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Here you find the differences in the way SC en CC rebreathers differ. updated 26 may 2010
  (active) Semi Closed Rebreather (electronic) Closed Circuit Rebreather
Working principle Constant gas injection Controlled gas injection
Oxygen measurement no measurement constant oxygen measurement
Gas type used Nitrox Oxygen and Diluent
Partial Oxygen pressure Varies with depth Constant
Oxygen Fraction in gas Constant Varies with depth
Removal of carbon dioxide Scrubber Scrubber
Circuit Half closed, the system has a surplus of nitrogen and thus produce bubbles Fully closed, only metabolized oxygen is replaced by new oxygen. No bubbles
Gas usage versus open circuit 2-4 times more efficient 15-50 times more efficient
Decompression obligation similar to Nitrox diving 3x longer DT in 15-25 meter zone.
3-1 x in 25 - 50 meter zone
Price (2010) new: 2500 - 4000 euro new: 7500,00 - 15.000,00 euro
Price second hand (2010) ~ 1500,00 (dolphin) ~ 2500,00 - 6000,00 euro
Maintenance (O-rings/Cells etc) ~ 250 - 500,00 p/y 500,00 - 800,00 p/y

Since I have been diving rebreathers I heart so many times that rebreather diving is extremely expensive. If you look at the price you have to pay when you buy a new closed circuit rebreather, diving lamp, dry suit analyzer filling station underwater camera etc etc, YES it is expensive. But there is another side. Suppose you dive open circuit, you will have the same problem. You also need diving lamp, dry suit analyzer filling station underwater camera etc etc. The additional cost is the rebreather. I tried to explain that when you use the rebreather in the right way it can even be cheaper than diving open circuit. These days on most tropical destinations oxygen, sofnolime bottles and weights are available. I made a trip to Egypt and took my rebreather in parts without the bottles and without sofnolime. On board there were filling facilities and sofnolime for sale. When you look at this table you might be surprised to see that if you look at the cost price per hour diving closed circuit is by far the cheapest way!

  Open circuit diver Semi closed rebreather diver Closed Circuit rebreather diver
  in euro in euro in euro
Trip to Egypt 1000 1000 1000
Price diving system 2500 5000 8000
Expected lifetime 10 years 10 years 10 years
write-off per year 250 500 800
2 dives per day during 7 days gasprice 112 56 36
sofnolime 6,00 euro per kilo - 84 135
realized dive time in hours 14 18 28
total costs involved 1362,00 1640,00 1971,00
total price per diving hour 97,00 91,11 70,39
Now you might be convinced! As a diving addict that when you pay 1971-1362 = 609,00 euro more for your week than the open circuit diver you will have DOUBLE DIVING FUN!

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