are already neutrally
buoyant, ready to ascend, and are consuming less air than they would
in an emergency. Referring to the above chart, you can see that this
would be possible for a diver who had a consumption rate of 1/2 cubic
foot per minute, and who left immediately upon switching to their
bailout bottle rather than taking time to get settled.
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