I realised when I was preparing the notes for this lecture that it will be pretty much impossible to discuss decompression without referring to nitrox diving & accelerated decompression using mixed gases. The intention of this lecture is to give an insight into decompression diving so I will restrict my references to nitrox & beyond as much as possible but you must realise that to truly understand decompression diving you have to understand the benefits of enriched air.
First of all, let's establish what it isn't. It is a dive that does not require any stops between the deepest part of the dive & the surface provided the ascent rate is within accepted limits (7m/min - 18m/min) because the body is capable of off-gassing the excess Nitrogen that has built up during the dive without injury.[Why, then, when we do no-deco dives do we do a safety stop?] The answer is that it provides an extra level of safety over & above that provided by the safe ascent rate & allows other factors to be catered for e.g. cold, fatigue, tiredness & so on. It is not mandatory, but is strongly recommended regardless of ability or experience & something that we as professional instructors will always do at the end of every dive. It is important to remember though, that it is not a 'stop' in movement, only a stop in vertical ascent & many of the dives that we do will allow us to poke around the shallows whilst doing the stop. Only on boat shot lines do we remain in one position but that is for logistical reasons.
[Difference between Safety Stop & Decompression Stop. Why must we do deco stops?] As we have heard, a safety stop is advised, a deco stop is mandatory - you must do it. There has been a build up of Nitrogen in your body & it cannot be safely off-gassed during the ascent. If you continue upwards you will risk injury or even death.
[Why?] As we dive deeper, the gas bubbles get smaller which is one reason why we consume more air - our lungs stay the same size but the gas bubbles are smaller so we need more to fill our lungs. On ascent, those bubbles start to expand but provided we ascend slowly enough there is no problem because they can escape as part of the natural respiratory system. The problem comes from two sources; either we ascend too fast in which case the bubbles expand to be larger than the vein or capillary in which they are moving & become jammed, or bubbles combine to form larger bubbles which again cannot escape. These latter bubbles are known as micro-bubbles or silent bubbles & in normal diving have little or no effect. A jammed bubble causes a 'bend' or Decompression Sickness to give it the modern term & we are in trouble. We cannot descend to clear it & it won't go on its own - if we are lucky, we will spend time in a recompression chamber & all will be well, if we are unlucky, we won't see the chamber, depending on where the bubble or bubbles are lodged.
How recompression chambers work is the subject of another lecture, but basically the patient is recompressed to a depth which will cause the bubbles to compress to a size small enough to allow them to escape the body.
By the way, there is a difference between Decompression Sickness & Decompression Illness.
[Anyone know?] The latter is DCS plus other related injuries such as Air Embolism, Pneumothorax etc.[Can anyone tell me what a Pneumothorax is?] Pneumothorax is an escape of air into the pleural cavity & can cause the lung to collapse. It can be diagnosed in several ways, but as with all lung injuries, is treated the same.
[which is?] Administration of 100% Oxygen. Hopefully, by now you can see why we do decompression stops. It is to allow the body time to expel the growing bubbles before they get too big to escape. Each stop depth is designed to allow this build up to safely exit the system which is why the levels are so close (3m apart).
[How deep is a deco dive?] Any dive deeper than 6m can become a decompression dive. The deeper you go the quicker the dive becomes a deco dive. For 10m it is in the order of 3 hours, for 50m it is about 2-3 minutes. These times can, of course, be extended by the use of Enriched Air, but that is the subject of another lecture.
Decompression diving has often been described as Glass ceiling diving. There is a depth beyond which you may not ascend until your body has off-gassed sufficiently to allow the ceiling to move. Many dive computers use this ceiling/floor concept to determine the optimum depth for off-gassing. As you ascend, you will be shown the ceiling depth. On arrival at a metre or two below that ceiling you will be warned not to ascend above it & are in the optimum zone for decompression. Eventually the ceiling moves shallower & you can follow it. Finally it clears & you are safe to exit the water. Some dive computers such as the Suunto D9 also add in a safety stop at the end of a decompression dive, but that then becomes optional with the argument previously.
Contrary to popular belief, Deco diving without a computer can be done. Don't feel you have to spend hundreds of pounds on a diving computer. A depth gauge & watch will do. All you need to be able to do is to move to a depth & stay there for a time. In fact some dive computers allow for a gauge mode to be entered, this just gives you time & depth & is designed so that you can run your own decompression profile which may be preferable to the one the computer is giving you. Remember,& this is vital - whether you go for a D9 with all the bells & whistles or a simple watch & depth gauge, you MUST plan in advance! A D9 is not a magic talisman! Decompression diving is serious, you cannot afford to run out of air on the ascent & abandon the stops - it could kill you!
There are numerous planners available for PCs or Palm Pilots such as VPlanner & Gap for the PC & JPlanner for the Palm Pilot. JPlanner is free to buy & use, VPlanner is shareware & Gap costs €130 to download. One point to bear in mind here; planning with a PC planner can be used either as the actual plan which will be run exactly, or it may be to give you an idea of the run time for the dive which will be done using your computer. Whatever you do, don't try to use both the planner & the computer, they are highly unlikely to produce the same results & may lead to errors in decompression. Planners & computers normally allow you to vary certain aspects of the decompression stops, such as the time, the first & last depths & so on. Also there are choices to include Pyle Stops, deep stop & shallow stops, so how do you decide?
