Rebreather mode (CCR)

MV-Plan handles closed circuit rebreather (CCR) diving. Turn CCR Mode on in the Settings to bring the rebreather-specific options out across the application.

Turning CCR mode on

In the Settings, turn CCR Mode on. That makes available:

  • the diluents on the Gases page;
  • the choice of diluent and setpoint in each dive segment;
  • the open circuit bailout simulation;
  • the stress consumption settings.

Setpoint mode

The setpoint is the oxygen partial pressure the rebreather holds. Two modes are offered:

  • Fixed: a single setpoint for the whole dive (0.7 to 1.6 bar).
  • Dynamic: three separate setpoints depending on the phase of the dive: descent, bottom and ascent (each from 0.5 to 1.6 bar).

Dynamic mode reproduces what really happens on a dive, where the setpoint is raised on the bottom and adjusted on the way up. The ascent setpoint applies to every ascent, including between two levels of a multi-level profile; the descent setpoint to every descent; the bottom setpoint at each level.

Setpoint per segment

On the Plan page, each segment added in CCR mode lets you choose:

  • the diluent used (among the enabled diluents);
  • the setpoint:
    • in fixed mode, you type the setpoint for the segment;
    • in dynamic mode, the setpoint follows the phase automatically, with the option to override it with a fixed setpoint for that particular segment.

Loop flushes

Turn Loop flushes (CCR) on in the Settings to bring a flush editor onto the Plan page. A flush is a deliberate pause during the ascent where the loop is emptied then refilled with a known mix, most often air once the bottom diluent has become too lean in oxygen to be breathed shallow.

  • Each flush has a depth, a mix (taken from the list of diluents) and a duration. The Compute button offers the depth that gives a target ppO2 or END with that mix, rounded to the shallower metre.
  • During the flush the loop leaves the setpoint: the ppO2 is the one the mix gives at that depth, and the line of the table shows the ppO2, the END and the density actually breathed. An alert flags a flush that falls outside your working ppO2 band.
  • By default the loop then goes back to the diluent in use. Tick Keep this mix to stay on the flushed mix to the surface, which is the deliberate helium purge before the decompression. The flush line says it on its own: it names the mix taken and the one it replaces, and the following lines carry the new mix.
  • The minutes of a flush count in the TTS like a stop. A flush planned deeper than the start of the final ascent never happens, and the plan says so.
  • The ppO2 of a flush is judged against your deco limit (1.6 bar by default) and not against the working limit, since a flush always happens during the ascent. It is read as a MOD, with the same rounding tolerance as everywhere else in the application: pure oxygen stays the gas of 6 m, at 1.62 bar.

Finishing the dive on 100% oxygen

This is the usual rebreather ending: at 6 m the loop is filled with pure oxygen, which gives 1.62 bar in the sea at sea level, and the decompression finishes on it. Rather than entering it on every dive, describe it once in the Settings, CCR Mode block:

  • Finish the dive on 100% oxygen turns it on for every rebreather dive. The depth (6 m by default) and the flush duration (1 min) are set just below, and the resulting ppO2, END and density are shown under the fields - they follow the water type and the altitude.
  • There is nothing to declare as a gas: a rebreather is dived on two cylinders, a diluent and pure oxygen, so oxygen goes without saying. Only diluents are entered on the Gases page.
  • The loop stays on oxygen to the surface, as it does in real life: once the loop is on oxygen, the electronics have nothing left to inject and cannot bring a ppO2 back down. The stops above the flush are therefore computed on oxygen.
  • If you entered a flush of your own at that depth in the editor of the Plan page, yours is the one that counts. If the dive never goes below the depth you set, the flush does not happen and the plan flags it.
  • The Plan page repeats that ending, read-only, under the editor, so the line appearing in the table can always be traced back to the setting that produced it.
  • Keep an eye on the CNS: a loop on pure oxygen at 6 m runs at 2.5 % per minute, against 0.6 % at 1.3 bar. A long oxygen ending therefore weighs heavily on the counter, and that is exactly what practice calls for breaking up with pauses on the diluent.

What the loop is really worth

The setpoint is an instruction, not a guarantee. The electronics of a rebreather can only add oxygen: for a ppO2 to come back down, the diver's own consumption has to eat into it and an inert gas has to take its place. Two consequences show up in the table:

  • On a loop carrying no inert gas - after an oxygen flush that was kept, or during the oxygen ending - the ppO2 is the one ambient pressure gives, whatever setpoint is set: 1.62 bar at 6 m, 1.32 bar at 3 m. That is the figure feeding the CNS and the OTU.
  • The Setpoint column stays empty on those lines, and also in water shallower than the setpoint itself. The table never shows a setpoint the loop does not hold next to a ppO2 that contradicts it.

On a loop carrying inert gas nothing changes: an air diluent at 60 m does hold the 1.3 bar setpoint, where air alone would give 1.49.

Bailout simulation

MV-Plan computes the open circuit way out should the rebreather fail. The plan then shows two profiles:

  • the CCR plan: the nominal rebreather dive;
  • the bailout plan: the ascent and the decompression done on your open circuit gases marked as bailout.

The bailout settings live in the Settings:

  • Bail delay: time spent at depth dealing with the incident before starting up on bailout.
  • Stress RMV: the raised consumption under stress.
  • Stress until: how far the stress consumption applies (first stop, gas switch or surface).

Bailout-specific alerts

On the switch from the rebreather to open circuit, MV-Plan can flag:

  • an increase in narcosis (higher END on the bailout gas);
  • an ICD risk (a sharp drop in helium that can affect the inner ear).

These alerts follow the options enabled in the Settings.

Bailout gas requirements

The gas consumption section of the plan estimates the bailout gas requirements per cylinder, from your stress consumption and the ascent profile. An alert appears if the capacity of a cylinder falls short for the way out. Cylinders are tracked while the ascent is built: when one runs dry, the plan moves on to the next at the depth where it empties, writes it as a gas change and flags it with an alert. The verdicts of the table (OK, TIGHT, INSUFFICIENT, NOT BREATHED, backup) are described on the Planning page.

For declaring diluents and bailout gases, see the Gases page. For how the calculation runs, see Planning.