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I'm no expert but here goes. There are two types of scr - active and passive. Active are the most common so I'll start there. A valve with a precise sized orifice, a constant mass flow valve, allows a constant bleed of fresh gas into the loop at a given rate. The gas is usually nitrox and no separate oxygen/diluent gases are used. As you breathe from the loop the oxygen fraction decreases and the constant flow allows enough nitrox in to replace it. An exhast valve is set to let a precice amount of gas to be vented from the loop to get rid of all that inert gas that is being injected. The actual gas breathed will have a lower oxygen fraction (and so a higher inert gas fraction) than the injected gas. So a bottle of 30% nitrox might give an FO2 of 23% or something. This can give a big gas saving over OC and can of course be used for trimix. The orifice used is set for the gas used and the dcesired FO2, and the diver is limited to a given depth range. There is no deco advantage over OC and FO2 is often variable making calculations a little tricky.
Passive scrs are similar but the counterlung is a bellows system with limited volume. As you breathe you trigger the injection of fresh gas in keeping with your breathing rate/tidal volume. As metabolism of O2 is closely linked to breathing rate this works fairly well. There are many more advantages to passive systems including a more stable FO2 and a much greater gas saving. But they're more comlicated to build. See the Halycon website for their RB80 passive do-da.
While scrs seem to lose many advantages of ccr (deco/gas efficiency/bubble free) they can run with no electronics at all making them robust and reliable. Various militaries use em as do some deep cave divers.  |