Block 3 · Practice: Dive Planning

Introduction

This part is about planning concrete dives: no-decompression dives, dives with decompression, and dives with a bit more decompression — different levels of complexity.
But the building blocks of planning are always the same: gas planning (how do we make sure we always have enough to breathe?), no-deco/deco planning (how do we organize the ascent as a team?), and emergency planning (what do we do when something goes wrong?).
What we cannot provide here is the logistics planning of a specific dive site — conditions around the world are simply too different, and that sits outside the scope of this course.
For any dive that goes beyond no-deco limits, you should use proper planning software. We recommend Subsurface, because it’s open source and accessible to everyone. Of course, you can use any planner you prefer.

Gas planning

A decompression theory course — and we put gas planning before deco planning? Exactly. DCS is unpleasant, but severe outcomes are relatively rare. “Out of breathing gas”, on the other hand, is a common trigger in fatal diving accidents. Gas planning is therefore, by far, the most important part of dive planning.
Of course, for more complex dives you need an idea of the profile before you can finalize gas planning — and in real planning you keep both in view.
In this section we assume you know your SAC/RMV (respiratory minute volume), you can calculate it, and you don’t just “dive until the tank is empty”. If you’re unsure, revisit the basics first.

Minimum gas / Rock Bottom

The foundation of gas planning is always the same question: how do we make sure we can still reach the surface safely if a breathing gas supply fails completely?
We know equipment can fail, so we plan for the worst case — at the worst moment: at maximum depth.
In the simplest scenario (which we start with) this is a recreational buddy dive on a single cylinder.
If one diver is out of gas, the team must be able to reach the surface together on the remaining gas from one cylinder. The amount of gas required is often underestimated.

Calculating minimum gas: the decisions

To calculate minimum gas in a meaningful way, you need a few facts — and you have to make conscious decisions at several points. The points below help you keep your assumptions transparent.

• Your SAC/RMV and your buddy’s: Some agencies teach a “standard SAC/RMV”. That can work — but it can also miss the mark, either too high (overly conservative) or too low (uncomfortably tight). Knowing your own SAC/RMV is always the better foundation.
• Time at depth until you actually start ascending: After a problem, you don’t automatically go up instantly. You organize, communicate, stabilize the team, maybe try a quick fix. Many plans assume 2 minutes here — depending on experience, training, and scenario, that can be slightly less or more.
• Breathing rate under stress: Hard to predict. Short-term SAC/RMV can jump dramatically (even 4–5×), but usually not for long — otherwise panic becomes the bigger issue. A practical assumption that often works: double the consumption under stress.
• Does stress breathing remain elevated during the ascent? Once the situation is under control, breathing can normalize. You can decide whether to plan the ascent at, for example, 1.5× normal SAC/RMV — or stay conservative and keep the “depth” stress value for the entire ascent.
• Additional reserve: Pressure gauges are not precision instruments, and the “last” bar are not fully usable: at some point the regulator will no longer deliver stably as supply pressure drops. An extra reserve of 10 to 30 bar is therefore sensible.

With the minimum gas calculator you can explore how these parameters affect your team’s minimum gas. How many bar must still be in the cylinder at the deepest point to guarantee a safe emergency ascent?

Gas planning with decompression gases

As soon as you carry an additional gas, gas planning changes. Your bottom gas no longer has to last all the way to the surface — it only needs to get the buddy to the depth where you can switch to the decompression gas. That reduces the required bottom gas significantly.
But it also adds another failure mode: the decompression gas can fail too. So you plan for both: getting the buddy to the gas switch, and compensating for the loss of your own deco gas. We do not assume both fail at the same time. In practice, “ascend with the buddy to the MOD of the deco gas” often requires more gas — but you should run both scenarios once.
Because cylinder sizes are usually fixed, planning typically starts with entering the gases and cylinder volumes you will dive, often together with your SAC/RMV (and sometimes your buddy’s).
Next comes the profile and the decompression schedule. Only once you have that can you see how much of each gas you will use if everything goes as planned. That is helpful: when you look at your SPG, it should confirm what you already know — including how many bar should be left at any point in the dive.
Then you add minimum gas. At the end of your bottom time, minimum gas must still be available. In this context, minimum gas is the larger of the two values: ascent with the buddy, or decompression with a failed deco gas.
In the real world, you’ll often plan the expected gas use, confirm you have enough, and then treat minimum gas as one of your hard limits. When someone hits minimum gas, you start the ascent.
The details differ between planners.
In Subsurface you can enter one SAC/RMV for bottom time and another for decompression, but not a separate value for your buddy. You choose a “consumption increase” factor — and important: this needs to include your buddy. A factor of 4 means “two divers breathing double their normal rate”. You also enter the problem-solving time. Additionally, you should run the plan once with the deco gas disabled and see how consumption changes.
In MultiDeco you can enter your SAC/RMV and your buddy’s separately and choose how you want to determine the turn point. The version shown here is closest to Rock Bottom; it typically assumes 2 minutes of problem-solving time and 1.5× normal breathing rate. MultiDeco can also generate loss-of-gas plans and variations in depth and time, or find the maximum bottom time based on available gas. That makes planning easier — but the software is not free.
Since the core rules are the same across planners, you can use the quiz below to check whether you’ve understood the key points.

