Open circuit and rebreathers compared

Chapter 19 showed how a rebreather keeps your breath in a loop and adds only the oxygen you use. This chapter puts it side by side with open circuit on the same dives. It compares 3 things: how much gas each uses, how much oxygen each gives you, and how much decompression each needs.

On open circuit (OC) you breathe from a cylinder through a regulator, and every breath you exhale leaves as bubbles. On a closed-circuit rebreather (CCR) your exhaled gas stays in a loop. A scrubber removes the carbon dioxide, and the unit adds oxygen to replace what your body used.

All the plans in this chapter were planned with DiveLogic, with the same decompression model and settings for both.

Gas use

On open circuit each breath comes out of the regulator at the ambient pressure, the total pressure around you. It is measured in ATA (atmospheres absolute, about the same as bar): 1 ATA at the surface, plus 1 ATA for every 10 m. So at 45 m it is 5.5 ATA. By Boyle’s law (chapter 1), a breath there holds 5.5 times as much gas as the same breath at the surface, so you use gas 5.5 times as fast.

Your RMV (respiratory minute volume, also called SAC rate) is the gas you breathe in a minute at the surface. On open circuit, your gas use per minute at any depth is your RMV scaled up by the pressure. This is the number that sizes your cylinders.

A rebreather adds only the oxygen your body uses. Your body uses the same amount of oxygen per minute at any depth. It depends on how hard you work, not on the pressure, so there is no $P$ in it. The rest of the loop gas is breathed again.

The loop also needs diluent, a second gas that keeps the loop full. As you descend, the loop gas is squeezed and the unit adds diluent. As you ascend, the gas expands and some is let out. So diluent use depends on depth changes and events such as clearing your mask, not on how long you breathe.

DiveLogic plans rebreather oxygen at 3 L/min while working and 1 L/min at rest. These are planning figures, set higher than most divers use, because running out of oxygen ends the dive. You can work out open-circuit gas for any dive with the gas needed tool.

Oxygen pressure

The PO2 (partial pressure of oxygen, also written ppO2) is the pressure of the oxygen alone in the gas you breathe. It is the FO2, the fraction of oxygen in the mix, times the ambient pressure (Dalton’s law, chapter 2).

On open circuit the FO2 is fixed, so the PO2 rises with depth. Air gives a PO2 of 0.21 at the surface and 0.21 × 5.5 = 1.16 at 45 m. So each open-circuit mix has a MOD (maximum operating depth). To breathe more oxygen on the ascent, you must carry a deco gas in a stage cylinder and make a gas switch to it.

An electronic CCR holds a fixed PO2 at any depth, called the setpoint. Think of it like a thermostat: when the PO2 drops below the setting, the unit adds oxygen, as a heater comes on when a room cools. A common working setpoint is 1.3. Many divers descend on a lower setpoint, such as 0.7, and switch to 1.3 at the bottom.

Near the surface the loop cannot hold a setpoint higher than the pressure around you. At 0 m the most it can hold is about 1 ATA. So the loop PO2 is the lower of the setpoint and the ambient pressure.

The figures in this chapter are for closed circuit only. A semi-closed rebreather’s loop oxygen changes with how hard you work.

Decompression

Inert gas is the part of your gas that the body does not use: nitrogen and helium. Your tissues take it up, called on-gassing, at a rate set by the inert gas pressure you breathe (chapter 10). Oxygen and inert gas share the ambient pressure. So a higher PO2 means a lower inert gas pressure.

A rebreather holds a high PO2 at every depth. So on most dives your tissues take up less inert gas and you have a smaller decompression obligation: fewer and shorter decompression stops on the way up. The tissue model is the same Bühlmann model for both (chapter 13). Only the inert gas pressure fed into it changes.

On open circuit, the inert gas pressure is the ambient pressure times the inert part of the mix (FN2 plus FHe). On a CCR, it is the ambient pressure minus the setpoint. Both first take off a small amount, about 0.06 ATA, for water vapour in the lungs. On the loop, the inert gas is split between nitrogen and helium in the same proportions as in the diluent.

At 45 m on 21/35, open circuit gives about 4.30 ATA of inert gas. A 1.3 setpoint gives about 4.14 ATA. Both plans count each 10 m of sea water as 1 ATA.

The comparison below uses one dive: 45 m for 30 minutes. 21/35 is trimix with 21% oxygen and 35% helium; it is the open-circuit bottom gas (the gas for the deepest part of the dive) and the rebreather diluent. Both plans use gradient factors 50/80, GF low 50 and GF high 80 (chapter 16), which set how close to its limit the model lets your tissues go.

Bailout

A rebreather can fail: the loop can flood, a sensor or valve can fail, or the scrubber can stop working. The standard response is to bail out: leave the loop and breathe open circuit.

From then on you use gas at the open-circuit rate, from the depth of the failure and through every decompression stop. So rebreather divers carry open-circuit bailout cylinders big enough for the worst point of the dive. DiveLogic plans these open-circuit ascents as bailout scenarios.

Try it

Choose the depth, the bottom time, the mix and the setpoint. The figure plans the dive on open circuit with EAN50 and on a rebreather, and the table compares them. Deeper and longer dives show a bigger difference. A higher setpoint shortens the rebreather plan and raises its oxygen exposure.