Hi Guys,
I have just returned from a very interesting week with the Royal Navy, some of which was spent at their diving and submarine training establishment in Gosport, Hampshire (formerly HMS Dolphin).
One of the demonsrtations at the submarine escape training tank (SETT) brought home to me the physics of a swimming ascent and I think it useful to describe for Scubaboarders, at least two of the many demonstrations we were shown.
The SETT instructor descended to 30 metres in the bell, took a breath of air at ambient pressure, exited the bell in swimming trunks alone, and then performed a perfectly controlled buoyant ascent at no more than 5 metres per minute, breathing out gently all the way, without moving a muscle!
It took him well over five minutes to ascend, and he controlled his rate of ascent perfectly through adjusting his buoyancy by means of exhaling sufficient air to keep his lung volume constant against the ever reducing ambient pressure in the water column. He made no swimming actions at all. When he reached the surface, he was not even short of breath and was entirely composed.
I was initially puzzled by this party trick, attributing it to his unique level of fitness, but of course he did not actually experience any degree of hypoxia because the Pa O2 at depth was close to 4 x 0.21 = 0.84 bar and the ascent simply reduced this back to the more normal level of 0.21 bar (less his metabolic needs);- Unlike the situation with free divers, his lungs continued to oxygenate his blood.
There would have been a certain amount of CO2 retention during his ascent but clearly he was able to counter the reflex urge to breath
1) by reducing CO2 build up by his low activity levels and
2) because of his fitness and training.
So if you do run out of gas at depth, there really does seem to be no need to panic (- as long as you remain buoyant).
Ths brings me on to another demonstation. On this occassion the SETT instructor took a deep breath on the surface and demonstated he could float unaided. He then repeated the exercise and pushed himself down to the 9 metre mark, where he remained neutrally buoyant for about 30 seconds - the lung compression had removed all effective buoyancy.
He then pushed himself down beyond the neutral position and sank, at an ever increasing rate, until he reached the bottom and entered the bell.
The point of the exercise?
Jumping off a high bridge is a effective way of committing suicide because if you hit the water fast enough it will take you below the neutral point and you will not float to the surface.