This, unfortunately causes many arguments in the diving community. Some believe that Pyle stops are best; essentially half depth/pressure stops between the maximum depth & the depth of the decompression stop, others believe you should do your stops as shallow as possible & others are of the view that a stepped ascent is best. Personally if I am doing a recreational deep dive & expect a few minutes of decompression I normally switch my computer to Deep Stops. If I am working then I normally do the stops shallow because I need to take account of the fact that I have clients with me who may be running low on air so a shallow stop will be best. Either way I always perform the 3 minute safety stop to be on the safe side & dive using Nitrox as an extra precaution, but run an air profile on the computer.
Okay, so we know what we have to do, now then, how does it work in practise? Let's take an example of a deep dive to illustrate how these approaches vary.
HMS Stubborn is a submarine off the coast of Malta sitting in about 60m of water. Let's say we are planning to dive to 60m for 25 minutes. For this discussion I am not concerned with air consumption, but suffice to say it would be an extremely important aspect of the dive.
[What is the bar pressure at 60m?] You would be consuming 7 times the amount of air compared to the surface. [Given a surface rate of 20 l/min, how much would be consumed at 60m] 140l/min & could be up as much as 175 l/min!
Clearly a single 12l cylinder will not last long at these depths!
12l x 200bar = 2400l
2400 / 140 = 17 minutes. This does not include descent or ascent time.
For all these profiles actual ascent time is not included which would add an extra 4 minutes. These profiles are for illustration purposes only & should not be taken as accurate.
| Depth (m) | Time (Aladdin Series:No gas switch) | Time RGBM (e.g. D9 with Gas switch) |
| 60 | 25 | 25 |
| 30 | 1 | |
| 27 | 1 | |
| 25 | 2 | 2 |
| 21 | 2 | |
| 18 | 6 | 2 (Gas switch to EANx50) |
| 15 | 6 | 3 |
| 12 | 6 | 6 |
| 9 | 12 | 6 |
| 6 | 66 | |
| 4 | 20 (Gas switch to EANx100) | |
| Total Time | 156 | 71 |
[Does anyone know why the decompression time gets longer as we get shallower?] There are two reasons; Firstly because the relative pressure change increases between shallower depths compared to those deeper which causes a greater expansion of bubbles requiring longer to off-gas successfully. Secondly, the deeper stops are kept shorter because at those depths the diver is still on-gassing, although it may be to different tissue compartments & may in fact extend the decompression requirements shallower. Of course, most deco dives will be considerably simpler than those & may only include a few minutes of decompression at 3-6m. Using computers, performing a decompression stop deeper than that required by the computer is acceptable. For example, if the computer wants a 3m stop but it is in the sea on a rough day, it may be more practical to perform the stop at 6m instead. This will work, but take longer. In this particular case the computer would typically add about 15-20 seconds per minute; not a problem for a short decompression but significant in the previous example where several minutes could be added. Deeper stops take progressively longer to decompress. Obviously there is a limit where decompression will no longer take place & in fact you may be on-gassing instead.
As you can see, deco diving can be complex. Also, as you all learned during your training, backup plans are important - more so in deco diving. What if we stayed an extra minute at 60m? What if we went to 62m instead of 60m? Using a wrist mounted slate is vital for this type of diving. I would urge anyone considering moving into this type of diving to buy one. By the way, 62m causes an extra 3 minutes of decompression & the extra minute at depth adds an extra minute at 9m.
General equipment requirements for deco diving: Ideally a Twin 12l back-gas set.
Warmer suit than normal due to potentially long stops.
Spare computer/timer
Slate for plan/backup plan
[The original question was 'What is decompression diving'.] Decompression Diving is: Diving that requires a diver to take planned stops during the final ascent to allow excess Nitrogen within the body to safely expel.
I have deliberately left out a discussion of tissue types, M-values & tissue saturation because, whilst it is useful to understand it, it is not vital for decompression diving since all computers & planners automatically take this into account. If you want to learn more about those aspects then a quick 'google' will bring up enough articles on the subject to keep you happy for a while.
So, how do you get trained? There are numerous agencies offering training courses, but a gradual build up of experience with a fellow diver is by far the best way. I learned a lot of what I know diving with the likes of Dave, Jason & John, particularly Dave who taught me a lot about decompression, Pyle stops & ascent profiles. In the world of diving, however, years of experience & ability count for nothing when trying to purchase gas for nitrox, extended range & accelerated decompression so at some stage you will need to take an officially recognised course & you have the choice between:
PADI/DSAT Tec Deep Diver
(Diving Science & Technology - PADI Subsidiary)
BSAC Advanced Nitrox/Extended Range
SAA
Other organisations include IANTD, TDI
(International Association of Nitrox & Technical Divers)
(Technical Diving International)
[Where to next?] Basic Nitrox / Advanced Nitrox / Extended Range/ Trimix.