I did a lot of scuba-diving and Technology Ireland asked if I would write an article on the physiological physics (!) of scuba-diving. I promptly agreed, neglecting to mention that I had practically been kicked out of Physics class during my Leaving Certificate for having such low marks. (I ended up in Business Studies and the rest, as they say, is history….).
It got published, I got paid and nobody wrote in to say I got anything wrong.
The French call it the 'granny sport'. Still, it's hard convincing people that they can spend twenty-five minutes under sixty feet of water, blissfully exploring the nooks and crannies of the seabed.
Yet there are now nearly 3,000 scuba-divers registered with Comhairle Fo Thuinn - the Irish Underwater Council. And the numbers are growing. Not surprising really: Ireland boasts of some of the best diving sites in Europe.
Like any adventure sport, diving has its share of associated risks. But with proper training and a proper understanding of diving physiology almost anybody can safely enter the mysterious depths of the watery underworld.
One of the diver's chief concerns is the increase in pressure as he submerges. On land, atmospheric pressure is exerted at the rate of 14.7 lbs per square inch. But 33 feet underwater, the combined or 'absolute' air pressure is 29.4 psi - double the air pressure at the surface: at 66 ft. absolute pressure will be 44.1 psi: while at 99 ft. pressure is exerted at a rate of 58.8 psi. Boyle's Law states that the volume of a gas will increase/decrease in proportion to a change in pressure. For the diver, this means that air spaces in the body - chiefly the middle ear and the sinuses - will be squeezed.
The small air space behind the eardrum - the middle ear - is connected to the back of the nose by the eustachian tube which balances the pressure between the middle and outer ear.
As the diver descends underwater, the pressure in the outer ear will become greater than the pressure within the middle ear. Although the pain is impossible to ignore. the eardrum will deflect inwards and eventually rupture if the pressure is allowed to grow. This can happen in as little as eight feet of water.
To counter this imbalance, the diver must equalise the air pressure by forcing air into his middle ear. To do this, he performs the Valsalva manoeuvre: he pinches his nose and blows, forcing air back through the eustachian tube directly to the middle ear. This is carried out repeatedly as the diver continues his decent and is often accompanied by a 'popping' noise.
On the other hand, the sinuses - air spaces in the forehead and on either side of the nose - must. and usually do, equalise naturally. Problems will occur however, if the sinus openings become congested or inflamed. A diver suffering from a cold should avoid diving as the eustachian tube is likely to be blocked, preventing the Valsalva manoeuvre.
Divers breath air and not pure oxygen as is commonly believed. Since air is a combination of oxygen and nitrogen, atmospheric pressure( l4.7psi) is the sum of the independent pressures exerted by these two gases (Dalton's Law). Thus nitrogen exerts an independent or partial pressure of 11.76 psi and oxygen a partial pressure of 2.94 psi.
Since absolute pressure increases with depth, the partial pressures of nitrogen and Oxygen will increase proportionately. Thus one cubic toot of air at sea level will be compressed to one third its volume at a depth of 66 ft. Absolute pressure at this depth is now 44.1 psi, so the partial pressure of nitrogen increases from 11.76 psi (sea level) to 35.28 psi (that is 44.1 x 80%).
Henry's Law states that gas becomes more soluble when pressure increases. Therefore, the nitrogen element of the air breathed becomes soluble and is absorbed into the diver's body tissues. Moreover, partial pressure increases threefold at a depth of 66 ft, so the body absorbs three times as much nitrogen than at sea level. (Oxygen on the other hand is burned up as energy).
The amount of nitrogen absorbed at sea level is equal to the partial pressure of nitrogen at atmospheric pressure. The deeper the dive, the more nitrogen is absorbed;so the problem is eliminating this excess from the diver's tissues before he returns to the surface. The amount of nitrogen dissolved in his tissues will depend on the depth and time span spent underwater.
A diver is said to be saturated when no more nitrogen can be absorbed at a given depth (pressure) and the only way to dissolve more nitrogen is to increase the pressure - by diving deeper.
Excess nitrogen is eliminated through the respiratory cycle: as the diver ascends, the pressure drops and the nitrogen comes out of solution and is passed on to the lungs. But complete elimination takes time. So the extent of the nitrogen saturation will determine the diver's rate of ascent. If the diver comes up too quickly, the sudden drop in surrounding pressure will cause the nitrogen to bubble out of solution. If this happens, the diver is said to be super-saturated.
