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Wednesday, 11 December 2013

Stops: Safety Vs Deco

So, I was recently asked by a fellow diver why it was possible to go through a safety stop but not a decompression stop? Basic Diving theory I hear some of you say, but here is the clarification.

A Safety Stop is NOT a mandatory stop when diving.

When we make deeper dives or stay longer underwater, we are strongly recommended that we complete a 3 minute safety stop at 5m to reduce the pressure gradient when surfacing.

What does that mean?

To explain that, we need a basic understanding of decompression, which, as divers, we should all have from our first training courses.

When we dive, we breathe compressed gas under pressure. At the surface the pressure around us is 1 bar and for every 10m we descend, the pressure around us (ambient pressure) increases by another bar. So, at 10m, the pressure is 2 bar, 20m the pressure is 3 bar etc...

Now, our bodies like balance (No, we aren’t hippy, pipe smoking, new-agers, we really do!)! And, as I am sure you all remember, nitrogen is the inert gas in our breathing mix that isn’t used in any metabolic process and is simply inhaled and exhaled... at the surface.

What our clever bodies try to do is balance the pressure of the gas dissolved in our bodies with the pressure of the gas we are breathing. So, as we go deeper underwater our body will dissolve more and more Nitrogen into our blood and tissues and, as we ascend it will release more and more nitrogen.

If we ascend too fast, the nitrogen is released too quickly and this is where bubbles can form and resembles that age-old example of the fizzy pop bottle shaken or champagne bottle opened too fast... which is bad!

A safety stop will slow down the ascent to allow a bit more nitrogen to come out of solution adding conservatism to our dives ie making it safer

Many dive computers will give you a 3 minute countdown when you reach 5-6m if you make any dive to 10m or beyond but it is just an added safety measure, which means, if there is an overriding factor such as gas loss, you DO NOT HAVE TO STOP!

On the other hand, a Decompression Stop is Mandatory.

When you have to complete decompression stop it means you have overstayed your allowable no-stop time. You have passed a no-decompression limit on your diving tables or dive computer and you MUST make a stop to reduce the nitrogen in your body sufficiently to ascend further or surface from your dive safely.


Some dive computers will add a safety stop onto the end of your 6m decompression stop for added conservatism for your decompression dives. It is often a good idea to add this extra conservatism if possible.

So, it is all there is the name really. A safety stop is for added safety and conservatism while a decompression stop is for needed decompressing. Unless you are a qualified decompression diver you should only ever have to be concerned with safety stops. However, in this day and age it is getting easier, as divers, to dive deeper and, once you get to around 35-40m your no decompression times are very short so you need to be aware of your times, depths and when you conservative safety stop becomes a mandatory decompression stop.