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Protection
of the underwater environment is a prime consideration when
diving, particularly when taking photographs. Major diving
agencies teach buoyancy skills and posture. Numerous articles
for photographers on how to approach and leave a coral reef
without causing damage are quite rightly a regular feature
in magazines, books and web sites. Achieving these skills
is ultimately a matter of practice.
However,
the most important time to achieve neutral is when? For one's
own safety it must be prior to surfacing, when undertaking
asafety stop or a decompression stop. Againall a matter of
practice. Well is it? Perhaps just a little bit of knowledge
may help.
We
know at the start of a dive our cylinder, hopefully full of
nitrox at between 20.9% to 40.0% oxygen, weighs more then
at the end of a dive. Hence neutral buoyancy and the art of
remaining at a decompression stop, which may be at a depth
of no more than 3 metres, relies on us having the correct
amount of overall weight just prior to surfacing. I've used
e fairly rough and ready rule of thumb that a 12 litre cylinder
is approximately 2 kg heavier when filled to 232 bar than
it is at 50 bar. A 15 litre cylinder circa 2.5 kg heavier.
However, what I didn't know was how accurate this was.
Recently
a non?diving colleague asked me about air density. In order
to help him with a demonstration he was preparing for a school
I did some approximations based on the above f igures, but
how close was I? So I tried to f ind a ref erence to air density
and found it wasn't too easy to track down a f igure. By chance
I was off to the Science Museum and decided to quiz them on
the subject.
After much searching an enthusiastic chap come up with the
answer.
Air
density
At 200 °C and 760 mm of mercury, i.e. I atmosphere, the
density of air is 1.204 mg/cu cm. Put in another way 1 litre
of air weighs 1.204 gms
Not
a lot of people know that.
So
to see what that means in the real world look at the Table
below:
| Size
- Litres |
Start
- Pressure |
End
- Pressure |
Weight
Difference |
| 15.0
l |
232
bar |
50
bar |
3.2869
kg |
| 12.0
l |
232
bar |
50
bar |
2.6295
kg |
| 10.0
l |
232
bar |
50
bar |
2.1913
kg |
| 03.0
l |
232
bar |
50
bar |
0.6574
kg |
Reproduced
from in
focus 78 (Sping 2004)
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