A small degree of super saturation can be tolerated. But if the pressure of the absorbed nitrogen is more than twice the surrounding pressure, the bubbles will be bigger and larger. And the diver will suffer from decompression sickness.
The most common analogy is the soda bottle which will bubble and fizz if opened too rapidly because of the sudden release in pressure (which is keeping the carbon dioxide in solution). The symptoms of decompression sickness can usually be seen an hour after surfacing, although a small percentage (about 15%) won't show for up to 12 hours, the time it takes to completely eliminate nitrogen.
The physical pain experienced by a diver during decompression sickness will depend on the location and size of the bubbles. Some of the symptoms include:
The notorious bends' occur when the bubble locate themselves in the body's joints so that the diver is literally bent over in pain - and possibly permanently deformed.
A diver suffering from decompression sickness needs immediate treatment in decompression chamber where the pressure is built up in order to force the nitrogen bubbles back into solution. At present, there are two chambers in Ireland - Galway and Craigavon - and there are plans to have another built in Dublin.
Decompression risks can of course be reduced by limiting the depth and duration the dive. Special tables have been drawn which gives estimates of how long a diver spend at any particular depth. The US dive tables are the most commonly used.
Some dives will have no stop' times: the diver can come straight up from a depth at a steady rate of ascent. Others will have graduated 'stop times': the diver will halt his ascent and spend a specified time at a predetermined depth before continuing his ascent to the next 'stop'.
Since it takes up to 12 hours for the excess nitrogen to be completely eliminated, the tables have worked out repetitive dive times for divers who wish to carry out a second or third dive during the day. Since nitrogen absorption is cumulative, repeat dives will be necessarily shorter and shallower.
Since these tables have been developed on the assumption that the diver is returning to surface pressure, it should be fairly obvious that flying is out of the question. Although they reduce the risks associated with decompression, these tables are not lOO% failsafe. The US Navy considers an accident rate of 5% acceptable.
The combination of pressure and nitrogen creates a further problem for the diver. As the partial pressure of nitrogen increases it begins to affect the central nervous system, causing a condition known as 'nitrogen narcosis'. Divers may begin to suffer the 'narcs' when they exceed 100 ft. The basic symptoms are giddiness and an inability to think logically.
The diver, who experiences a sense of euphoria, is the last person to realise what is happening. This poses a serious threat as the diver may become so narcotic, he may forget what he is doing.
Fortunately, nitrogen narcosis is a phenomenon of depth and will disappear without any after-effects once the diver returns to a shallower depth. One of the first instructions received by a trainee diver is that he must breathe properly. Some people erroneously believe that if they skip breaths - holding their breath between shallow breaths- they will conserve their air. Nothing could be further from the truth. One of the immediate dangers of breath- holding is that gas expands as pressure drops (Boyle's Law). Imagine a diver who breathes his compressed air at the bottom, holds his breath and begins to ascend. Because of the drop in surrounding pressure. the air expands in his lungs.
If he fails to exhale, the lungs will over- expand and the air bubbles will escape into his bloodstream. These air bubbles will most likely be carried to the brain where they will block blood reaching the brain tissue - causing permanent injury within minutes. Furthermore, breathing is triggered off mainly by the level of carbon dioxide in the lungs. A 5% level of CO, causes discomfort. At 10%, the diver loses consciousness. Hypercapnia (CO2 excess) happens when the diver's breathing rate is too low to eliminate the carbon dioxide from the lungs. Breath-holding would almost certainly induce hypercapnia, especially at depths where the partial pressure of CO, is increased so that even a 2% increase would cause major problems.
Snorkellers, however, have a technique of prolonging their stay underwater by blowing off the CO2 in their lungs by hyperventilating. This removes the trigger to breathe and they can stay much longer underwater, with one breath, without feeling the need to come up for air.
There are great dangers with this technique as the diver may start his ascent too late and blackout before he returns to the surface. Still, the physiology sounds a lot worse than it actually is.
As long as the diver knows what is happening and how to prevent serious accidents, he is likely to remain safe underwater. Fortunately, the standard of training is very high in Ireland and is carried out by diving clubs which are all registered with Comhairle Fo Thuinn.
