What is sCCR in diving?
An sCCR is a semi-closed rebreather: it recycles most of your exhaled gas from a single nitrox supply and vents the rest as bubbles — simpler than a full CCR, several times more gas-efficient than open circuit.
An sCCR (semi-closed circuit rebreather, usually abbreviated SCR or pSCR) recycles your exhaled gas through a CO2 scrubber, fed by a single supply gas, almost always nitrox. Unlike a fully closed loop it deliberately vents part of the gas, so some bubbles always escape: hence "semi-closed".
How does an sCCR add and vent gas?
Two ways. Active SCRs inject nitrox at a constant mass flow through an orifice matched to the mix; the loop vents the surplus. Passive SCRs discard a fixed fraction of every breath — about a tenth on the Halcyon RB80's bellows — and add supply gas mechanically, so consumption tracks your breathing.
How is it different from open circuit and CCR?
It sits between them. It uses several times less gas than open circuit, yet is far simpler than a CCR: purely mechanical, often with no electronics or oxygen sensors. The trade-off is that the loop's oxygen fraction varies: your body consumes oxygen, so you breathe a leaner mix than the tank holds — leaner still under heavy workload or on ascent. Hypoxia is the classic SCR risk; you must know what you are actually breathing.
Who dives semi-closed rebreathers?
The Dräger Dolphin and Atlantis brought SCR diving to recreational nitrox divers around 2000; the passive RB80 remains a proven cave-exploration tool today.