Is a rebreather safe at any depth or just deep depths? I thought I read somewhere that rebreather can be dangerous at shallow depths but my fiance said that you can use them at any depth. He said that military uses them at shallow depths and that is how he knows you can use a rebreather at shallow depths. So can you SAFELY use a rebreather at say, 40' and above or is it only for deep diving?
Has your question been answered or are you wondering more than when you posted the question?
I'll give it a shot.
First off, not every rebreather can be used at any depth. Different types of rebreathers were developed for different needs.
The O2 rebreathers used by combat swimmers are a tool for moving stealthily below the surface. For that they work well, and are also used by researchers, film makers etc for shallow water diving. Shallow water being 6 meters/18 feet or less. And before some jumps in and argues that they're taken much deeper than that by the military its simply a matter of training and taking chances. When the alternative is getting shot at or captured, the risk of O2 toxicity becomes a much more viable option.
In general it is, IMHO, not a good idea to equal military and civilian rebreather use. Military use of rebreathers is restricted to very specialized assignments, may it be EOD work or moving about unseen, and both the quality and the quantity of the training don't compare to civilian rebreather classes. Most importantly, the state of mind and level of discipline don't quite compare in most cases I believe. Last but not least, the diving protocol for mixed gas rebreathers is on a very different level in the military compared to the average CCR outing.
Rebreathers developed for using only pre-mixed gas or gases can be operated in the depth range(s) of the respective gases, same as open circuit. Those are usually semi-closed rebreathers, their purpose range from recreational (like the Dolphin or UBS 40) to extreme deep cave exploration.
Mixed-gas rebreathers are, as the name replies, gas mixing machines carried along by the diver. Depth range depends on the gases used, for recreational diving depths usually O2 and air (= nitrox), for deeper dives trimix or heliox.
Within those ranges rebreathers can be used. If your "SAFELY" is supposed to mean failsafe, I'm afraid none of them is. Neither are the divers using them. A fatality with a rebreather for "recreational" diving has occurred in a pool, with a "deep cave exploration" unit in less than 10 feet of water. Victims of fatal rebreather accidents have ranged from more or less untrained, uncertified, to well trained and highly experienced on their respective units.
As I said, rebreathers can fail, and so can their divers.
Something rebreather diving shares with open circuit diving, actually.
Hope that helps.