CCR Diving (Closed Circuit Rebreather)

DiveToolbox supports full CCR dive planning alongside open circuit (OC). The CCR planner uses the Buhlmann ZH-L16C tissue model with gradient factors, calculating equivalent gas at each depth based on your setpoint configuration.

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Switching to CCR mode

In the dive planner, select CCR in the circuit mode selector at the top of the configuration panel. The interface adapts to show CCR-specific inputs: setpoints, diluent, and rebreather tank configuration.

Setpoints

CCR diving uses two setpoints that control the partial pressure of oxygen (PPO2) delivered by the rebreather:

Setting	Range	Description
Low setpoint	0.4–1.0 bar	Used during descent. Typically 0.7 bar.
High setpoint	0.9–1.6 bar	Used during bottom time and ascent. Typically 1.3 bar.
Switch depth	Optional	Depth at which the unit switches from low to high setpoint. If left blank, switches at maximum dive depth.

The planner calculates the equivalent open-circuit gas at every depth based on the active setpoint. This equivalent gas is used for all decompression calculations, CNS, OTU, gas density, and END computations.

Diluent

The diluent is the gas that fills the breathing loop alongside oxygen. Configure it using:

• Preset selector: Quick-select common diluent mixes (Air, Trimix 18/45, Trimix 21/35, etc.)
• Custom O2 % and He %: Manual input for custom mixes. Nitrogen is calculated automatically.

Hypoxic diluent

If your diluent O2% is below 16%, a warning is displayed. Hypoxic diluents require a pre-breathe on oxygen before descent to avoid hypoxia at shallow depths.

Tank configuration

CCR tank configuration differs from open circuit. Two separate tanks are configured:

O2 supply tank

The oxygen tank feeds the rebreather's oxygen addition system. Configure:

• Volume (liters): Tank size
• Start pressure (bar or psi): Pressure at the beginning of the dive
• Reserve pressure (bar or psi): Minimum pressure to keep as safety margin

The planner shows total available gas and usable gas (accounting for reserve).

Diluent tank

The diluent tank compensates for volume loss during descent. Same settings as the O2 tank.

Metabolic O2 rate

This is the diver's oxygen consumption rate in liters per minute, regardless of depth. The default is 0.85 L/min, which represents a relaxed diver. Typical range is 0.5–1.5 L/min. Increase this value for high-exertion dives.

tip

Unlike open circuit SAC rate, the metabolic O2 rate is depth-independent because the rebreather maintains a constant PPO2 in the loop.

Gas consumption

The Gas Consumption Summary panel shows usage for each tank:

• Progress bar with color coding: green (adequate), amber (marginal), red (insufficient)
• Percentage used with remaining volume
• Summary metrics: O2 consumption rate, equivalent O2 at depth, diluent composition, and configured setpoints

Bailout planning

Bailout planning simulates an emergency switch to open-circuit breathing from any point in the dive. This is critical safety planning for CCR divers.

Enabling bailout

Toggle Bailout Planning in the configuration panel. When enabled, additional options appear:

Stress factor

During a bailout, the diver's gas consumption increases due to stress. Select a multiplier applied to the base SAC rate:

Factor	Usage
2.0x	Standard
2.5x	Conservative
3.0x	Very conservative

Bailout tanks

Add one or more open-circuit bailout tanks. Each tank card shows:

• Gas name and mix
• Volume and total gas
• Maximum Operating Depth (MOD)
• Edit and delete controls

Click Add Tank to configure a new bailout gas. You can add both bottom gases and decompression stage gases.

Bailout analysis

Once configured, the Bailout Analysis panel shows a complete safety assessment:

• Overall status: Adequate (green), Marginal (amber), or Insufficient (red)
• Per-tank assessment: Progress bar showing gas needed vs. available for each bailout tank
• Errors and warnings: Specific issues like insufficient gas for a particular tank
• Bailout runtime and decompression time: How long the emergency ascent takes
• Bailout schedule (expandable): Full phase-by-phase breakdown with depths, durations, and gas switches — including a visual dive profile chart

How it works

The CCR planner performs the following calculations at each depth:

1. Determines the active setpoint (low or high) based on current depth and switch depth
2. Calculates the equivalent open-circuit gas: the O2% and He% that would produce the same PPO2 at that depth
3. Feeds the equivalent gas into the Buhlmann ZH-L16C tissue model for inert gas loading
4. Computes decompression obligations, ceiling, and ascent schedule
5. Tracks O2 consumption (metabolic rate, depth-independent) and diluent consumption (descent only)
6. If bailout is enabled, recalculates the entire ascent on open circuit from the worst-case point

info

The planner automatically clamps PPO2 when the setpoint exceeds the ambient pressure at shallow depths. It also accounts for hypoxia risk when using low setpoints near the surface.

What's next?

• Configure your Diver Profile with appropriate gradient factors for CCR diving
• Review the safety warnings displayed in your plan results
• Learn about Successive Dives with residual tissue loading from a previous CCR dive