| They work by adding a premixed gas at a rate that will maintain a ppO2 as you breathe the loop down. Because this means adding more gas than the loop can hold some of the excess must be vented every now and again so you get some bubbles.
The big downside of active SCRs is that there are no overt failure modes. If something goes wrong then you keep breathing until the loop goes hypoxic. They depend on an adequate flow of gas through a sonic orifice, with different orifices for different depths and gas mixes. These have to be preset on the surface and there is your first weakness. Gas flow depends on flow through the orifice being at sonic velocities. There is no filter so salt and debris can partially clog the orifice and reduce the flow. Weakness number two, gas still flows but not at a rate adequate to support life, you pass out and die while everything appears to be working. Also, as the gas flow is preset and not controlled by your breathing rate as in a passive SCR (or your metabolism in a CCR) you can overbreathe the unit if working hard, i.e. you consume O2 faster than its added, it becomes more of a risk the shallower you get. Weakness number three and again there are no warning signs.
I don't see the point of a RB for shallow diving, they take more maintenance than OC, they require more training, more discipline, more prep, more hassle and air/EAN isn't exactly expensive. On the plus side, the Draegers are extremely well made.
I wouldn't use one personally but if I did I'd certainly make sure I put some kind of ppO2 monitoring system on it.  |