Pressure in relation to diving and aviation
It should be clear from the previous paragraph that for every 10.3 m (approx. 10 m in sea-water) a diver descends the pressure on his body increases by one atmosphere. The aqualung diving-suit incorporates a rubber helmet fitted with a circular window and supplied with air from compressed-air cylinders carried on the wearer’s back. Using this apparatus, experienced divers can descend for very short periods to a maximum depth of about 60 m, where the total pressure is seven atmospheres. At depths in the neighbourhood of 45 m they can work for periods of about 15 minutes. It is dangerous to stay longer at these depths, since, as a result of the high pressure, an excess of nitrogen dissolves in the blood, and on return to the surface nitrogen bubbles form in the blood in the same way that bubbles form in a bottle of soda water when the cap is removed. Such a-condition causes severe pain or even death. and in cases of emergency the diver is immediately placed in a decompression chamber. This is a steel tank full of compressed air and, by slowly reducing the pressure over a long period, the nitrogen becomes gradually eliminated from the blood without forming bubbles.
The danger to health from the painful diver’s bends, as this condition is called, is greatly reduced if a mixture of 8 per cent oxygen and 92 per cent helium is used in the gas cylinders. An American diving team claim to have achieved, in 1981, a safe working depth of 686 metres using Trim ix, a mixture of oxygen, nitrogen, and helium.
In contrast with the problems encountered by the diver, the crew and passengers in aircraft flying at high altitude would experience difficulty in breathing and consequent danger owing to the low atmospheric pressure. The problem is overcome by “pressurizing” the aircraft. All openings are sealed, and a normal atmospheric pressure is maintained inside by the use of air pumps.