caveseeker7
Contributor
All rebreathers share certain features:
counterlung(s) to breath into and out of, scrubber to absorb the CO2 you generate, DSV (mouthpiece) to open and close the loop for the diver to breath from, hoses to connect it all.
The main difference are the gases used, and the mode of injection:
SCRs expel some of the gas from the loop, hence they're called semi-closed. They use pre-mixed gases, either Nitrox or for deeper dives He mixes, same as you would with OC scuba gear.
Active addition or constant mass flow (cmf) SCRs add gas through an orifice or needle valve. More gas than the diver needs is fed into the loop, the difference is periodically "burbed" via an over pressure valve. Those units, for most part, are made for nitrox mixes, thus have the same maximum operating depth as OC nitrox would. Gas savings towards OC is about 4:1-5:1, the leaner the gas, the deeper the MOD, the higher the flow rate.
Passive addition SCRs are respiratory minute volume keyed. On every inhalation, a set amount of gas is dumped via a one way valve, the missing gas volume is then replenished by a Schrader valve. This type of SCR can be used with any pre-mixed gas(es) you use on OC circuit, and all manufacturers I can think of offer gas blocks that allow to switch the gas supply. Gas savings depends on the relation between the counterlung bellows and dump bellows. In stock form most units offer about 8:1 gas savings. The ratio has been pushed to 15:1 on deep dives with scooters by highly experienced exploration divers.
CCRs use two different gases, pure O2 and a second gas mix to dilute it so it can be breathed deeper than 6 meters safely. The diluent can be air, heliox or trimix. The diluent is only used to maintain loop volume (like adding gas to your BC when descending) and dilute the O2. O2 is only added to maintain a chosen partial pressure of O2.
Those are two important items:
O2 use is the same at the surface as it is at 10 ft, 100 ft or 1000 ft depth. Since only O2 gets replenished at depth, gas savings are higher the deeper you dive.
The partial pressure of O2 being stable means the mix in the loop is highly dynamic. You have a nitrox (or whatever gases you're using) mixing machine on your back. You'll always have the "best mix" for the depth you're at, which can extend your NDL and shorten your deco.
Downside is the gas in the loop is highly dynamic, you'll need to stay on top of it. There'll be gas in the loop, but it may not sustain life (too low pO2) or become toxic (to high pO2). Hence the first rule of RB diving, ALWAYS KNOW YOUR PO2!
And while you need only a very small gas supply for the loop, you'll need bailout of some sort should there be an emergency, negating the weight savings on a dive.
Other disadvantage are shared by all RBs: scrubber failure/CO2 hit, caustic cocktail, added gear, maintenance and expense, and absolute need for discipline.
counterlung(s) to breath into and out of, scrubber to absorb the CO2 you generate, DSV (mouthpiece) to open and close the loop for the diver to breath from, hoses to connect it all.
The main difference are the gases used, and the mode of injection:
SCRs expel some of the gas from the loop, hence they're called semi-closed. They use pre-mixed gases, either Nitrox or for deeper dives He mixes, same as you would with OC scuba gear.
Active addition or constant mass flow (cmf) SCRs add gas through an orifice or needle valve. More gas than the diver needs is fed into the loop, the difference is periodically "burbed" via an over pressure valve. Those units, for most part, are made for nitrox mixes, thus have the same maximum operating depth as OC nitrox would. Gas savings towards OC is about 4:1-5:1, the leaner the gas, the deeper the MOD, the higher the flow rate.
Passive addition SCRs are respiratory minute volume keyed. On every inhalation, a set amount of gas is dumped via a one way valve, the missing gas volume is then replenished by a Schrader valve. This type of SCR can be used with any pre-mixed gas(es) you use on OC circuit, and all manufacturers I can think of offer gas blocks that allow to switch the gas supply. Gas savings depends on the relation between the counterlung bellows and dump bellows. In stock form most units offer about 8:1 gas savings. The ratio has been pushed to 15:1 on deep dives with scooters by highly experienced exploration divers.
CCRs use two different gases, pure O2 and a second gas mix to dilute it so it can be breathed deeper than 6 meters safely. The diluent can be air, heliox or trimix. The diluent is only used to maintain loop volume (like adding gas to your BC when descending) and dilute the O2. O2 is only added to maintain a chosen partial pressure of O2.
Those are two important items:
O2 use is the same at the surface as it is at 10 ft, 100 ft or 1000 ft depth. Since only O2 gets replenished at depth, gas savings are higher the deeper you dive.
The partial pressure of O2 being stable means the mix in the loop is highly dynamic. You have a nitrox (or whatever gases you're using) mixing machine on your back. You'll always have the "best mix" for the depth you're at, which can extend your NDL and shorten your deco.
Downside is the gas in the loop is highly dynamic, you'll need to stay on top of it. There'll be gas in the loop, but it may not sustain life (too low pO2) or become toxic (to high pO2). Hence the first rule of RB diving, ALWAYS KNOW YOUR PO2!
And while you need only a very small gas supply for the loop, you'll need bailout of some sort should there be an emergency, negating the weight savings on a dive.
Other disadvantage are shared by all RBs: scrubber failure/CO2 hit, caustic cocktail, added gear, maintenance and expense, and absolute need for discipline.
