DIVE PLANNING
Introduction
This section is about planning specific dives: no-decompression dives, dives with Deco, and even those with a bit more Deco – varying in complexity.
Nevertheless, dive planning always has the same components: Gas planning – how do I ensure I always have enough to breathe? Deco or no-decompression planning: How do we, as a team, organize the ascent? And emergency planning: How do we manage the situation if something goes wrong?
What we naturally cannot provide here is the planning of logistics for a specific dive – the waters worldwide are simply too diverse for that, and it’s beyond what this course aims to teach.
For dive planning beyond no-decompression limits, you should use planning software. We recommend Subsurface, which is accessible to everyone as open-source software. Of course, you can use any other planner.
Subsurface
Subsurface is recommended as a planner for more demanding dives. The software is completely free and open source. In the planner, you can display heatmaps to monitor tissue saturation.
Gas planning
A course on Deco theory, and we’re putting gas planning before Deco planning? There’s a good reason for that: while DCS is unpleasant, it rarely takes a severe course. “No breathing gas left,” however, is a frequent trigger for fatal diving accidents. Gas planning is therefore by far the most important part of dive planning.
Of course, for more complex dives, you need to know the dive profile before you do the gas planning. In practice, you will usually keep both in mind simultaneously.
In this section, we assume that you know your SAC (Surface Air Consumption), can calculate it, and don’t just dive until your tank is empty. If you are unsure about this, you can review the basics.
Gas Planning: Basics
In this section from our Science of Diving course, you can review the basics.
Minimum Gas – Rock Bottom
The basis of all gas planning is always the same idea: How can I ensure that I can still safely reach the surface even if one breathing gas supply completely fails?
We know that equipment can fail, and we assume the worst-case scenario – and that at the worst moment of the dive, namely at maximum depth.
In the simplest case, which we will look at first, it’s about a recreational dive in a buddy team with a single tank.
If one of them runs out of breathing gas, the team must be able to reach the surface together with the remaining gas from one tank. How much gas is needed for this is often underestimated.
Calculate Minimum Gas: Decisions
To meaningfully calculate Minimum Gas, you need a few facts – and you must consciously make decisions at several points. The following points will help you make your assumptions transparent.
- Your and your buddy’s SAC: Some agencies work with a “standard SAC.” This might be appropriate in practice, but it can also be off – in doubt, too high (overly cautious) or too low (uncomfortably tight). Knowing your own SAC is always a better basis.
- Time at depth until actual ascent: After a problem, you don’t automatically start the ascent immediately. You organize yourselves, communicate, sort out the team, perhaps try a quick solution. Many plan 2 minutes here – but depending on experience, training, and scenario, this can be a bit less or more.
- Breathing rate under stress: This is difficult to predict. Temporarily, the SAC can increase sharply (even 4 to 5 times), but usually not for long – otherwise, the resulting panic is the bigger problem. A practical assumption often works: doubling of consumption under stress.
- Does stress consumption persist during ascent? If the situation is back under control, breathing can normalize. You can decide here whether to calculate with, for example, only 1.5 times the normal SAC for the ascent – or whether to conservatively stick to the value from depth.
- Additional reserve: A pressure gauge is not a precision instrument, and the “last” bars are practically not fully usable: at some point, the regulator no longer delivers stably below the set intermediate pressure. An additional reserve of 10 to 30 bar is therefore sensible.
With the Minimum Gas Calculator, you can simulate how the different factors affect your team’s minimum gas. How many bar must remain in the tank at the deepest point to guarantee a safe ascent in an emergency?
Gas Planning with Deco Gases
As soon as you carry an additional gas, gas planning changes as well. The bottom gas no longer has to last all the way to the surface, but only needs to get the buddy to the depth where you can switch to the Deco gas. That reduces the required amount significantly.
However, there’s another possible complication: the Deco gas can also fail. So you have to plan for both: getting the buddy to the gas switch, or compensating for the loss of your own Deco gas. We do not assume that both fail at the same time. In most cases, ascending with the buddy to the MOD of the Deco gas will require more gas, but you should run the numbers for both scenarios.
