ahhh but GB500NZ you oversimplify, and you are talking about a pure Oxygen rebreather like they used to use in WW2 (and some forces still use
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Yes Roger, you're basically bang on, although there are a variety of ways to spoil your day with one... all rebreathers I am aware of use a chemical scrubber (soda lime, sofnolime, dragersorb etc can't remember the proper chemical name) to "scrub" CO2 out of the breathing gas, the gas you breather is then recorculated round the loop and Oxygen added by a variety of means dependent on the rebreather.
Roughly speaking there are three main types of rebreather,
O2 rebreather, where neat oxygen is added, probably in a random or unmetered quantity, to make up for the Oxygen used by your metabolism. This means that at times it is possible to find yourself breathing neat O2. They are therefore limited by the fact that beyond a certain depth (dunno what 33ft is in metres but it sounds roughly right, 9mish ish from memory) the increased ambient pressure means that the Partial pressure of the oxygen will increase beyond 1.4 or 1.6 Bar (depending which group taught you). This in turn presents a significant risk of something called an "O2 hit", or properly, "Central Nervous System Toxicity" where the O2 becomes basically poisinous to your system and you will convulse, blackout, and become tonic in a relatively random sequence. Obviously underwater and needing a regulator held in your mouth to provide breathing gas this will commonly result in death
Above 9m they are ok, which is fine for checking your battleship for limpet mines (or attaching the same) but not a lot of use for "sports" diving.
The next sort is a "Semi closed circuit rebreather" which also uses a scrubber but the Oxygen is replace because a small sonic orifice allows a metered amount of Nitrox (Air with less nitrogen and extra Oxygen in simple terms) to be added to the breathing loop constantly. Approximately every four breaths the loop vents overpressure and all is right with the world
They are normally good to forty metres (130 feet) but any diving which needs decompression stops is normally prohibited which limits them severely. Nice simple mechanical system though.
The last type is CCR, or closed circuit rebreather. Complicated and clever and good for as deep as you dare in the right configuration and with the right gases plumbed in. Proper exotica and I don't have time to describe one as I need to go and put tea on!! But I'll continue later if anyone is still awake.....
