Which CCR? Understanding differences in Rebreather design.
Solenoid or constant mass flow, which is best for me?
In a previous article I looked at the ways that closed circuit rebreather (CCR) differs from open circuit SCUBA (OC) as well as the basic operating concepts of CCR. This time I would like to look in more detail at the two most popular methods of introducing oxygen into the loop and maintaining the desired partial pressure of oxygen (PO2) during the dive. These two types of rebreather can be split into electronically controlled CCR (ECCR) and constant mass flow (CMF) CCR, (sometimes called diver controlled or manually controlled CCR) and are quite different in their design.
With an ECCR we have a computer which is connected to the oxygen sensors (cells), this functions as both a display, giving the diver all the necessary information, including PO2, depth, time etc. and as a controller for the oxygen solenoid. This is a magnetic/electric valve which delivers measured doses of oxygen to the loop when triggered by the controller. The desired PO2 is called the setpoint and is programmed into the controller by the diver. Usually we have two setpoints, low and high and usually they are 0.7 and 1.3 respectively. Sat on the boat just before the dive, we would be breathing on our CCR at low setpoint, with a PO2 of 0.7, at the surface we are at 1 atmosphere of pressure, so our fraction of oxygen is also 0.7 or 70% (PO2=FO2xATA). Once we reach our dive depth, in this case lets say 25 meters, we switch to high setpoint (by pressing a button on the controller), the solenoid fires in bursts, bringing the PO2 up to 1.3 and maintaining it there, at 25m we now have an FO2 of 37% (1.3/3.5). As we swim around the solenoid will fire short bursts several times a minute, keeping the PO2 in a very tight range, during ascents it will fire much more often due to decreasing ambient pressure causing the PO2 to drop, this is where novice CCR divers can struggle with buoyancy as you need to vent quite a bit of gas from the loop as well as venting your wing.
CMF units share the same basic principles as ECCR’s, the difference being instead of having periodic injections of O2 into the loop, there is a constant trickle coming in through a ‘leaky valve’, usually around 0.75 to 1 litre a minute. This is normally slightly less than a diver will metabolise during a dive so there is a manual addition button which adds O2 when pressed. The diver monitors the PO2 during the dive via a computer connected to the sensors in the unit, if it is lower than desired, press the manual add button to bring it up, if higher, add diluent. This might sound like a lot of work but when swimming around at a fairly constant depth the drop in PO2 is very slow due to the constant O2 addition. We still have the same concept of high and low setpoints, only now they are much more elastic. An experienced diver can keep a tight range of PO2 with little effort but you will always get greater variation of PO2 with CMF units compared to ECCR, particularly during ascent. The CMF ‘leaky valve’ is supplied with oxygen by a regulator with a fixed intermediate pressure, meaning there is no increase in intermediate pressure corresponding to increased depth which is normally the case, this means that CMF units will have depth limitations shallower than ECCR, typically in the range of 60 to 80 meters.
CCR diving has historically been the preserve of hard core technical divers, however over the last decade or so, manufacturers have attempted to make CCR’s easier and safer to dive in order to introduce them to the recreational diving public. The mainstream thought behind this was to make an ECCR with fully redundant computer control, remove as many decisions as possible from the diver and have two states of operation, all good-diving on the loop and problem-bail out to OC. The most popular unit of this type is the Poseidon Mk 6/7. Early models struggled with firmware issues and poor reliability, this seems to have been sorted out now, but as with all ECCR’s they are considerably more expensive than CMF units. Poseidon units also suffer from a bit of an identity crisis in that they are not considered a hard core tech diving unit as they lack manual overrides on many functions while still having the complexity and expense of top of the line technical ECCR’s.
Conversely, I believe that CMF units like the KISS Spirit are far more suitable for recreational diving as they are simpler, have less to go wrong, cheaper, often smaller and lighter and as they do not have a solenoid they are a little easier to master buoyancy on. ECCR’s are more complex machines with a heftier price tag, but if deep long decompression diving is your thing they are very hard to beat. Holding a tight range of PO2 makes them more efficient from a decompression standpoint and solenoid control leaves hands free for other tasks like scootering, photography or salvage as well as not having depth limited by the intermediate pressure of the oxygen feed.
So by now I hope you can see that it is impossible to say which type of unit is empirically ‘better’, but easy to see which type will suit a particular type of diving. Here at Deefer Diving you can try both APD Inspiration and Kiss Spirit and decide for yourself, one thing is certain for me, either type of CCR beats open circuit diving hands down!
About the author: Jon is a highly qualified rebreather and recreational dive instructor. He loves talking about rebreathers almost as much as diving on rebreathers, and is a great source of information on the technical and mechanical aspects of CCR.
For more information about diving in Carriacou and the rebreather training we offer, please visit our website,