If you’re a PADI-trained diver, your instructor will have explained to you the wisdom of a safety stop at the end of every dive. BSAC divers are given similar advice. What’s the rationale behind this?
There are purely practical reasons for making a short pause before your final ascent. It allows you to double-check your bottom time and confirm that you’re within the no-decompression limits for the dive. It also gives you the opportunity to establish close contact with your buddy so that you can arrive at the surface together and thus minimise the possibility of being separated by adverse surface conditions (swell, currents, fog, etc). Most importantly, however, it gives you time to fine-tune your buoyancy so that you can ascend the final few metres at a controlled and preferably much slower pace.
So why is it so important to make safety stops and why should you ascend more slowly for the last few metres?
During any dive, your body is continually absorbing nitrogen from the air you breathe. This should not be news to you whichever agency you trained with – it’s one of the fundamentals of scuba diving. Even at relatively shallow depths, this nitrogen saturates your blood and nervous system – termed “fast” tissues – in a fairly short space of time. As soon as you start to ascend at the end of the dive, this nitrogen starts to come out of solution into your blood which then carries it away to your lungs so that you can breathe it out. All this happens without your noticing it.
Although your blood and nervous system are able to release dissolved nitrogen fairly quickly, it’s important not to ascend so quickly that they’re unable to do so in a safe manner. A good analogy is a bottle of fizzy drink. The liquid in the bottle is saturated with carbon dioxide at pressure: if you open the cap gradually the gas is allowed to come out of solution in a controlled manner and the operation is unspectacular; if you release the pressure suddenly, bubbles will form because you’re forcing the gas to come out of solution too quickly.
The human body behaves in a very similar way to the bottle of fizzy drink. Experiments have shown that where the pressure of the nitrogen dissolved in your body tissues is higher than the surrounding water pressure by a factor in excess of 1.5, instead of being released harmlessly, it’s more likely to accumulate into bubbles which can lead to decompression sickness (DCS). Even tiny – so-called “silent” – bubbles which don’t cause overt signs of sickness, can contribute towards what is known as “sub-clinical DCS” – the feeling of fatigue which you sometimes experience when you’re driving home after a weekend’s diving – longer, deeper dives causing greater fatigue. This is why nitrox, or “enriched air” divers, claim they don’t feel so tired after a series of dives: nitrox contains proportionately more oxygen than ordinary air and less nitrogen is therefore absorbed during diving.
PADI’s recommended depth for safety stops is five metres. This figure wasn’t chosen at random – the water pressure at five metres is 1.5 times the pressure at the surface, and this is exactly the figure above which experiments have shown that bubbles are more likely to form if you subject saturated tissues to a sudden pressure drop. However, if you pause your ascent for three minutes at five metres, you give these tissues time to “catch up” – that is, to release enough nitrogen to bring them to roughly the same pressure as the surrounding water (five metres = 1.5 bar). When you make your final ascent to the surface, especially if you do it slowly in order to release the remaining nitrogen in your “fast” tissues in a gradual and controlled manner, you’re greatly reducing the likelihood of DCS.
Of course you still have nitrogen dissolved in your slower tissues (such as bone and muscle), especially after longer, deeper dives, but these are less of a concern since these tissues release nitrogen much more slowly and are less likely to be affected by bubble formation during a normal ascent. This is a complex subject: if you’re interested, it’s covered in more detail in The PADI Encyclopedia of Recreational Diving (Chapter 2: The Physiology of Scuba Diving).
If your diving tends to be mostly around or beyond the 30m mark or you occasionally carry out dives involving staged decompression procedures, you may be interested in reading about deep safety stops: there are a number of useful articles on the Web, two of which are listed below.
Note: you should not attempt to dive below the depth limit for your level of certification, nor should you plan decompression dives without the appropriate training and equipment.
“The Importance of Deep Safety Stops: Rethinking Ascent Patterns From Decompression Dives” by Richard L. Pyle
“Clearing Up The Confusion About Deep Stops” by Erik C. Baker