Scuba & Nitrox Glossary: R
Nitrox and breathing-gas terminology is prioritized first. General scuba terms follow below. Definitions are educational summaries and do not replace formal training, manufacturer instructions, or an operational dive plan.
Nitrox & Gas Terms — R
- Reference Gas
- A gas of known composition used to calibrate or validate an analyzer.
- Residual Oxygen Exposure
- Oxygen dose carried forward from earlier dives when evaluating repetitive exposure.
- Rich Mix
- Informal term for a mixture with a relatively high oxygen fraction; it must always be stated numerically.
- ROX
- Recreational oxygen-enriched air, a term used by some training systems.
- Oxygen-Rich Mix
- A gas mixture with a higher oxygen fraction than another compared mixture.
- Rebreather Setpoint Change
- A planned adjustment to the target loop oxygen partial pressure.
- Residual Blend
- Gas remaining in a cylinder before additional components are added.
- Residual Oxygen Calculation
- Accounting for oxygen already in a cylinder during blending.
- Rolling a Cylinder
- A legacy practice intended to encourage mixing after partial-pressure blending.
General Scuba Terms — R
- Rebreather
- A breathing apparatus that recycles exhaled gas after removing carbon dioxide and managing oxygen.
- Recompression
- Increasing ambient pressure, generally in a chamber for treatment or controlled procedures.
- Regulator
- Equipment that reduces cylinder pressure and delivers breathing gas on demand.
- Repetitive Dive
- A dive made while residual inert gas from a previous dive remains relevant.
- Reserve Gas
- Gas intentionally protected for emergencies or ascent.
- Reverse Block
- Pain or injury risk when expanding gas cannot vent during ascent.
- Rule of Thirds
- A gas-management method allocating one third out, one third back, and one third reserve; appropriate use requires training and context.
- Reciprocal Heading
- A compass direction 180 degrees opposite the outbound heading.
- Reef Hook
- A device sometimes used in current; its use may be restricted and can damage reefs.
- Planned Reserve Gas
- Breathing gas deliberately retained for contingencies and ascent.
- Rip Current
- A concentrated flow moving away from shore through the surf zone.
- Rocking Boat Entry
- An entry adapted to vessel movement and sea conditions.