Dive planning
All courses at some point are about planning your dive, sometimes they talk about it – but what exactly is meant by that? Do you need to know anything about it if you’re following a guide anyway?
We think: Yes. As a trained diver, you are responsible for your own dive, so you must ensure that you are comfortable with the plan and confident that you want to and can do the dive in that way. And you want to know that you can still cope even if something doesn’t go as planned.
To do this, you should have thought about three things: no-decompression limits or deco planning, gas planning, and a plan B for emergencies.
Sounds complicated? It is not at all. It’s only ever as complicated as the dive you’re planning.
Deco planning
Deco planning? Isn’t this something only for very advanced and technical divers?
If by that you mean planning quite accurately beforehand the time at a certain depth and the ascent with deco stops: Yes. But deco planning also means understanding what a reasonable dive profile looks like and making a decision about what you think is safe enough. To do this, you need to know a few things.
Where do the no decompression limits come from?
No-decompression limits are not like the Ten Commandments that fell from the sky, and they do not represent a hard, unchanging limit. No-decompression limits are a risk assessment: approximately one in 10,000 dives, probably slightly more, results in DCS. That is the commonly accepted risk.
But where do these boundaries come from?
First of all, from experience: Divers have become ill. Research has come out of that, and from that has come knowledge to a certain extent about what leads to decompression sickness. You want to reduce the risk of getting them – that’s what the whole deco theory is about. An introduction to this can be found in the SSI Science of Diving Manual and Decompression Diver and of course in the Punkfish course “Decompression Theory”.
Here is just a brief summary on where the no-decompression limits come from.
No-stop limits: Reach the surface without mandatory stop
Diving within no-decompression limits means that you can easily (slowly!) surface at any point in time. Upon reaching the surface, one is only supersaturated to the extent that the M-values, the critical limits, are not exceeded.
These limits can be found in every dive table, or of course in every dive computer. At any point during the dive, it shows how much time you can still spend at that depth. On deeper dives, you can reach this limit even on normal recreational dives, but if you ascend within the no-decompression limits, it will become longer again.
When surfacing after a certain time, you are not completely saturated, but according to the table you reach a certain “repetition group”, or the computer displays a residual saturation. This is a sign of how much nitrogen is in the body after the dive.
The deeper you go, the less time you are allowed to spend there. But: If you ascend slowly after the maximum time at depth, you will reach the surface with a smaller repetition group or a lower residual saturation than if you dive shallower for a longer time. Why is that?
If you want to see how saturation and desaturation progress during a dive, it can be useful to use the Subsurface planning software. Here, we show some dives that represent typical examples. Dives with rectangular profiles at depths of 18, 30, and 37 meters and a multilevel dive should suffice as examples here.
We descend rapidly and remain at depth until we reach the no-decompression limit according to the SSI table. Then we surface at 9m/min. The graphic then also shows the first five minutes on the surface.
Below the dive profile we see a heat map – a representation of tissue saturation. You can see what this means in the explanation for each profile.
Subsurface Dive Planner and Logbook
If you want to plan your dives in advance, planning software is significantly better than a table. Subsurface is freely available and has a very good visualization of saturation with the heatmaps. We highly recommend this tool.
Heatmap 50 minutes at 18m: The middle tissues are close to their M value when they reach the surface.
During a fairly leisurely dive to 18m to the no-decompression limit, the allowed tissue supersaturation (i.e., the M value) is reached in the middle tissues upon reaching the surface. The upper lines, the very fast tissues, are below their M value, the slow tissues are only very slightly saturated.
Heat map after 20 minutes at 30 meters. This is where fast bir medium tissues come into their own.
If we dive to 30m until reaching the no-decompression limit, quite fast tissues reach a saturation that makes a safety stop important. Upon reaching the surface, slightly faster tissues are at M-value, compared to the shallower but longer dive at 18m.
Heatmap for 10 minutes at 37 meters. At this depth, the fast tissues end the dive.
When we go deeper into the no-decompression limits, here at 37m, it’s the fast weaves that slow us down. When their M value is reached, we have to show up. The middle tissues are not yet particularly saturated.
Go to 40m, dive for 45 minutes and still come to the surface less saturated than after a dive to 18m? That’s exactly what you do on a multilevel dive.
To really savor the no-decompression time, you can of course do multilevel dives with the help of the dive computer – that’s what you actually do almost all the time in real diving. In this process, the fastest tissues are saturated at depth, but are allowed to desaturate again as they ascend, while the middle tissues continue to saturate.
When surfacing, the fast tissues have long ceased to be a problem. Nevertheless, several such dives in a row are suspected of triggering “undeserves DCS hits” disproportionately often.
Model tissues or compartments
Our body absorbs nitrogen at different rates in different places. The blood is saturated very quickly via the lungs, and less well perfused parts of the body saturate more slowly. Because the human body consists of an infinite number of different tissues, saturation is modeled in compartments, i.e. in theoretical tissues.
The 16 model tissues in the most well-known decompression model, Bühlmann ZHL-C, are represented in the heatmap by the lines that lie close together. The fast tissues at the top, the slow tissues at the bottom. The color indicates whether and to what extent they saturate and desaturate.
