CCR (rebreather) dive planning: step by step

“After my first 10 hours on a rebreather, I was a real expert. Another 40 hours of dive time later, I considered myself a novice. When I completed about 100 hours of rebreather diving, I realized I was only just a beginner.”
— Richard Pyle, reflecting on the steep learning curve of rebreather diving blog.padi.com

When diving, everything is interconnected, and the plan must form a coherent whole. It is therefore essential to establish a clear starting point. Naturally, the starting values are depth and bottom time. We generally dive to different depths for varying lengths of time. While we usually have a clear idea of the intended depth, determining bottom time is not always simple, as it is often limited by available gas supply and the required decompression obligation. Sometimes we must use the method of halving the interval, choosing a bottom time that fits within our gas reserves. When we talk about gas supply in closed-circuit diving, we mostly mean bailout gas. Especially for beginners, it is not assumed that dives will be carried out at the limits of rebreather gas capacity. The simplest scenario, of course, is when we have a very specific bottom time required to achieve our goal.

Starting Point – Depth and Time

To make things clearer, let’s plan a wreck dive. This is simple because wreck dives usually begin from a boat, descending down a line directly to the target depth. Time is then spent at roughly the same depth, give or take a few meters (unless it’s a very large wreck). Let’s assume this wreck lies at 35 meters. I chose this depth deliberately, as I consider it the maximum for air diluent CCR diving.

How long do we need to spend at the wreck? Suppose the wreck is 50 meters long. The typical swimming speed of a technical diver ranges from 10 to 15 meters per minute. Since we’re not racing but rather observing and exploring, we’ll choose a more conservative value—5 meters per minute. To swim around the 50-meter wreck, that gives us about 20 minutes.

Choosing the Setpoint

To plan decompression, we must first decide on the setpoint—the partial pressure of oxygen during the CCR dive. Looking at CNS exposure tables, we find that at a PO2 of 1.3

bar, the maximum exposure is 180 minutes. This is more than enough for our purposes. A higher constant PO2 would not be safe. Lower setpoints are used for dives deeper than 50 m or under demanding conditions (currents, cold water, heavy workload).

For descent, we use a lower setpoint, as high PO2 is unsuitable at the surface and would rise dangerously during descent. A common descent setpoint is 0.7 bar. With the Liberty CCR, we can also set a “descent setpoint” that increases proportionally with depth, saving oxygen. For example, this could be set at 0.4 bar.

Gas Selection

In this example, we will use air as the diluent, since the article is aimed at beginners. Nitrox is not suitable as a diluent because it would exceed our planned setpoint. However, nitrox is better suited as bailout gas. At the planned maximum depth of 35 m, the pressure is 4.5 bar. At a maximum PO2 of 1.6 bar for the bailout cylinder, this gives us nitrox 35 (1.6 / 4.5 = 0.35). We will use the standard nitrox 32.

Ascent Planning

We now know enough to plan ascent and decompression. The best tool is a decompression planner. A good choice is divesoft.app, which provides advanced CCR dive planning. Using gradient factors 40/75 (a conservative and safe setting), the result is a single stop at 6 m for 2 minutes (with ascent speed of 9 m/min).

Gas Management

Gas planning for this dive is straightforward. Diluent is calculated by multiplying depth pressure and loop volume, then conservatively by 2.5. In our case: 4.5 bar × 10 liters × 2.5 = 112.5 liters. In a 3-liter cylinder, that is 37.5 bar. We also know dives should not begin with less than 100 bar.

Oxygen is even simpler. Total dive time is 27 minutes. At a conservative consumption of 2 liters/min, that equals 54 liters, or 18 bar in a 3-liter cylinder.

Scrubber Duration

We must also consider scrubber duration. The minimum duration is 2 hours 47 minutes, even under heavy workload and cold water. Our 27-minute dive is well within this.

Plan B – The “Shit Plan”

The most important aspect of CCR planning is preparing for emergencies where the unit cannot be used to ascend. This could be a flooded unit or (although unlikely with Liberty) a total electronics failure.

At any point, we must be able to ascend on open circuit. We need sufficient bailout gas to cover the remaining bottom time plus a safety margin (usually 3 minutes), with breathing rates elevated due to stress—often 50 liters/min. Hypercapnia, very relevant in CCR diving, must be considered.

On open circuit, PO2 will decrease during ascent, so decompression will be less efficient than with CCR. Adding 3 minutes bottom time and switching to open circuit extends decompression to a 1-minute stop at 9 m and 9 minutes at 6 m.

Calculating gas use for this profile: 2076 liters of nitrox 32 are required. In a 12-liter cylinder, that is 173 bar; in an AL80, it is 187 bar.

Conclusion

This example shows that even a relatively short, simple dive can become bailout-gas-limited in an emergency. If air were used instead of nitrox, gas supply would not suffice. Always plan dives carefully to avoid unpleasant or dangerous situations

Author: Jakub Šimánek