A CCR is a closed loop rebreathing device, which permits the addition of oxygen either manually or automatically.

There are basically two types of mCCrthose which feature a flow adjustment to maintain a constant flow rate such as the KISS unit and the rEvo. Another type is one in which the flow rates may be adjusted on thefly ernabling one to dive deeper such as the Heat or Pelagain. The diver themn fine tunes po2 by manual addion of oxygen.