Since the tanks you have available are usually fixed, you start planning by entering the gases and tank sizes you will be diving with into the planner. You can usually enter your SAC, and sometimes your buddy’s SAC as well.
Next comes the dive profile and the Deco. Only once you know that can you see how much of which gas you’ll use if everything goes as planned. That’s helpful: when we look at the SPG, it should only confirm what we already know, and we should have checked beforehand how many bar should still be in the cylinder at each point in time.
Then you add the minimum gas. At the end of our bottom time, that must still be left. Minimum gas here is the larger value: ascent with the buddy, or Deco with failed Deco gas.
In practice, you’ll often estimate how much gas you’re likely to use, make sure you have enough with you, and then set minimum gas as one of the limits. If someone reaches their minimum gas, you start the ascent.
Gas planning can differ from planner to planner.
In Subsurface, you can enter one SAC rate for bottom time and another for Deco, but not a separate one for the buddy. You specify how much higher the consumption is—and note: the buddy must be included here; times 4 would be “two people breathing twice as much as normal.” And you enter the time needed to resolve the problem. In addition, you then need to disable the Deco gas once and see how the planned consumption changes.
In MultiDeco, you can enter your own SAC rate and the buddy’s separately and then decide which concept you want to use to determine the turn point. The version presented here is closest to Rock Bottom DIR; the default assumption is 2 minutes to resolve the problem and a 1.5× higher consumption than normal. You can also display plans for a gas loss and variations in depth and time, or find the maximum bottom time based on the gas supply. That makes planning a bit easier, but the program also costs some money.
Since the basic rules are the same for all planners, you can use the following quiz to check if you have understood the most important points.
Ascending: No-Deco, Deco, More Deco
The second important part of any dive plan is setting the limits – be it no-decompression limits, the GFs the team uses, the complete runtime, or the maximum TTS.
No-Deco Dives
Most dives take place without extensive planning. The maximum depth and often the maximum time are set, sometimes air consumption also determines the time. And then it is agreed to stay within the no-decompression limits.
As long as you have agreed on what minimum gas you should still have at each depth, this is completely sufficient for planning – no one needs to run through every dive on the computer beforehand.
What is important, however, is that “no-decompression” depends on the dive computer and personal settings. If you have decided to use a more conservative setting, this must be communicated within the team. You might then suddenly have “Deco” while everyone else is still within no-decompression limits. This is not a big deal, quite the opposite, but it does require more time in shallow water at the end of the dive, and that should be discussed.
This is especially important for guides: some divers will have Deco on their computer while you are actually guiding a no-decompression dive. That’s fine, but it’s always good to know beforehand – so, simply asking if anyone is using different settings and then being able to work with that is always good.
Deco Dives with and without Deco Gases
As soon as the no-decompression limit is exceeded, the dive should be planned a bit better. Simply “slipping into Deco” is not bad if it’s only for a few minutes and you are sure to have enough gas – but you should know what you are doing. And the longer the Deco becomes, the more thorough the planning must be.
Without Deco gas, the gas supply and the minimum gas requirements will very quickly limit the dive. If two people also have to do Deco together from one tank, you quickly reach the limit. Therefore, for more than a few minutes of Deco, a Deco gas is really sensible.
In the section on Deco gases, we have already seen how Deco gases improve and shorten decompression, and given tips on how to select them. In dive planning, we use our planner to create the profile we want to dive, with the gases we have chosen.
We don’t have to plan a rectangular profile; we can include multiple levels. Especially for shore dives, it’s normal to start shallower before reaching depth. To make the plan as close to reality as possible, we should calculate this. And if we know in advance that we don’t want to spend the entire time at maximum depth, but a part of it is shallower – such as when exploring a wreck from bottom to top – then we can plan that too. This makes the plan as close to reality as possible.
The plan is output in a runtime table: where you are at each minute of the dive. You can then simply follow this runtime rigidly – but this sacrifices flexibility.
