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Thermal Considerations at Great Depth
By: Steve Burton In the last few articles in this series we have looked at the effects of decompression, oxygen toxicity, narcosis and breathing gases selection, which concerns all divers even at recreational diving depths. An observation common to all parameters, is that predicting a diver’s physiological response for bounce dives below 50m (165ft) begins to be a bit of a gamble…and beyond 100m (330ft) ‘all bets are off’.
The large body of formal man tested research available for planning Heliox saturation dives contrasts starkly with the hearsay anecdotal evidence whispered in internet chat rooms. These Chinese whispers mixed together with harebrained decompression schemes tested on salmon or in gel samples…prove to be a predictably risky mix that causes several faces to go missing from my photo album every year in the quest to push the world deep dive records down a few metres more.
One area which we might consider to be reasonably predictable is our bodies’ response to thermal exposure. But this too proves to be just as problematic when descending through 313m (1,027ft) of water and 31 bars of pressure change in just 12-minutes. Consider a recreational scuba dive made almost anywhere off the coast of Thailand in warm tropical seas. The diver has a normal body core temp 37ºC (98.6ºF) and the warm coral seas average 28ºC (82ºF). Although most divers will wear a 3mm shorty wet suit for comfort on this dive, anyone wearing just a bathing suit could comfortably complete a 1 hour dive at these temperatures, and expected survival times with floatation aids in a ‘lost at sea scenario’ would be in excess of 12 hours. In comparison, let’sconsider an extreme Trimix bounce dive to 313m (1,027ft) carried out a few kilometres further out to sea, still in tropical Thailand. As the 12-minute decent progresses, the sea temperature becomes anything but tropical… Notes: - Temperatures quoted are typical summer temperatures.
- Much of the Baltic Sea freezes over during the winter months!
The particular Trimix decompression requirements for this open ocean dive required the following thermal exposures:- - 10 mins spent below 200m (656ft) immersed in water colder than 6º Celsius (43º Fahrenheit)
- 30 mins spent below 100m (330ft) immersed in water colder than 14º Celsius (57º Fahrenheit)
- 90 mins spent below 50m (165ft) immersed in water colder than 22º Celsius (72º Fahrenheit)
- 6 hours 40 mins spent in-water decompressing in average temperature of 25º Celsius (77º Fahrenheit)
If immersion cooling wasn’t enough, the additional cooling produced by breathing the thermally conductive Helium based Trimix breathing gases at high ambient pressures results in more than half the bodies heat producing capabilities being used up just to by inhaling and exhaling the breathing gas.
The body’s expected survival times in cold water with light clothing are given in the table below:
It is apparent then that significant thermal protection is required for a diver contemplating this dive. Although underwater breathing gas heaters and battery powered heating pads could have been used, the successful compromise for short duration Artic conditions on the bottom and long duration tropical conditions near the surface has proved to be a tri-laminate dry-suit and Thinsulate undergarment.
A wet suit of any type would have proven wholly inappropriate for this dive, due to suit compression problems at depth. The 31bar ambient pressure would have resulted in even a 5mm suit being compressed to a ‘paper thin’ 0.16mm thick at the bottom depth. Unconsciousness due to severe hypothermia and another missing face in my photo album would be the likely outcome in this case..
So how do commercial divers manage to live and work underwater for up to 28 days in freezing conditions? The answer is simple. Resting periods are spent in a dry pressurized thermally controlled environment, with working shifts undertaken using a commercial diving suit utilising both a hard hat to protect the head and to keep it dry and warm, and with the body heated by a ‘hot water suit’ that receives a continual supply of heated water from the surface. The diver can manually adjust the temperature of the suit at any time by the use of a manual ‘Flow/Dump’ valve on his suit that varies the amount of hot water flowing into the suit.
| Deg. C | Deg. F | Exhaustion/Unconsciousness Sets in within | Time (with flotation) | | 0 | 32 | under 15 min. | under 15 to 45 min. | | 0-5 | 32-41 | 15 to 30 min. | 30 to 90 min. | | 5-10 | 41-50 | 30 to 60 min. | 1 to 3 hours |
| Shallow Water | 28'C (82'F) | Tropical conditions. | 3mm. Shorty | | 50metres (165ft) | 2'C (72'F) | The Red Sea in low season. | 5mm Full Length. | | 100 metres (330ft) | 14'C (57'F) | 15 to 30 min. | 7mm Wet or Dry Suit | | 313 meteres (1,027ft) | 41-50 | 30 to 60 min. | Day suit + woolly bear. |
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