Half-lives
The individual model tissues saturate at different rates. In the beginning, the pressure from the outside is much greater than that in the tissues – saturation is fast. The more the tissues saturate, the smaller the pressure difference. They then saturate more slowly.
The time it takes for a tissue to travel half the distance of the pressure difference between gas and tissue is called the half-life. For the fast tissues, a half-life of 5 minutes is assumed, others up to one hour follow in the fast and medium range, the slowest has a half-life of over 10 hours. Relevant for recreational divers:inside are the fast and medium tissues.
M values
Any tissue can withstand a certain amount of supersaturation. The M-value indicates how high the inert gas pressure can be in the tissue: From which overpressure in the tissue does the risk of DCS become too great?
The heatmap shows how close the saturation in each individual tissue is to the M-value. Red means that 100% of the M-value has been reached; the further it goes towards yellow or even green, the further below this limit the inert gas pressure lies in this tissue.
What does a low-risk dive profile look like?
What we do at depth almost doesn’t matter – for the ascent, the following always applies: simply follow the desaturation curve….
How do you get that to work?
As a simple rule of thumb: Even after no-decompression dives, it’s good to plan for at least five minutes for the last ten meters. We take one minute to get from 10 to 5m, three for the safety stop, and then at least one more minute to the surface.
Yes, another whole minute to the surface! The end of the safety stop does not mean that you now shoot to the surface via power inflator. You just elegantly take another minute and go up slowly, really: SLOWLY. The pressure change is strongest in the last few meters, and it is precisely there that we can ensure with little effort that we desaturate noticeably in the last few meters and reach the surface with as little supersaturation as possible.
How well do I need to plan my dive?
As long as you dive within the no-decompression limits, you can simply surface at any time if there is a problem. Slow and controlled, but you just go up and then look further.
When diving like this, it is sufficient for planning purposes to estimate how much gas you still need at what depth to get up safely in case of a problem. How to do this exactly follows in the next section. Then you agree on a maximum depth, check the computer and fini regularly, go up on time, and all is well.
It is helpful to have a rough idea of the no-decompression limits: 45 minutes at 20m, 20 minutes at 30m, 8 minutes at 40m, something like that – the order of magnitude helps to know the dive profile in advance.
The situation is different if a direct ascent is not possible, e.g. if you make a deco dive. In that case, you should be sure that the air is not only bs to the surface, but also for the necessary stops. This makes planning much more complex – the knowledge for this belongs in a different course, the Decompression Diver or the Tek Extended Range course.
How much air (gas) is enough?
How much do I consume if everything goes well?
What do I need if something goes wrong?
How much do I need to get me and my buddy to the surface in one piece?
My planned gas consumption
To estimate which cylinder to choose for a particular dive, you need to know how much you are likely to use. For this, you need to know two things: Your own consumption, and the planned dive profile.
How much do I consume? Surface Air Consumption (l/min)
1. How many liters have I consumed?
At the beginning of the dive I had a full tank with 200 bar, at the end I had 50 bar left. So I have used 150 bar.
With this pressure was the air in a diving bottle, which has a certain volume.
150 bar from a 10l bottle is 150bar x10l=1500 barl
2. At what depth?
To know how much I breathe, I have to convert the consumption to what it would be on the surface – so, a volume.
Average depth 12m corresponds to 2.2 bar
1500 barl : 2.2 bar = 682l
3. In what time?
Last but not least, I need to know what I breathe in a certain period of time – in one minute can be measured quite well. Therefore, we calculate what volume we breathe per minute – thesurface air consumption (SAC rate).
Dive 45 minutes
682l : 45 min = 15.2 l/min
Liters consumed by pressure by time makes SAC (Surface Air consumption)
How much do you consume when things are not going well?
We know our SAC on a normal dive. But what happens when everything is no longer calm, but we are stressed and breathing hard?
When you breathe with exertion, your breathing rate increases. Normally it is 12-16 breaths per minute. During heavy work, it can rise to as many as 50 breaths/minute. In addition, the respiratory volume , normally about 0.5l, also increases up to 3l.
There is a wide range between 12×0.5= 6l SAC at rest and 16x3l=48l. Our normal SAC is already above the 6l resting breath. Can it really go up to 50l?
Some assume this and calculate their reserves according to it. In the case of firefighters, such high values are also quite measurable; they breathe significantly more in an operation on land, where they work hard with heavy equipment on their backs. But if you look at what is physically feasible underwater, it becomes clear that you can only sustain such breathing for a few breaths. A bit more controlled and above all not physically overexerted you are under water already. How much, you have to estimate for yourself – planning a little more conservatively than you actually believe is certainly always a good idea.
Rock Bottom: How much reserve is enough?
There is, of course, a reason that we think so much about the sufficient amount of gas: Having nothing left to breathe has fatal consequences under water. Even if it is a rare problem, the consequences can be so serious that we must take good precautions at this point.
If we know our SAC and that of our partner, we can calculate how much air we need at what depth to safely surface together at any time. This is the hard limit, the rock bottom.
