mCCR - manual or mechanical rebreathers - are based on the fundamental principle of close circuit diving: human metabolizm consumes a fixed amount of oxygen (at constant effort) and it's about 1 liter per minute. A diver with 3l tank filled with oxygen at 200 bar can - in theory - spend 10 hours under water. In fact oxygen is "wasted" during ascend and some other procedures but all in all we use it very little. Furthermore, the depth does not affect the oxygen consumption. So all we need is the system providing fixed amount of oxygen to the breathing loop, exactly as much as is metabolized. CMF (Constant Mass Flow) is such a system, providing fixed oxygen flow to the loop by constant flow orifice. In ideal conditions (during relaxed swimming) the amount of oxygen provided by CMF is the same as amount metabolized by the diver, which ensures fixed amount of oxygen in the breathing loop. At higher effort and during ascend the diver has to add more oxygen to the loop, using manual inlation valve, to keep its required amount. The advantages of mCCR are simple construction and relatively small quantity of possible failure sources. These units don't have computers, electrovalves and other elements susceptible to failure. It's just an orifice (a hole of microscopic diameter) providing oxygen to the system regardless of the other elements. Drawback of this sollution is the need for constant monitoring of ppO2 and manual "refilling" it to the setpoint, which - in stressfull situation and during ascend - significantly increases the risk of hypoxia (too small amount of oxygen).

eCCR - electronic CCR - is a system equipped with solenoid, an electrovalve which opens on demand, adding oxygen to the breathing loop. Electronic rebreathers are controlled by computer connected to oxygen sensors, using special algorythm to maintain oxygen setpoint in the breathing gas. Let's say that we want, during all the dive, to breath the gas with oxygen partial pressure at 1.3 bar - that's what we call setpoint. After correct setup the computer is monitoring ppO2 and in case opens the solenoid to add oxygen in an amount providing ppO2 of 1.3 bar. The advantage of this system is convenience - the diver doesn't have to manually maintain fixed ppO2 in the breathing loop - the computer does it for him. The drawback is higher possibility of critical failure - for example solenoid blocked in open possition or electronic failure. 

What is the hCCR?

Why not use in the same time the advantages of mCCR and eCCR while minimizing their drawbacks? hCCR - hybrid CCR - is such solution. rEvo is the first rebreather in the world which is mCCR and eCCR at the same time. Constant flow orifice adds oxygen to the breathing loop all the time. There is manual oxygen inflation valve of course but  we also have the solenoid, controlled by the computer, which opens to maintain the setpoint when ppO2 drops below the setpoint. What this solution gives us?

How to use hCCR efectively?

It's very simple - diving with rEvo we setup its eCCR part (Shearewater electronics) for setpoint of 1.20 - 1.25 bar and then "manually" maintain required ppO2 in the breathing loop at 1.3 bar. During normal, stresless dive the constant flow orifice adds the same amount of oxygen as metabolized by the diver. In that way ppO2 once set manually (at 1.3 bar for example) stays constant so the solenoid doesn't work at all. In stresfull situation or when diver's effort rises or during ascend, the ppO2 drops below 1.25 bar (oxygen loss is higher than its constant in-flow) so the solenoid opens to stop further ppO2 drop. This solution minimizes the possibility of solenoid blockage (because it doesn't work almost at all), without sacrificing the comfort of being protected by the computer watching the oxygen partial pressure not to drop below safe level.