Calibration gas – A gas of a known composition used to calibrate gas sensors, particularly PO2 and PCO2 sensors.
Carbon dioxide (CO2) – Waste gas generated by the process of metabolism and exhaled by the diver into the breathing loop.
Carbon dioxide retention − Condition in which arterial CO2 is seen to increase in divers due to insufficient ventilation, excessive dead space in the breathing loop, or ineffective CO2 scrubber filtration.
Catastrophic loop failure – A complete failure of the breathing loop of a rebreather such that it cannot be recovered in closed-circuit mode; usually occurring from a ripping or tearing and subsequent flooding of a unit or a carbon dioxide emergency.
Central nervous system (CNS) – The human brain, spinal cord, and associated major neurological pathways that are critical for basic life-support processes, muscular and sensory systems.
Central nervous system oxygen toxicity − A serious form of oxygen toxicity, usually caused by exposure to breathing mixtures with an oxygen partial pressure in excess of 1.6 ata. Symptoms may include visual disturbances, hearing anomalies, nausea, twitching, dizziness and severe convulsions.
Chain of custody − Refers to the chronological documentation that captures the seizure, custody, control, transfer, analysis, and disposition of physical or electronic evidence, typically for legal purposes.
Channeling (of scrubber canister) − Condition in which improper packing or excessive settling forms channels that allow some CO2 to pass through the scrubber without being absorbed.
Check valve – A one-way, non-return valve that directs gas to move in only one direction through the breathing loop.
Closed-circuit rebreather (CCR) – A type of rebreather that usually includes some form of oxygen control system and generally only vents gas upon ascent.
CO2 absorbent – A material that chemically binds with CO2 molecules (Sodasorb, Drägersorb®, lithium hydroxide, Sofnolime®, Micropore ExtendAir, etc.).
CO2 absorbent canister – A canister in the breathing loop containing CO2 absorbent.
Condensation – Water that forms when water vapor cools and forms liquid droplets. In a rebreather, heat conduction through the breathing hoses and other components of the breathing loop lead to condensation. This process may be exacerbated by materials with greater heat conductivity and lessened with insulation of the breathing loop components.
Conduction (thermal) – Heat flow between objects in physical contact; the inverse of insulation.
Constant mass flow valve – A type of valve that allows a constant mass of gas molecules to flow at a fixed rate.
Constant volume flow − A type of valve that delivers a constant volume, independent of ambient pressure, thus a flexible number of gas molecules.
Convection (thermal) – Heat flow through circulating currents in liquid or gas environment.
Counterlung – A collapsible bag connected to a rebreather breathing loop, which expands as a diver exhales and collapses as a diver inhales.
Cubic feet (ft3) – A unit measure of volume, defined as the space occupied by a cube one foot on each side; 1 ft3 = 28.3 L.
Current limited (oxygen sensor) − A condition in which a change in the load applied to a sensor is not met with a change in the current supplied by the sensor.