Since we dive with computers today, and often with a backup computer, we can naturally use it. From our plan, we extract the information on how long the planned ascent will take in total. This is our maximum Time to Surface (TTS). We have ensured that there is always enough gas for this at our maximum depth, and we can simply dive until either the Minimum Gas or the agreed TTS is reached. The planned runtime then only serves as a backup.
Plan B: What if something goes wrong?
In both gas planning and Deco planning, we have already seen that you ALWAYS consider that something can go wrong. Any gas can fail, you can accidentally or due to an incident take longer than planned, and we have already provided for this in the basic dive planning. Even if something goes wrong, we can still safely reach the surface.
Here is a summary of the Plan B options for gas planning and Deco, and some considerations for equipment redundancy.
Gas Loss
We are able to compensate for the loss of bottom gas or a Deco gas.
We have calculated that our bottom gas is also sufficient for Deco if only one Deco gas is carried; or that one Deco gas is sufficient if two are actually carried. This ensures that everyone can safely reach the surface alone.
However, in a team, you have the option of getting Deco gas from your buddy. Either after they are finished with that gas, or you pass it back and forth multiple times. How exactly you want to handle this should be discussed beforehand – and practicing passing a stage bottle is always a good idea.
It was deeper/longer than planned
In planning, we calculated an emergency plan for 5 minutes more at depth and consider it acceptable if necessary. If it turned out to be a bit deeper than planned, you should keep in mind that the Minimum Gas calculation still holds at that deeper depth and the gas is still breathable. As long as you do not exceed the planned TTS, you can end the dive as planned.
Since the gases and our entire planning are designed for a specific maximum depth, you should not simply exceed it. If you want to go deeper – it’s better to plan correctly for it. Planning for what your certification allows and then diving something completely different is definitely not good practice.
Tips for More Flexibility
Sometimes you don’t yet know exactly where the most beautiful spots are, and you want to keep a certain range of depths open. In that case, for planning, you take the absolute maximum depth you are willing to go to and plan gases and Deco accordingly. TTS and Minimum Gas are set – but you can stay shallower than planned, which means you can dive longer until that limit is reached. If it’s cold or other factors limit the total time, you can additionally decide that already dived time plus TTS should not exceed a certain limit.
Redundancy: Everything Can Break
For simple dives within no-decompression limits, the topic of redundancy is not very common. Guides usually carry a bit of extra weight, a spare mask, perhaps even an extra fin strap, but for most divers, redundancy lies with the buddy or the guide.
The more complex the dive, the more we rely on equipment truly working. Therefore, we carry many items in duplicate, and for some dives, even in triplicate.
The basic rule applies: as much as necessary, as little as possible. Anything that would pose a serious problem if it failed must be carried in duplicate – but no more. I can’t continue diving without a mask, let alone read the computer? Second mask. If I have to cut myself free and a knife falls out of my hand, I want to be able to pull a second one. If the computer fails and I can’t just surface, I need a second one – or runtime and bottom timer.
And of course, redundancy is particularly important for gas supply. At depths greater than 40m, it is more than just sensible not to rely exclusively on your buddy. So, you will usually dive with a twinset or two sidemount tanks – this way, even with a gas loss, you still have part of your own breathing gas available. Help from your buddy is then good, but no longer essential for survival – everyone is redundant for themselves.
What if…? Real Emergencies
Unfortunately, even with the best planning and all necessary caution, a real diving accident or a medical emergency can occur during a dive.
Discussing this in detail is beyond the scope here. However, dive planning must also include how to proceed if something truly goes wrong.
- How do you get someone out of the water?
- 112 or other number, cell phone reception, time until emergency services arrive?
- Emergency oxygen – extra and/or from dive gases?
- Insurance hotline?
And what you can also discuss as a team: What risk would you be willing to take for certain problems underwater? You certainly don’t have to decompress a GF 40/60 if someone is having a serious medical problem – but where would you draw the line in each case?