In a critical situation – a medium-pressure hose bursts, there are only bubbles around you – you need a moment to sort yourself out. We assume 2 minutes here, which is rather little. Then we ascend together, at no more than 9m per minute, i.e. in a controlled manner.
We take our SAC into account, but take into account that we will no longer be able to breathe easily in this position. For this example calculation, we decided to assume that we breathe twice as much during problem solving as we normally do during diving, but only 1.5 times as much during ascent. This can be discussed and adapted for your own safety needs.
What this means can be clearly seen in the table for a SAC rate of 15l.
We assume a SAC of 15l/min here. This can of course be higher or lower. In any case, it makes sense to do the calculation yourself with your own SAC rates and those of your buddy and to define your own limits.
We have rounded up the values for the pure consumption for the ascent a little. Why? The last 10 bar are not so easy to breathe, and a pressure gauge does not always display as precisely as one would like. You can plan an even larger additional reserve here, we consider rounding up plus 10 bar to be the minimum that you should plan for.
If you would like to play through yourself which reserve might be useful, you can find a very simple Minimum Gas Calculator here. In contrast to the table, we have simplified things somewhat here and assume that breathing doubles for the entire time – this is also a common way, and we assume that both have the same AMV.
To be able to determine your basic assumptions yourself, you can use our Advanced Minimum Gas Calculator.
And if something goes wrong?
Even when we were planning for gas, we saw that we were basically planning for a severe problem – even in the really unlikely event that a gas supply fails completely, we have enough with us to reach the surface safely.
Of course, a similar thing applies to everything else that can go wrong. How exactly we deal with it depends on two factors: How likely is the problem? And what are the consequences when it happens? A torn fin strap is far more likely than a burst medium-pressure hose, but it’s such a small problem that most of us don’t take a spare strap into the water, and rightly so.
Probable triggers of accidents
Diving accidents always have a trigger – a problem that started the chain of events that led to an incident. DAN has compiled what that is from 500 accidents in 2017.
The two lists here come from the DAN Annual Diving Report 2019 Edition, which is freely available and really worth reading.
We see here that “No more breathing gas” is by far the most common. That’s why we take the topic of “Rock Bottom” so seriously: Suddenly having nothing more to breathe can really be avoided. Immediately after that comes entrapment – you get stuck somewhere. This can be partially avoided if you don’t dive in caves that are too narrow or for which you are not trained, and a cutter can help to free yourself from lines or fishing nets. And then in third place come the equipment problems.
Equipment problems
Even though our equipment is safe and well maintained, problems can always occur. Just a grain of sand in the inflator can cause the jacket to inflate permanently; a regulator can blow out for various reasons; every piece of plastic breaks at some point and every O-ring bursts.
Such problems repeatedly lead to incidents that are reported to DAN. As you can see in the list, these are problems that can usually be solved. The correct reaction is part of the OWD course – but be honest: Have you practiced it again afterwards?
Free-flowing Regulator
Apparently, Free-flowing regulators have repeatedly led to accidents. The main way to avoid this is to do one thing: From time to time, practice continuing to breathe from a regulator that is blowing off. The first time you should have practiced this in the OWD course – but to not forget it afterwards, you have to do it again and again.
A regulator can suddenly blow off under water because it is icing up. This can happen if there is some moisture in the bottle, or if ice forms in the second stage. It can also block something for another reason – but whatever happens: When a regulator is free-flowing, you keep getting air. This is still enough for a regular promotion.
BCD inflates
If the jacket inflates by itself, it is usually because the inflator button is stuck. Pushing and pulling the first can solve the problem. If not, the inflator hose must come off: You practiced this once in OWD. But if you never did it again after that – try it again. It doesn’t require strength, but technique, and you have to remember that every now and then.
Nothing left to breathe
If this happens, there has been a massive error in your gas planning – equipment problems alone cannot cause this. You always have a buddy, or redundant equipment, and you know where to get something to breathe if your supply should fail.
Of course, it can happen that a single controller no longer delivers anything. For example, when rust from the bottle clogs the filter. Or when water blocks the valve from the inside. In the Regulator itself, this occurs, if at all, as a freak accident – incredibly rare.
When hoses burst
Which is worse: a broken high-pressure hose or a leaking medium-pressure hose?
High pressure sounds much more dangerous, but in this case it is not. The cylinder pressure, i.e. the high pressure, comes out of the first stage only through a tiny hole in the hose. Even if this bursts, the air only flows out very slowly.
The medium pressure hose, on the other hand, supplies us with air, and delivers a large amount of it at 10 bar above ambient pressure. The hose diameter and the outlet from the first stage are many times larger than those of the high-pressure hose – the air flows out completely within a few minutes.
With a burst high-pressure hose, you can still finish the dive calmly; with a burst medium-pressure hose, the path leads directly upwards.
Stay in training
Dive planning also includes ensuring that you and your buddy are able to deal with problems. What helps here is regular training. You can repeat a few skills with each other in the shallows at the beginning or end of the dive, or do a training session with a diving instructor. It is your responsibility not to let the skills learned in the OWD and later rust. If something goes wrong, your quick reaction is important.
