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WHAT IS TRIMIX DIVING?
by Pieter Venter
Deep diving on compressed air is potentially dangerous for two
reasons: one, a high partial pressure of nitrogen becomes narcotic, and
two, at high partial pressures (such as would prevail at depths over 70m)
oxygen becomes toxic. An oxygen 'hit' resembles an epileptic fit and
usually leads to drowning of the victim.
One solution, therefore, is to add an inert, non-narcotic diluent gas to
the air thereby reducing the partial pressures of both oxygen and nitrogen.
Complications now arise because the lowered oxygen fraction can make the
Trimix gas hypoxic at moderate depth (oxygen levels too low to sustain
life), therefore a travel gas is required from surface to depth and vice versa.
More complications arise because of the rapid absorption of the inert gas
into the diver's tissues at these depths requiring the diver to make a
series of decompression stops to expel the inert gasses from his
tissues. Decompression can be accelerated by breathing gasses with an
elevated oxygen partial pressure (Nitrox) or pure oxygen, at shallow depths
(to avoid oxygen toxicity).
Trimix divers diving to depths of 100m or more therefore require three
different gas mixes; a bottom mix consisting of a helium-air mix, a travel
mix of plain compressed air (with or without added oxygen) and an
oxygen-rich decompression mix. Each is formulated to have minimal narcotic
effect and minimum decompression penalties. Decompression stops and times
are carefully calculated using a special computer software package.
The deep divers of Deep Diving CC generally use a gas mix similar to:
| Gas |
Helium |
Nitrogen |
Oxygen |
Volume |
| |
% |
% |
% |
(litres) |
| Air |
0.0 |
79.0 |
21.0 |
2198 |
| Oxygen |
0.0 |
0.0 |
100.0 |
890 |
| Heliair |
50.0 |
35.0 |
10.5 |
5227 |
Requiring a decompression schedule that may look like this (but usually
with longer stops for safety):
| Depth (m)
|
Volume |
Time
Min Required (min) |
RTime
(min) |
Gas |
Volume
(litres) |
| 30 |
|
2 |
2 |
Air |
169 |
| 100 |
|
13 |
15 |
Heliair10.5 |
3385 |
| 45 |
|
8 |
23 |
Heliair10.5 |
1376 |
| 40 |
|
3 |
25 |
Heliair10.5 |
192 |
| 35 |
|
4 |
29 |
Heliair10.5 |
240 |
| 30 |
|
1 |
29 |
Heliair10.5 |
34 |
|
5227 |
|
|
|
|
| 30 |
|
2 |
31 |
Air |
120 |
| 25 |
|
5 |
36 |
Air |
240 |
| 20 |
|
8 |
43 |
Air |
342 |
| 15 |
|
14 |
57 |
Air |
510 |
| 10 |
|
27 |
83 |
Air |
799 |
| 5 |
|
1 |
84 |
Air |
799 |
|
2198 |
|
|
|
|
| 5 |
|
39 |
123 |
Oxygen |
12 |
| 0 |
|
1 |
124 |
Oxygen |
12 |
|
890 |
|
|
|
|
Total dive time: 123 minutes
Time to fly: 8 hours
Time to clear: 17 hours
Breathing: 25.0 l/min diving; 15.0 l/min decompression.
THESE NOTES ARE FOR INTEREST ONLY AND MUST NOT BE USED FOR DIVE
CALCULATIONS. DEEP TRIMIX DIVING MUST NOT BE ATTEMPTED BY ANYONE WHO IS NOT 100% MEDICALLY FIT AND WHO HAS NOT UNDERGONE FULL TRAINING AND
CERTIFICATION.
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