WHAT IS A DEEP DIVE? It means different things to different people. The idea of deep is not a constant but a state of mind.
Deep for new divers is a dive beyond their previous maximum depth. Deep for a relatively experienced diver in clear, warm water might be 35m, whereas the same diver in murky water on a wreck in the English Channel might feel too deep at 20m. For some technical divers these days, dives don’t qualify as deep until they are beyond 100m.
No matter what the actual depth is, however, techniques for approaching and dealing with a deep dive are essentially the same.


The deeper you go, the more air you use, so the risk of exhausting your breathing supply is more acute on a deep dive. So you need to know, before the dive, that you have enough air for the planned depth and time.
To do this, you have to know your consumption rate. Then you can work out how much you need, allowing for an adequate reserve just in case there is an emergency and you find yourself having to ascend with another diver breathing from your supply too.
I explained how to calculate your consumption rate earlier in this series (Five Things Tec Divers Do, February). Essentially you find out the water volume of the cylinder you’re using, then do a timed swim at a fixed depth.
You record how much air you used in bar and, referring to the cylinder volume, convert this figure to a surface rate expressed in litres per minute.


Unless you have been trained in planned decompression diving and are properly equipped for it, you should not deliberately plan to stay at depth beyond no-decompression-stop limits.
This is standard advice and makes complete sense. Getting into a situation in which you have nothing left to breathe but still have deco stops to complete before you can safely go to the surface is the stuff of nightmares. And it can easily happen if you don’t know what you’re doing.
However, when you do deeper dives, no-deco-stop times become shorter and there is always the chance that you may go beyond the limits accidentally.
It happens to divers every day because they’re having fun, are only human and tend to get distracted. Narcosis plays a role in this too (see below).
When you go into deco, you will see that your computer display changes. Instead of the remaining dive-time figure you’re used to seeing, you’ll see new depth and time readings.
The depth shown will usually be 3m or 6m. The time may be 3 minutes or more when you first notice it. If you don’t know what your computer’s “deco” screen looks like, check the operations manual now. It is far better to see this for the first time in the comfort of your own living room than when you’re under water at 30m.
When you’re on a dive and see this screen, the first thing to remember is that the computer is not telling you to ascend immediately to the depth shown. So don’t shoot up in a blind panic.
The computer is just letting you know that you should go no shallower than this. This depth is your “decompression ceiling”. On a no-deco-stop dive, the surface is your ceiling.
The new time reading is the number of minutes you have to wait at your decompression ceiling before going to the surface. This is your decompression-stop time. On some computers the figure might include the time it takes to ascend from your current depth to your ceiling.
The longer you spend at the depth you are at, the more deco-stop time you will accumulate. So start ascending gradually, keeping a close eye on your computer.
If the deco-stop time keeps increasing or your decompression ceiling becomes deeper, then slowly move shallower.
When the reading stops increasing and starts to fall away instead, continue your dive at that depth, air-supply permitting, but don’t go deeper again. Neither your computer nor your body will like that.
What you will find is that eventually your deco-stop time will disappear and you will once again have the remaining dive-time reading that you normally see. This will happen quite quickly once you reach a depth of 10m or shallower.


A deep dive takes you further from the sanctuary of the surface, so it is even more important than usual to be sure you have the contingencies covered and know exactly what to do if anything goes wrong.
This applies particularly to air-supply emergencies. Some options available to you on a shallow dive are not viable when you’re deep.
For instance, an uncontrolled freeflowing regulator will empty a cylinder in just over two minutes. To ascend safely to the surface from 36m, at a rate of 9m per minute, will take you four minutes. So at 36m, a direct solo ascent to the surface is no longer something you can include in your emergency plan.
If you have only a single cylinder with a single valve, you will be reliant upon a dependable, experienced, suitably equipped buddy to bail you out in the event of an air-supply emergency.
If you don’t fancy putting your life in your buddy’s hands, especially when narcosis at depth (see below) tends to make teamwork more difficult, then you need to take your own back-up supply.
A good choice for deep no-deco-stop diving is a 2- or 3-litre 200 bar pony cylinder. It is attached by brackets to the main cylinder on your back and has its own regulator.
This gives you a genuine alternative source of air and removes the need for an octopus second stage on your primary regulator. So if you already own standard scuba, all you need to buy is a first stage, and the cylinder of course.


Air is an intoxicating cocktail. Nitrogen, its major constituent gas, has a depressant effect on your central nervous system. Divers call it narcosis. Jacques Cousteau called it “l’ivresse des profondeurs”, the drunkenness of the depths.
As Cousteau poetically suggested, the effect is similar to alcohol. And like alcohol, it is dose-related.
The degree to which you are affected depends on the quantity you consume. The effects of narcosis are progressive and increase with time and depth.
At 30m a diver breathing air will experience symptoms such as mild euphoria and slow reactions. At 50m the diver’s judgement will be significantly impaired.
The subjective symptoms differ but the objective effect is the same. Often, it manifests itself as overconfidence, fearlessness or over-relaxation.
In an emergency this can produce confusion and slow response time. For instance, a diver who is deep on air when he finds that his regulator is freeflowing may, in his befuddled state, remain at depth as he tries to fix the problem instead of ascending immediately or swimming to a team-member for assistance.
You may hear divers claim that they don’t suffer from narcosis. Yet again, the alcohol analogy applies. Someone who has had a couple of drinks will often be heard to say that they think they drive better when inebriated.
Similarly a diver at depth will often feel more relaxed and comfortable, confident and capable than usual.
In fact, as with the drunk driver, a deep diver is actually euphoric and perceptually impaired, has a tendency to be less cautious, will accomplish tasks less effectively, react to events more slowly and have an inaccurate sense of the passage of time.
However, the good news is that to a certain extent you can apply techniques to accommodate your impaired state.
Yet again the alcohol analogy is useful. A man sitting at a bar, having had a few drinks already, will reach for his glass and knock it over to the general hilarity of all around him.
He learns his lesson, however, and the next time he is in the same situation he recognises the danger of just reaching out, knowing that his perception is clouded by the alcohol in his system and that there is a good chance that if he just extends his hand casually he will knock the glass over again.
So he concentrates fully on the task, slides his arm slowly across the bar until his fingers gently touch the bottom of the glass, then, closing his hand around it, he pulls the glass across the bar towards him slowly.
He then tilts his head to drink from it instead of testing his hand-to-mouth co-ordination by lifting the glass unnecessarily.
Similarly, a diver who recognises intellectually that he is impaired at depth and who is familiar with the effects of the narcosis has taken the first steps towards being able to deal with it.
The next key stage is to learn to concentrate on important issues such as time, depth and the tasks to be performed, instead of getting carried away with euphoria and allowing it to cloud the mind.
It’s important on a deep dive to concentrate on maintaining a tight focus. Slow down and exercise mental control over every movement.
Moving steadily and deliberately is especially important when trying to perform a task at depth. Success is far more likely if the action is performed in a rehearsed sequence of steps rather than in one continuous, flowing movement.
Similarly, it is essential that, before you start deep diving, you should spend time rehearsing emergency drills and self-rescue skills until they become instinctive.
Developing automatic correct responses to emergencies is the best way to combat the narcosis-assisted confusion that will prevent you finding the right solution if you rely on your intellect alone.