Ascending: no-deco, deco, more deco

The second key part of planning is setting limits — no-deco limits, the team’s gradient factors/settings, the full runtime, or a maximum time to surface (TTS).

No-decompression dives

Most dives happen without detailed planning. You set a maximum depth and often a maximum time; sometimes gas consumption defines the time. And you agree to stay within no-deco limits.
If you have agreed on minimum gas for the relevant depth, that is often enough — nobody needs to run every single dive through a planner.
What matters, though: “no-deco time” depends on the dive computer and personal settings. If someone uses more conservative settings, the team must communicate that. Otherwise one diver suddenly has “deco” while everyone else is still no-deco. That’s not a problem — it’s often a good thing — but it does require more time shallow at the end of the dive, and that should be agreed.
This is especially relevant for guides: some divers will show deco on their computer during a dive that is being guided as “no-deco”. That’s fine, but it’s good to know beforehand — so: ask what settings people use, and be able to work with the answer.

Decompression dives with and without deco gases

Once you exceed no-deco limits, you should plan a bit more carefully. “Sliding into deco” isn’t automatically terrible if it’s just a few minutes and you have enough gas — but you should know what you’re doing. And the longer the decompression, the more thorough the planning must be.
Without deco gas, gas supply and minimum gas requirements quickly become the limiting factors. If two divers have to complete decompression together from one cylinder, you reach hard limits fast. That’s why for anything beyond a few minutes of deco, a deco gas is genuinely useful.
In the deco gas block we already saw how deco gases improve and shorten decompression, and how to choose them. For planning, we build the profile we actually want to dive, with the gases we’ve chosen.
That doesn’t have to be a square profile: we can plan multi-level dives. Especially on shore dives, it’s normal to spend time shallower before going deeper. If we want the plan to be close to reality, we should include that. And if we already know we won’t spend the full time at maximum depth — for example on a wreck you explore from bottom to top — we can plan that too.
The plan is output as a runtime table: where you are at each minute of the dive. You can follow that runtime strictly — but you give up flexibility.
Since we dive with computers (and often a backup computer), we can use them. From our plan, we take the key information: how long the planned ascent takes in total. That becomes our maximum time to surface (TTS). We ensured that enough gas is available at maximum depth for that ascent, and then we can simply dive until either minimum gas or the agreed maximum TTS is reached. The planned runtime becomes a backup.

Plan B: what if something goes wrong?

In both gas planning and deco planning, we always assume that something can go wrong. Any gas can fail, you may need longer than planned (by accident or due to an incident) — and that’s exactly why those scenarios are built into proper planning. Even when something goes wrong, we can still get to the surface safely.
Here is a concise summary of Plan B options for gas planning and decompression, plus a few thoughts on redundancy in equipment.

Gas loss

We are able to compensate for the loss of bottom gas or a decompression gas.
We have verified that our bottom gas can cover decompression if we only carry one deco gas; or that the remaining deco gas is sufficient if we normally carry two. This ensures that each diver can reach the surface safely, even alone.
In a team, you also have the option of using your buddy’s deco gas: either after they’re done with it, or by passing it back and forth. How exactly you want to do this should be agreed beforehand — and passing a stage is something you should practice from time to time.

It ended up deeper / longer

Redundancy: anything can break

On simple no-deco dives, redundancy is not a major theme. Guides often carry a little extra weight, a spare mask, maybe even a spare fin strap — but for most recreational divers, redundancy is the buddy (or the guide).
The more complex the dive, the more we depend on equipment actually working. That’s why we carry many items twice — sometimes even three times.
Rule of thumb: as much as necessary, as little as possible. Anything that becomes a serious problem if it fails should be redundant — but not more than that. Can’t continue without a mask, let alone read the computer? Carry a spare mask. Need to cut yourself free and you drop a knife? You want a second cutting device. If the computer fails and you can’t just ascend, you need a second computer — or runtime plus bottom timer.
And of course redundancy matters hugely for gas supply. Below roughly 40 m, it’s more than sensible not to rely solely on your buddy. You’ll typically dive doubles or two sidemount cylinders — so even with a gas loss you still have part of your own breathing gas available. Buddy support remains useful — but it’s no longer the single point of survival. Each diver is self-redundant.

What if… real emergencies

Unfortunately, even with good planning and all necessary caution, real accidents and medical emergencies can happen.
Covering this in full detail is beyond the scope here — but dive planning should still include agreement on what you do if something seriously goes wrong.

• How do you get someone out of the water?
• 112 (or local emergency number), phone reception, time until EMS arrives?
• Emergency oxygen — extra unit and/or from breathing gases?
• Insurance hotline / emergency assistance?

And one more team discussion that’s worth having: what level of decompression risk are you willing to accept in specific scenarios underwater? You don’t have to “ride out” a full GF 40/60 decompression if someone has a serious medical problem — but where do you draw the line, in which case?