chippy:
I'm a very keen UW photographer, and rebreathers seem like a great way to extend bottom time and of course, a bit more fish friendly.
Either semi- or fully-closed rebreather will do that. A semi-closed unit will do it to a lesser extend. Rebreather diving is quite different from open circuit diving and needs to be learned first. If you want to go with a CCR don't even think about taking a camera during the first 50 dives. You need to gain experience and become proficient before you start sharing attention with other activities.
Emergency procedures as well as general safe RB diving procedures (such as checking your pO2) have to become second nature first.
chippy:
Firstly, what qualifications to I need to take a rebreather course? Presumably AOW and a nitrox course?
That depends on both the unit and the certifying agency. You may have to have advanced nitrox and/or rescue/first aid, and a minimum of logged dives (usually 50). The exception is the RB80 for which you have to show GUE DIR-F and GUE Tech-1.
chippy:
How much is a decent rebreather setup?
There is a broad range of RBs, the least expensive one should be the Dräger Ray at about $1700. Similar operating SCRs go up to about $4500, which is where manual controlled CCRs start. eCCRs start around $7500 and go up to about $20,000. Those are list prices, many units do not come complete. You may have to add things like bailout, O2 monitor, BC, tanks etc.
If you're buying in Europe the RB needs to have EC rating, as far as I know currently only the Drägers Ray and Dolphin, the Azimuth SCR and the Inspiration CCR have gotten that mark.
chippy:
Whats the defferences between semi closed and fully closed?
Semi-closed and fully closed describes the breathing loop.
SCR dump some of the gas from the loop, how much/often depends on the model you're using. There are two operating principles currently available: Active addition (Ray, Dolphin, Azimuth) and passive addition (RB80).
Active addition has a device to meter a constant mass flow which is higher than your matabolism (should be). The additional amount is the amount dumped by the over pressure valve. As the name implies, the flow of gas is constant once you open the tank's valve. Doesn't matter if you dive or just forgot to close it afterwards. For most part these SCRs are used with Nitrox mixes between 32% and 60%, which limits depth to recreational profiles. The lower the amount of O2 in the mix the larger the orifice and the higher the flow rate. So the amount of time you have on a tank depends on the mix you use.
Passive addition is keyed to your respiratory minute volume. Gas injection occurs at the rate you breath. During every breathing cycle a fixed amount is dumped. Just about any mix that can be used OC can be used here, the RB80 has been used very sucessfully deep cave penetrations.
Upon decent all RBs need to add gas to make up for increased ambient pressure (just like a BC or drysuit). Likewise, when ascending all RBs need to vent gas.
CCRs keep all the gas in the loop and only the metabolized O2 is added. This can either be done manually (Jetsam KISS) or electronically (Inspiration, Prism Topaz, Megalodon, MKseries etc.). CCRs generally have three O2 sensors to monitor the amount of O2 in the loop. With the KISS O2 is added constantly below your metabolic needs and occasionally topped of manually. You need to both monitor and add, there are no alarms. eCCRs have a computer (or two or three) that monitor the three sensor readouts, vote between them how much O2 is needed to stay at the chosen set point and control a solenoid valve that does the injection. They usually have some kind of alarm, visual, audible or both. In case the electronics or solenoid misfunction they can be dived manually in closed or semiclosed (just exhale through your nose) mode, and are all used with OC bailout.
Getting a rebreather is something you should consider carefully. As you breath in a loop there is usually always a breath to take ... but not necessarily one that can sustain your well being.
My recommendation:
Read up on the subject and the units available (where you are), if and how they fit your need and abilities, find an instructor and do an intro. That's a brief lecture and some time in the pool in most cases. You might even be able to try different systems. It'll give you a better idea if they are for you.
There is plenty of good info on this board (search function) as well as on the net.
